Enhancing entomopathogenic nematode efficacy with Pheromones: A field study targeting the pecan weevil

Consistent efficacy is required for entomopathogenic nematodes to gain wider adoption as biocontrol agents. Recently, we demonstrated that when exposed to nematode pheromone blends, entomopathogenic nematodes showed increased dispersal, infectivity, and efficacy under laboratory and greenhouse conditions. Prior to this study, the impact of entomopathogenic nematode-pheromone combinations on field efficacy had yet to be studied. Steinernema feltiae is a commercially available entomopathogenic nematode that has been shown to increase mortality in insect pests such as the pecan weevil Curculio caryae. In this study, the pecan weevil was used as a model system to evaluate changes in S. feltiae efficacy when treated with a partially purified ascaroside pheromone blend. Following exposure to the pheromone blend, the efficacy of S. feltiae significantly increased as measured with decreased C. caryae survival despite unfavorable environmental conditions. The results of this study highlight a potential new avenue for using entomopathogenic nematodes in field conditions. With increased efficacy, using entomopathogenic nematodes will reduce reliance on conventional management methods in pecan production, translating into more environmentally acceptable practices.

Efficacy of entomopathogenic fungi, nematodes and spinetoram combinations for integrated management of Thrips tabaci

BACKGROUND

Two consecutive field trials using a blend of entomopathogens in combination with a new chemistry insecticide were conducted to determine treatment effects on onion thrips (Thrips tabaci Lindeman) populations, crop damage, plant development, crop yield and impact on natural enemies. Products were tested in an onion cropping system and included the insect pathogenic fungus Beauveria bassiana (isolate WG-11), an entomopathogenic nematode Heterorhabditis bacteriophora (strain VS) and the new-chemistry chemical insecticide spinetoram.

RESULTS

In all treatments, a significant decrease in thrips per plant population was detected in both trials. Overall, dual application of entomopathogens and insecticide was more effective than singly applied treatments. The lowest number of thrips larvae (1.96 and 3.85) and adults (0.00 and 0.00) were recorded when treated with dual application of B. bassiana and spinetoram at 7 days post application (DPA) after the second spray application in 2017and 2018, respectively. Damage on onion plants was considerably decreased in all treatments relative to the control. The lowest damage was observed on onion plants treated with B. bassiana + spinetoram at 7 DPA after the second spray application during both years. A significant decrease in the number of natural enemies (beetles, spiders, mites, lacewings, ants and bugs) on onion plants was recorded during both years. Insect pathogens when applied alone and in combination with each other considerably protected arthropod natural enemies compared to insecticide application applied alone. Significant increase in plant agronomic traits was observed compared to the control. Among all the treatments, B. bassiana + spinetoram produced maximum leaf length, leaf weight, total leaves, neck diameter, bulb diameter, number of rings per bulb, bulb weight, dry matter and plant yield following the 2017and 2018 applications, respectively.

CONCLUSION

The findings of the study reveal the potential of using insect pathogens and insecticide for control of T. tabaci. However, combinations containing spinetoram are harmful to nontarget organisms, whereas biological control agents help in protecting biodiversity in onion agroecosystems. © 2023 Society of Chemical Industry.

Post-Application Field Persistence and Efficacy of Cordyceps javanica against Bemisia tabaci

Previously, Cordyceps javanica Wf GA17, a causing agent of whitefly epizootics in southern Georgia, demonstrated superior temperature tolerance and higher virulence against the whitefly Bemisia tabaci than commercial strains in the laboratory. The post-application persistence and efficacy of this fungus against B. tabaci were compared with that of the commercially available C. javanica Apopka97 strain over a two-year field study in cotton and vegetable crops. When blastospores of both strains were applied alone, whitefly populations were not effectively suppressed. Thus, JMS stylet oil was added to fungal treatments for enhancing efficacy and persistence. For 0-day samples, all fungal treatments caused similar but significant levels of immature mortality regardless of fungal strain, propagule form (conidia vs. blastospores), and application method (alone or mixed with JMS). In follow-up samplings, Wf GA17 blastospores + JMS achieved higher control levels than other treatments in some trials, but the efficacy did not last long. The JMS oil alone caused significant mortality and suppressed whiteflies. Over 90% of spores lost viability 24 h after treatment in all fungal treatments. Across evaluation times, there was no difference between the two fungal strains (conidia or blastospores, alone or combined with JMS), but conidia persisted better than blastospores for both strains. Overall, the field persistence and efficacy of C. javanica did not last long; therefore, improved delivery methods and formulations are needed for enhancement.

Tolerance of Steinernema carpocapsae infective juveniles in novel nanoparticle formulations to ultraviolet radiation

Entomopathogenic nematodes (EPNs) are susceptible to abiotic environmental factors including ultraviolet (UV) radiation, which affects the survival and efficacy. This study evaluated nanoparticle (NP) formulations for protecting Steinernema carpocapsae infective juveniles (IJs) from UV radiation. First, silica-NH₂ NPs at oil-to-water ratios of 2:8, 3:7 and 4:6 were compared with Barricade Fire Gel (1 % and 2 %) and a water control (aqueous IJs) by exposing IJs to UV light (254 nm) for 0, 10 and 20 min. Barricade gel (especially 2 % Barricade) significantly improved IJs viability after UV treatment, while all three NPs had adverse effects on IJ viability after UV radiation. Subsequently, two silica (SiO₂ basic and advanced) and one titania (TiO₂) based formulations were tested with Barricade (1 % and 2 %) and a water control. The titania-NH₂ NPs provided the highest UV protection, and IJ viability and virulence were not reduced even after 20-min UV. Except TiO₂, only 2 % Barricade at 10-min UV and SiO₂ basic at 20-min UV had lower IJ mortality than the water control. Only TiO₂ formulated IJs caused higher insect mortality and infection levels than aqueous IJs after UV treatment. The UV tolerance of TiO₂ was further examined by assessing the number of nematodes invading the hosts. Consistent with virulence tests, the number of invading nematodes in titania-NH₂ NPs did not decrease after UV radiation for 10 or 20 min compared with the no-UV control. The anti-UV capability of titania-NH₂ NPs has promise as a tool to enhance biocontrol efficacy of EPNs under field conditions.

Combined application of entomopathogenic nematodes and fungi against fruit flies, Bactrocera zonata and B. dorsalis (Diptera: Tephritidae): laboratory cups to field study

BACKGROUND

The peach fruit fly, Bactrocera zonata, and the Oriental fruit fly, B. dorsalis (Diptera: Tephritidae), are economically important fruit fly species in various regions of the world. We evaluated the effects of separate and combined applications of the entomopathogenic fungi (EPF) Beauveria bassiana (WG-18) and Metarhizium anisopliae (WG-02), and the entomopathogenic nematodes (EPNs) Heterorhabditis bacteriophora (VS strain) and Steinernema carpocapsae (ALL strain) against larvae, pupae and pharate adults, of B. zonata and B. dorsalis under laboratory, glasshouse and field cage conditions.

RESULTS

Combined applications of EPF and EPNs produced greater mortality than individual treatments under all conditions. Against both species, the combination of B. bassiana and H. bacteriophora consistently exerted strong effects that were similar to the combined application of B. bassiana and S. carpocapsae whereas M. anisopliae applied with S. carpocapsae was least effective in all combinations. In a laboratory bioassay, synergistic interactions were detected between B. bassiana and H. bacteriophora applied against larvae and pharate adults of both fly species, between B. bassiana and S. carpocapsae against larvae of both species and pharate adults of B. zonata, and between M. anisopliae and H. bacteriophora against B. zonata larvae. Other combined treatments resulted in additive effects, especially against fly pupae. In a potted soil bioassay, there were only additive interactions in all combinations against different stages of both flies. The 3rd instar of both flies was more susceptible than pharate adult and pupal stages. Additive interactions between EPNs and EPF were detected in the glasshouse against 3rd instars and pupae, and under field conditions against 3rd instars of both fly species.

CONCLUSION

These results indicate how particular combinations of entomopathogenic fungi and nematodes could be deployed in integrated pest management of tephritid fruit flies in orchard agro-ecosystems. © 2022 Society of Chemical Industry.

Comparing Production and Efficacy of Cordyceps javanica With Cordyceps fumosorosea

A newly discovered entomopathogenic fungus Cordyceps javanica (Friedrichs & Bally) Samson & Hywel-Jones (Hypocreales: Cordycipitaceae) strain Wf GA17 was compared with the commercial Cordyceps fumosorosea Wize (Hypocreales: Cordycipitaceae) Apopka 97 strain for liquid-culture production, formulation, insecticidal efficacy, and storage stability under laboratory conditions. We compared culture media with carbon:nitrogen (C:N) ratios of 10:1, 30:1, and 50:1 for these two isolates. A third strain, C. fumosorosea strain ARSEF 3581, had previously been optimized for liquid-culture production of blastospores at 10:1 C:N served as an added control. These seven cultures were processed by spray drying with skim milk powder, stored at 25oC to evaluate storage stability, and assayed for insecticidal activity against Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae) neonates. Final blastospore concentrations were not significantly different among cultures, ranging from 4.47 to 9.88 × 108 spores/ml. Fungal biomass decreased and final glucose concentrations increased with increasing C:N ratios, indicating better fungal growth with higher nitrogen concentrations. Product yields from the spray dryer (grams per liter culture) increased with increased C:N ratios while spore concentrations decreased, ranging from 2.27 to 7.17 × 109 spores/g. There were no significant differences for insecticidal efficacy among the seven treatments. Spores produced in 10:1 C:N ratio media retained viability longer than spores produced in other media. Cost of ingredients decreased with increasing C:N ratios, such that the 30:1 media may yield the most economical product. The raw material cost needed for application was 1.4× greater for Wf GA17 compared with Apopka 97, a difference that could be erased by optimization of culture conditions.

Laboratory virulence of entomopathogenic nematodes to the sweetpotato whitefly, Bemisia tabaci

The sweetpotato whitefly, Bemisia tabaci Middle East-Asia Minor 1 (MEAM1), is a major insect pest on vegetable crops worldwide. Enormous economic losses result from direct and indirect plant damage caused by MEAM1. Biological control using entomopathogenic nematodes (EPN) may be an effective alternative strategy against MEAM1 because this pest has developed resistance to most insecticides. First, nine EPN species (Heterorhabditis bacteriophora, H. indica, H. georgiana, H. floridensis, Steinernema feltiae, S. carpocapsae, S. riobrave, S. glaseri, and S. rarum) were investigated for virulence to MEAM1 third instar nymphs on snap bean leaves under laboratory conditions. The mortality of MEAM1 nymphs was evaluated at 3 days post-inoculation (dpi). Compared to the water control, the application of the nine EPN species except S. glaseri resulted in significantly higher mortality of MEAM1 nymphs, such as H. bacteriophora (66.31%), H. floridensis (56.38%), S. carpocapsae (54.54%), and S. rarum (57.80%). Subsequently, the four virulent EPN species, H. bacteriophora, H. floridensis, S. carpocapsae, and S. rarum were evaluated further for virulence to MEAM1 nymphs on snap bean and tomato leaves. The mortality of MEAM1 nymphs was assessed at 3 dpi and 7 dpi. There were no significant differences in MEAM1 nymphal mortality between tomato and snap bean at either 3 dpi or 7 dpi. The mortality of MEAM1 nymphs caused by the application of H. floridensis (99.25%) was significantly higher than the other three EPN species and the water control at 7dpi. The results indicate that H. floridensis is a very promising biocontrol agent for B. tabaci management.

Identification and Virulence of Cordyceps javanica Strain wf GA17 Isolated From a Natural Fungal Population in Sweetpotato Whiteflies (Hemiptera: Aleyrodidae)

A new strain of the entomopathogenic fungus, identified as Cordyceps javanica (Frieder. & Bally) Kepler, B. Shrestha & Spatafora (Hypocreales: Cordycipitaceae) wf GA17, was found naturally infecting the sweetpotato whitefly, Bemisia tabaci (Gennadius) MEAM1 in southern Georgia, US, in September 2017. The fungus was tested for pathogenicity and virulence in comparison with commercially available entomopathogenic fungal strains against several insect species in the laboratory. In specific, it was compared with Cordyceps fumosorosea (Wize) Kepler, B. Shrestha & Spatafora (Hypocreales: Cordycipitaceae) Apopka 97, Beauveria bassiana (Bals.-Criv.) Vuill. (Hypocreales: Cordycipitaceae) strain GHA, and Metarhizium brunneum Petch (Hypocreales: Clavicipitaceae) strain F52 for virulence against B. tabaci (4th instars) and cotton aphid, Aphis gossypii Glover (Hemiptera: Aphididae) (4th instars with or without wingbuds), on leaf-discs, and against last instars of pecan weevil Curculio caryae (Horn) (Coleoptera: Curculionidae) and citrus root weevil Diaprepes abbreviatus (L.) (Coleoptera: Curculionidae) in soil cups. Against B. tabaci, C. javanica exhibited higher mortality and mycosis development at 5 d post inoculation than other fungi. In assays against A. gossypii with and without wingbuds, C. javanica and C. fumosorosea had the highest mortality and mycosis levels and B. bassiana had the lowest; nymphs with wingbuds were more susceptible to some fungal infection than those without. Against C. caryae, B. bassiana was more effective than other fungi. For D. abbreviatus, B. bassiana also caused the highest mortality while M. brunneum had the lowest, with Cordyceps spp. being intermediate. Overall, the findings suggest high potential of the new strain, C. javanica wf GA17, for managing whiteflies and aphids, while it was not as effective as B. bassiana against the curculionids.

Quantification of pH tolerance levels among entomopathogenic nematodes

Soil pH affects the availability of nutrients, which impacts plant growth and development. Similarly, soil pH may also influence microorganisms in the soil, either beneficial or nonbeneficial. One such group of beneficial microorganisms is entomopathogenic nematodes (EPN), parasites of soil-inhabiting insects. Entomopathogenic nematodes have a number of attributes that make them good alternatives to chemical insecticides. The objective of this study was to investigate pH tolerance of 11 steinernematids and six heterorhabditids post exposure to different pH levels. Entomopathogenic nematode populations were exposed to varying pH levels (pH2 to pH11) made up from two different chemical solutions (ammonium-acetate and citrate-phosphate). Entomopathogenic nematode populations are expected to have varying tolerance to different pH levels. The highest infective juvenile survival was obtained from pH3 to pH10 in citrate-phosphate, where all populations displayed >50% survival. Steinernema carpocapsae populations had >90% survival at pH3 to pH11 in citrate-phosphate solutions. Overall, the steinernematids had a higher survival range in ammonium-acetate pH solutions compared with the heterorhabditids. Moreover, Steinernema spp., S. carpocapsae (ScCxrd, ScAll, and ScItalian) and S. riobrave showed consistently higher survival in both acidic and alkaline solutions, when compared to the other steinernematids, suggesting that they may be applied in both acidic and alkaline soils. These findings can be of use when selecting EPNs for biological control purposes in the two countries, respectively.

Evaluation of Barricade® to enhance survival of entomopathogenic nematodes on cowhide

Entomopathogenic nematodes (EPNs), Steinernema riobrave and Heterorhabditis floridensis are under evaluation for eradication of the southern cattle fever tick, Rhipicephalus microplus infesting nilgai antelope, in South Texas. Cattle fever ticks are a significant threat to the U.S. livestock industry. Although they have been eradicated in the U.S. they frequently re-invade along the Texas-Mexico border. Remotely operated field sprayers have been developed to directly treat nilgai antelope with EPNs as they transit fence crossings and as they contact wetted foliage and soil from the surrounding area. EPNs are known to be susceptible to mortality from ultraviolet light (UV) and desiccation. A sprayable fire gel, Barricade®, has been reported to protect EPNs from UV and desiccation but has not been tested on animal hides. Barricade® at 1 and 2 percent rates was mixed with the water solution of S. riobrave and H. floridensis and applied to cowhides (to mimic direct treatment of nilgai) and filter paper and then these substrates were placed out of doors in 0, 30, 60 or 120 min of sunlight. Wax moth larvae, Galleria mellonella, were exposed to the cowhides and filter paper to determine efficacy of the EPNs. Efficacy of S. riobrave with 1 and 2% Barricade® gel applied to cowhides was significantly improved at 30 and 60 min as compared to the control. At 120 min mortality of the wax moth larvae was near zero for both the control and the treatments. Similar results were found with the filter paper test. In contrast, efficacy of H. floridensis with Barricade® applied to cowhides or filter paper was not significantly improved at 30, 60 or 120 min as compared to the water only control. Barricade® has the potential to improve the efficacy of S. riobrave and other EPNs by reducing mortality and desiccation, especially when used in the remotely operated sprayer developed for treatment of cattle fever tick infested nilgai.

Antifungal activity of Xenorhabdus spp. and Photorhabdus spp. against the soybean pathogenic Sclerotinia sclerotiorum

The fungus, Sclerotinia sclerotiorum, causes white mold disease and infects a broad spectrum of host plants (> 500), including soybean with yield losses of up to 70%. Biological control is a potential alternative for management of this severe plant pathogen, and relative to chemical fungicides, provides broad benefits to the environment, farmers and consumers. The symbiotic bacteria of entomopathogenic nematodes, Xenorhabdus spp. and Photorhabdus spp., are characterized by the production of antimicrobial compounds, which could serve as potential sources for new bio-fungicides. The objectives of this study were to assess cell-free supernatants (CFS) of 16 strains of these bacteria cultures on S. sclerotiorum mycelium growth; assess the volatiles of X. szentirmaii cultures on the fungus mycelium and sclerotium inhibition; and evaluate the X. szentirmaii cultures as well as their CFS on the protection of soybean seeds against the white mold disease. Among the 16 strains, the CFS of X. szentirmaii showed the highest fungicidal effect on growth of S. sclerotiorum. The CFS of X. szentirmaii inhibited > 98% of fungus growth from mycelium and sclerotia, whereas the volatiles generated by the bacterium culture inhibited to 100% of fungus growth and 100% of sclerotia production. The bacterial culture diluted to 33% in water and coated on soybean seeds inhibited S. sclerotiorum and protected soybean plants, allowing 78.3% of seed germination and 56.6% of plant development. Our findings indicate potential for a safe and novel control method for S. sclerotiorum in soybean. Moreover, this is the first study to indicate that volatile organic compounds from Xenorhabdus spp. can be used in plant disease suppression.

Mortality of native and invasive ladybirds co-infected by ectoparasitic and entomopathogenic fungi

Harmonia axyridis is an invasive alien ladybird in North America and Europe. Studies show that multiple natural enemies are using Ha. axyridis as a new host. However, thus far, no research has been undertaken to study the effects of simultaneous infection by multiple natural enemies on Ha. axyridis. We hypothesized that high thallus densities of the ectoparasitic fungus Hesperomyces virescens on a ladybird weaken the host’s defenses, thereby making it more susceptible to infection by other natural enemies. We examined mortality of the North American-native Olla v-nigrum and Ha. axyridis co-infected with He. virescens and an entomopathogenic fungus—either Beauveria bassiana or Metarhizium brunneum. Laboratory assays revealed that He. virescens-infected O. v-nigrum individuals are more susceptible to entomopathogenic fungi, but Ha. axyridis does not suffer the same effects. This is in line with the enemy release hypothesis, which predicts that invasive alien species in new geographic areas experience reduced regulatory effects from natural enemies compared to native species. Considering our results, we can ask how He. virescens affects survival when confronted by other pathogens that previously had little impact on Ha. axyridis.

Environmental Tolerance of Entomopathogenic Fungi: A New Strain of Cordyceps javanica Isolated from a Whitefly Epizootic Versus Commercial Fungal Strains

Simple Summary

Whiteflies are significant pests of cotton and vegetables in southeastern USA. In previous studies, we isolated and identified a new strain of entomopathogenic fungus that caused epizootics among whiteflies in cotton fields of Southern Georgia, USA. The objective of this study was to test the level of tolerance of this new strain to environmental conditions as compared to commercial fungal strains. We exposed the new strain and three commercially available strains of biopesticides (BotaniGard, Met52, and PFR-97) to different temperatures and strong ultraviolet (UV) radiation before examining post-treatment viability and virulence against a common model organism for insect pathology, the greater wax moth larvae. We found that the new strain had similar levels of activity to commercial strains at moderate temperatures, but higher tolerance than PFR-97 to extremely low and high temperatures and strong UV intensity. These findings suggest that the new fungal strain has potential for commercial development as an alternative to PFR-97 for managing certain types of insect pests.

Abstract

A new strain of Cordyceps javanica (wf GA17) was observed causing widespread epizootics among whiteflies in Southern Georgia in 2017. The tolerance of conidia to environmental factors including variable temperature and ultraviolet (UV) light was compared between this strain and three commercial strains of entomopathogenic fungi (Metarhizium brunneum F52, Cordyceps fumosorosea Apopka97, and Beauveria bassiana GHA). Under 10–30 °C, C. javanica wf GA17 responded similarly to other fungi, with the highest virulence against Galleria mellonella at 25 °C, followed by 20, 30, and 15 °C; lowest virulence was observed at 10 °C. At 35 °C and 40 °C, C. javanica wf GA17 had lower tolerance than M. brunneum F52 and B. bassiana GHA, but was superior to C. fumosorosea Apopka97 in conidia viability and post-treatment virulence. After exposure to −20 °C for 56 d, C. javanica wf GA17 exhibited lower germination than M. brunneum F52 and lower virulence than M. brunneum F52 and B. bassiana GHA, but higher germination and virulence than C. fumosorosea Apopka97. Following exposure to strong UV light, viability and virulence of all fungi were reduced with increasing exposure periods. Increased environmental tolerance of C. javanica wf GA17 over C. fumosorosea Apopka97 suggests that the new strain could have applicability for commercial pest management.

Environmental tolerance of entomopathogenic nematodes differs among nematodes arising from host cadavers versus aqueous suspension

Environmental factors such as temperature and desiccation impact the survival and efficacy of entomopathogenic nematodes (EPNs). Most studies on environmental tolerance have focused on EPNs applied in aqueous suspension. Another approach for EPN application is via infected host cadavers. Emergence in host cadavers is also more representative of nematodes in natural populations. In prior studies, certain advantages in fitness have been observed with the cadaver application approach relative to aqueous application, yet the impact of environmental stress on these approaches requires investigation. In this study, we compared the effects of various temperatures (heat and cold) and desiccation intervals (48 and 72 hr) on the survival, virulence and reproductive capacity of Heterorhabditis bacteriophora and Steinernema glaseri when applied via cadaver versus aqueous suspension. In the heat tolerance bioassays, following exposure to 30 °C, 35 °C and 37. 5 °C, nematodes (from both species) in the cadaver treatments exhibited higher survival, and reproductive capacity compared with aqueous application. No survival was observed above 37.5 °C regardless of species or application approach. In cold tolerance, no differences were observed between the cadaver and aqueous treatments after a sequence of exposures from 10 °C to -2 °C. In desiccation assays, following exposure to 85% relative humidity for 2 or 3 days, nematodes (from both species) exhibited higher survival and reproduction in the cadaver treatment than in the aqueous treatment, whereas no differences were observed in virulence. This is the first study to find differential stress tolerance among nematodes emerged from infected host cadavers versus those applied in aqueous suspension. Our findings indicate additional advantages when using the cadaver approach for biocontrol applications, and suggest EPNs existing in natural populations may have broader environmental tolerance than those applied via aqueous suspension.

Preferential infectivity of entomopathogenic nematodes in an envenomed host

Entomopathogenic nematodes and parasitoid wasps are used as biological control agents for management of insect pests such as the Indian meal moth, Plodia interpunctella. The parasitoid wasp Habrobracon hebetor injects a paralytic venom into P. interpunctella larvae before laying eggs. A previous study reported that the entomopathogenic nematode Heterorhabditis indica preferentially infects P. interpunctella that have been envenomed by H. hebetor while results in this study showed a similar preference by the entomopathogenic nematode, Steinernema glaseri. We therefore tested four hypotheses for why nematode infection rates are higher in envenomed hosts: (1) elevated CO₂ emission from envenomed hosts attracts nematodes, (2) paralysis prevents hosts from escaping nematodes, (3) volatile chemicals emitted from envenomed hosts attract nematodes and increase infection, and (4) reduced immune defenses in envenomed hosts increase nematode survival. Results showed that envenomed P. interpunctella larvae emitted lower amounts of CO₂ than non-envenomed larvae. Physical immobilization of P. interpunctella larvae did not increase infection rates by S. glaseri but did increase infection rates by H. indica. Emissions from envenomed hosts were collected and analyzed by thermal desorption gas chromatography/mass spectrometry. The most abundant compound, 3-methyl-3-buten-1-ol, was found to be an effective cue for S. glaseri attraction and infection but was not an effective stimulus for H. indica. Envenomed P. interpunctella exhibited a stronger immune response toward nematodes than non-envenomed hosts. Altogether, we conclude that different mechanisms underlie preferential infection in the two nematode species: host immobilization for H. indica and chemical cues for S. glaseri.

Pheromone extracts act as boosters for entomopathogenic nematodes efficacy

Inconsistency in entomopathogenic nematode (EPN) efficacy is still one of the biggest challenges for the wider adoption of EPNs as biocontrol agents. Previous studies demonstrated that extracts from EPN-infected hosts enhance dispersal and efficacy, two key factors in success of EPNs. Some active components in the insect host cadavers responsible for dispersal, ascarosides, have been identified as nematode pheromones. We hypothesized that pheromone extracts increase dispersal of EPN infective juveniles (IJs) leading to increased efficacy. First, we determined whether pheromone extracts improved IJ movement/dispersal in soil columns baited with Tenebrio molitor larvae. We found that pheromone extracts induced higher numbers of Steinernema carpocapsae and Steinernema feltiae IJs to move towards T. molitor larvae in the bottom of the column compared to IJs treated with infected cadaver macerate and water, positive and negative controls, respectively. Furthermore, the number of S. carpocapsae IJs that invaded T. molitor larvae was higher for the pheromone extract treatment than the controls. S. feltiae IJs that were pretreated with pheromone extracts and macerate (positive control) infected T. molitor at the same rate but invasion was superior to IJs that were treated with water. Consistent with the soil column tests, both S. carpocapsae and S. feltiae IJs treated with pheromone extracts performed better in killing larvae of two economically important insect larvae, pecan weevil, Curculio caryae, and black soldier fly, Hermetia illucens, in greenhouse tests compared to IJs treated with water. We demonstrated pheromone-mediated behavioral manipulation of a biological control agent to enhance pest control potential. Conceivably, nematodes can be exposed to efficacy-enhancing pheromones prior to field application.

Comparative Assessment of Four Steinernematidae and Three Heterorhabditidae Species for Infectivity of Larval Diabrotica Virgifera Virgifera

Larval Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae) were exposed to seven different entomopathogenic nematode species to test their potential infectivity in a laboratory setting. Known D. virgifera-infecting nematode species Heterorhabditis bacteriophora Poinar, Heterorhabditis megidis Poinar, Jackson & Klein, Steinernema feltiae Filipjev, and Steinernema carpocapsae Weiser were tested in a concerted experiment alongside Steinernema diaprepesi Nguyen & Duncan, Steinernema riobrave Cabanillas, Poinar & Raulston, and a Missouri wild-type H. bacteriophora which have not been previously tested on D. virgifera. The species S. rarum Doucet was tested separately for D. virgifera infectivity. Third-instar D. virgifera were exposed to either 60 or 120 nematodes per larva for 6 d. Following exposure, mortality was recorded and larvae were examined to determine the presence of active nematode infections. Results indicated a significantly higher proportion of larvae with active infections from the Heterorhabditidae species and S. diaprepesi than the other Steinernematidae species for both exposure rates; mortality data indicated a similar trend. Steinernema rarum showed almost no infectivity in laboratory experiments.

Control of Pecan Weevil With Microbial Biopesticides

The pecan weevil, Curculio caryae (Horn) (Coleoptera: Curculionidae), is a key pest of pecans Carya illinoinensis ([Wangenh.] K. Koch) (Fagales: Juglandaceae). Control recommendations rely on broad spectrum chemical insecticides. Due to regulatory and environmental concerns, effective alternatives for C. caryae control must be sought for pecan production in conventional and organic systems. We explored the use of microbial biopesticides for control of C. caryae in Georgia pecan orchards. Three experiments were conducted. The first investigated an integrated microbial control approach in an organic system at two locations. Three microbial agents, Grandevo (based on byproducts of the bacterium Chromobacterium subtsugae Martin, Gundersen-Rindal, Blackburn & Buyer), the entomopathogenic nematode Steinernema carpocapsae (Weiser), and entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin, were applied to each treatment plot (0.6 ha) at different times during the season. A second experiment compared the effects of S. carpocapsae and B. bassiana applied as single treatments relative to application of both agents (at different times); survival of C. caryae was assessed approximately 11 mo after larvae were added to pots sunk in an organic pecan orchard. In a conventional orchard (with 1.0 ha plots), the third experiment compared Grandevo applications to a commonly used regime of chemical insecticides (carbaryl alternated with a pyrethroid). All experiments were repeated in consecutive years. The combined pest management tactic (experiment 1) reduced C. caryae infestation relative to non-treated control plots in both locations in 2014 and one of the two locations in 2015 (the other location had less than 1% infestation). In experiment 2, no differences among combined microbial treatments, single-applied microbial treatments or different numbers of application were observed, yet all microbial treatments reduced C. caryae survival relative to the control. In the third experiment, both Grandevo and standard chemical insecticide applications resulted in lower weevil infestation than the control (both years) and there was no difference between the insecticide treatments in 2014 although the chemical insecticide regime had slightly lower infestation in 2015. These results provide evidence that microbial biopesticides can substantially reduce pecan weevil infestations in organic and nonorganic systems.

Trans-cinnamic acid and Xenorhabdus szentirmaii metabolites synergize the potency of some commercial fungicides

Development of novel approaches for the control of fungal phytopathogens is desirable. In this study we hypothesized that the combination of commercial fungicides with certain enhancing agents could result in synergistic levels of control. Prior research has indicated that trans-cinnamic-acid (TCA), a metabolite of the bacteria Photorhabdus luminescens and metabolites of Xenorhabdus szentirmaii are particularly toxic to various phytpathogenic fungi when compared to metabolites of other Xenorhabdus or Photorhabdus spp. In this study we explored the efficacy of commercial fungicide interactions when combined with either TCA or X. szentirmaii. Fungicides (active ingredient) included Abound® (Azoxystrobin), Serenade® (Bacillus subtilis), Elast® (dodine), Regalia® (extract of Reynoutria sachalinensis), Prophyt® (potassium phosphite) and PropiMax® (propiconazole). In laboratory experiments, singly-applied or combined agents were assessed for fungicidal activity against four plant-pathogenic fungi, Monilinia fructicola, Rhizoctonia solani, Colletotrichum gloeosporioides and Fusarium oxysporum. Fungicidal activity was measured by the phytopathogen's growth on potato dextrose agar with and without fungicide. The interactions between fungicidal agents were determined as antagonistic, additive or synergistic. For suppression of M. fructicola, synergy was observed between TCA when combined with certain concentrations of Elast®, PropiMax®, Regalia®, Prophyte® or Serenade®, and for combinations of X. szentirmaii with Abound®. For suppression of R. solani, synergy was observed between TCA combined with Regalia® or Serenade®. Additionally, when TCA was combined with X. szentirmaii synergistic levels of suppression to M. fructicola were observed. Other combinations of TCA or X. szentirmaii with the fungicides or using alternate concentrations were either additive or occasionally antagonistic in nature. Our results indicate that TCA and X. szentirmaii can each act as strong synergists to enhance fungicidal efficacy. These results may be used to reduce negative environmental impacts of pesticide use while improving control of plant diseases. Additional research is needed to explore the diversity of the synergistic effects and confirm our observations under field conditions.

A Comparison of Novel Entomopathogenic Nematode Application Methods for Control of the Chive Gnat, Bradysia odoriphaga (Diptera: Sciaridae)

Bradysia odoriphaga Yang & Zhang (Diptera: Sciaridae) is the most serious pest of Chinese chive, Allium tuberosum Chemical pesticide application has become a necessary step to control B. odoriphaga after each of the four to six harvests during the growing season. We compared the effects of application type (nematode-infected cadaver, aqueous nematode suspension, and mixture of cadaver and aqueous suspension) and nematode species (Heterorhabditis bacteriophora vs. Steinernema carpocapsae) on B. odoriphaga control. Nematode species combinations and the use of the cadaver method has only been tested in a relatively few studies, and has not been tested for this target pest, B. odoriphaga. Furthermore, this is the first report of combining application methods (aqueous and cadaver). Results indicated that the cadaver treatment produced higher mortality of B. odoriphaga than the aqueous treatment, and H. bacteriophora caused higher mortality of B. odoriphaga than S. carpocapsae. The mortality of B. odoriphaga was 96.7% in H. bacteriophora cadaver treatment and only 27.5% in control treatment without nematode addition. Cadaver treatments caused higher biomass of A. tuberosum than that of aqueous treatment. The plant biomass in H. bacteriophora cadaver treatment was 10 folds as much as that in the control treatment. The mixed aqueous suspension of H. bacteriophora and S. carpocapsae did not increase the mortality in pot trial. Our findings indicate that entomopathogenic nematodes applied as cadavers might be a potential alternative biological agent in the integrated management of B. odoriphaga for Chinese chive production.

The Effects of Nutrient Concentration, Addition of Thickeners, and Agitation Speed on Liquid Fermentation of Steinernema feltiae

Entomopathogenic nematode production in liquid fermentation still requires improvements to maximize efficiency, yield, and nematode quality. Therefore, this study was aimed at developing a more suitable liquid medium for mass production of Steinernema feltiae, by assessing the effects of nutrient concentration, thickeners (primarily agar), and agitation speed on infective juvenile (IJ) yield. Base medium (BM) contained yeast extract (2.3%), egg yolk (1.25%), NaCl (0.5%), and corn oil (4%). All media were inoculated with Xenorhabdus bovienii, and 2 d later, with 2-d-old S. feltiae juveniles. For the nutrient concentration experiment, we evaluated the base medium versus a modified base medium containing all the components, but with 3× concentrations of yeast extract (6.9%), egg yolk (3.75%), and corn oil (12%). The nematodes and bacteria were cultured in 150-ml Erlenmeyer flasks containing 50 ml of liquid medium at (25°C) and 180 rpm on a rotary shaker incubator. To assess the effect of thickeners, IJs were inoculated in BM with agar (0.2%), carrageen (0.2%), and carboxymethyl cellulose (0.2% and 0.5%). The addition of 3× more nutrients relative to the BM resulted in a significantly lower yield of nematodes. For agar and agitation speed experiments, five levels of agar in the BM (0%, 0.2%, 0.4%, 0.6%, and 0.8% agar) and two agitation speeds (180 and 280 rpm) were evaluated for production. Increasing agitation speed from 180 to 280 rpm and higher levels of agar in the medium (> 0.2%) significantly increased the yield of bacteria. At the lower agitation speed, media amended with 0.4% and 0.6% agar produced higher nematode yields compared to media without agar. Media with 0.2% and 0.8% agar resulted in intermediate levels of nematode production. At the higher agitation speed, media supplemented with 0.8% agar resulted in the lowest yield of nematodes when compared to the other media tested. Results indicated that increasing nutrient concentration levels was detrimental to nematode production. Also, media containing agar (0.4% and 0.6%) increased nematode yields when cultures were grown at low agitation speed. When IJs were used as the inoculum, 0.2% agar also enhanced recovery and nematode yield at the higher agitation speed.

Multifaceted effects of host plants on entomopathogenic nematodes

The success of parasites can be impacted by multi-trophic interactions. Tritrophic interactions have been observed in parasite-herbivore-host plant systems. Here we investigate aspects of multi-trophic interactions in a system involving an entomopathogenic nematode (EPN), its insect host, and host plant. Novel issues investigated include the impact of tritrophic interactions on nematode foraging behavior, the ability of EPNs to overcome negative tritrophic effects through genetic selection, and interactions with a fourth trophic level (nematode predators). We tested infectivity of the nematode, Steinernema riobrave, to corn earworm larvae (Helicoverpa zea) in three host plants, tobacco, eggplant and tomato. Tobacco reduced nematode virulence and reproduction relative to tomato and eggplant. However, successive selection (5 passages) overcame the deficiency; selected nematodes no longer exhibited reductions in phenotypic traits. Despite the loss in virulence and reproduction nematodes, first passage S. riobrave was more attracted to frass from insects fed tobacco than insects fed on other host plants. Therefore, we hypothesized the reduced virulence and reproduction in S. riobrave infecting tobacco fed insects would be based on a self-medicating tradeoff, such as deterring predation. We tested this hypothesis by assessing predatory success of the mite Sancassania polyphyllae and the springtail Sinella curviseta on nematodes reared on tobacco-fed larvae versus those fed on greater wax moth, Galleria mellonella, tomato fed larvae, or eggplant fed larvae. No advantage was observed in nematodes derived from tobacco fed larvae. In conclusion, our results indicated that insect-host plant diet has an important effect on nematode foraging, infectivity and reproduction. However, negative host plant effects, might be overcome through directed selection. We propose that host plant species should be considered when designing biocontrol programs using EPNs.

Curative Control of the Peachtree Borer Using Entomopathogenic Nematodes

The peachtree borer, Synanthedon exitiosa (Say 1823), is a major pest of stone fruit trees in North America. Current management relies upon preventative control using broad-spectrum chemical insecticides, primarily chlorpyrifos, applied in the late summer or early fall. However, due to missed applications, poor application timing, or other factors, high levels of S. exitiosa infestation may still occur and persist through the following spring. Curative treatments applied in the spring to established infestations would limit damage to the tree and prevent the next generation of S. exitiosa from emerging within the orchard. However, such curative measures for control of S. exitiosa do not exist. Our objective was to measure the efficacy of the entomopathogenic nematode, Steinernema carpocapsae, as a curative control for existing infestations of S. exitiosa. In peach orchards, spring applications of S. carpocapsae (obtained from a commercial source) were made to infested trees and compared with chlorpyrifos and a water-only control in 2014 and 2015. Additionally, types of spray equipment were compared: nematodes were applied via boom sprayer, handgun, or trunk sprayer. To control for effects of application method or nematode source, in vivo laboratory-grown S. carpocapsae, applied using a watering can, was also included. Treatment effects were assessed 39 d (2014) or 19 d (2015) later by measuring percentage of trees still infested, and also number of surviving S. exitiosa larvae per tree. Results indicated that S. carpocapsae provided significant curative control (e.g., >80% corrected control for the handgun application). In contrast, chlorpyrifos failed to reduce S. exitiosa infestations or number of surviving larvae. In most comparisons, no effect of nematode application method was detected; in one assessment, only the handgun and watering can methods reduced infestation. In conclusion, our study indicates that S. carpocapsae may be used as an effective curative measure for S. exitiosa infestations.

In Vivo Production of Entomopathogenic Nematodes

In nature, entomopathogenic nematodes in the genera Heterorhabditis and Steinernema are obligate parasites of insects. The nematodes are used widely as biopesticides for suppression of insect pests. More than a dozen entomopathogenic nematode species have been commercialized for use in biological control. Most nematodes intended for commercial application are produced in artificial media via solid or liquid fermentation. However, for laboratory research and small greenhouse or field trials, in vivo production of entomopathogenic nematodes is the common method of propagation. Additionally, small companies continue to produce nematodes using in vivo methods for application in niche markets. Advances in mechanization and alternative production routes (e.g., production geared toward application of nematodes in infected host cadavers) can improve efficiency and economy of scale. The objective of this chapter is to describe basic and advanced procedures for in vivo production of entomopathogenic nematodes.

Viability and Virulence of Entomopathogenic Nematodes Exposed to Ultraviolet Radiation

Entomopathogenic nematodes (EPNs) can be highly effective biocontrol agents, but their efficacy can be reduced due to exposure to environmental stress such as from ultraviolet (UV) radiation. Our objectives were to 1) compare UV tolerance among a broad array of EPN species, and 2) investigate the relationship between reduced nematode viability (after exposure to UV) and virulence. Nematodes exposed to a UV radiation (254 nm) for 10 or 20 min were assessed separately for viability (survival) and virulence to Galleria mellonella. We compared 9 different EPN species and 15 strains: Heterorhabditis bacteriophora (Baine, fl11, Oswego, and Vs strains), H. floridensis (332), H. georgiana (Kesha), H. indica (HOM1), H. megidis (UK211), Steinernema carpocapsae (All, Cxrd, DD136, and Sal strains), S. feltiae (SN), S. rarum (17C&E), and S. riobrave (355). In viability assessments, steinernematids, particularly strains of S. carpocapsae, generally exhibited superior UV tolerance compared with the heterorhabditids. However, some heterorhabditids tended to be more tolerant than others, e.g., H. megidis and H. bacteriophora (Baine) were most susceptible and H. bacteriophora (Vs) was the only heterorhabditid that did not exhibit a significant effect after 10 min of exposure. All heterorhabditids experienced reduced viability after 20 min exposure though several S. carpocapsae strains did not. In total, after 10 or 20 min exposure, the viability of seven nematode strains did not differ from their non-UV exposed controls. In virulence assays, steinernematids (particularly S. carpocapsae strains) also tended to exhibit higher UV tolerance. However, in contrast to the viability measurements, all nematodes experienced a reduction in virulence relative to their controls. Correlation analysis revealed that viability among nematode strains is not necessarily related to virulence. In conclusion, our results indicate that the impact of UV varies substantially among EPNs, and viability alone is not a sufficient measure for potential impact on biocontrol efficacy as other characters such as virulence may be severely affected even when viability remains high.

An insect pupal cell with antimicrobial properties that suppress an entomopathogenic fungus

Soil-dwelling insects have developed various mechanisms to defend against pathogen infection. The pecan weevil, Curculio caryae, spends two to three years in the soil inside an earthen cell. We hypothesized that the cell may possess antimicrobial properties. In a laboratory study, we tested the hypothesis using the fungus Beauveria bassiana as a model. B. bassiana is a common endemic pathogen of C. caryae. We compared the number of colony-forming-units on selective media when B. bassiana was exposed to autoclaved soil, non-autoclaved soil, or soil from a C. caryae pupal cell. Soil from C. caryae cells was suppressive to B. bassiana. To our knowledge this is the first report of antimicrobial properties associated with an insect soil cell. The findings expand our knowledge of host-pathogen relationships. Additional research is needed to determine the basis for the suppressive effects observed.

Characterization of Biocontrol Traits in Heterorhabditis floridensis: A Species with Broad Temperature Tolerance

Biological characteristics of two strains of the entomopathogenic nematode, Heterorhabditis floridensis (332 isolated in Florida and K22 isolated in Georgia) were described. The identity of the nematode's symbiotic bacteria was elucidated and found to be Photorhabdus luminescens subsp. luminescens. Beneficial traits pertinent to biocontrol (environmental tolerance and virulence) were characterized. The range of temperature tolerance in the H. floridensis strains was broad and showed a high level of heat tolerance. The H. floridensis strains caused higher mortality or infection in G. mellonella at 30°C and 35°C compared with S. riobrave (355), a strain widely known to be heat tolerant, and the H. floridensis strains were also capable of infecting at 17°C whereas S. riobrave (355) was not. However, at higher temperatures (37°C and 39°C), though H. floridensis readily infected G. mellonella, S. riobrave strains caused higher levels of mortality. Desiccation tolerance in H. floridensis was similar to Heterorhabditis indica (Hom1) and S. riobrave (355) and superior to S. feltiae (SN). H. bacteriophora (Oswego) and S. carpocapsae (All) exhibited higher desiccation tolerance than the H. floridensis strains. The virulence of H. floridensis to four insect pests (Aethina tumida, Conotrachelus nenuphar, Diaprepes abbreviatus, and Tenebrio molitor) was determined relative to seven other nematodes: H. bacteriophora (Oswego), H. indica (Hom1), S. carpocapsae (All), S. feltiae (SN), S. glaseri (4-8 and Vs strains), and S. riobrave (355). Virulence to A. tumida was similar among the H. floridensis strains and other nematodes except S. glaseri (Vs), S. feltiae, and S. riobrave failed to cause higher mortality than the control. Only H. bacteriophora, H. indica, S. feltiae, S. riobrave, and S. glaseri (4-8) caused higher mortality than the control in C. nenuphar. All nematodes were pathogenic to D. abbreviatus though S. glaseri (4-8) and S. riobrave (355) were the most virulent. S. carpocapsae was the most virulent to T. molitor. In summary, the H. floridensis strains possess a wide niche breadth in temperature tolerance and have virulence and desiccation levels that are similar to a number of other entomopathogenic nematodes. The strains may be useful for biocontrol purposes in environments where temperature extremes occur within short durations.

Freezing and desiccation tolerance in entomopathogenic nematodes: diversity and correlation of traits

The ability of entomopathogenic nematodes to tolerate environmental stress such as desiccating or freezing conditions, can contribute significantly to biocontrol efficacy. Thus, in selecting which nematode to use in a particular biocontrol program, it is important to be able to predict which strain or species to use in target areas where environmental stress is expected. Our objectives were to (i) compare inter- and intraspecific variation in freeze and desiccation tolerance among a broad array of entomopathogenic nematodes, and (ii) determine if freeze and desiccation tolerance are correlated. In laboratory studies we compared nematodes at two levels of relative humidity (RH) (97% and 85%) and exposure periods (24 and 48 h), and nematodes were exposed to freezing temperatures (-2°C) for 6 or 24 h. To assess interspecific variation, we compared ten species including seven that are of current or recent commercial interest: Heterorhabditis bacteriophora (VS), H. floridensis, H. georgiana, (Kesha), H. indica (HOM1), H. megidis (UK211), Steinernema carpocapsae (All), S. feltiae (SN), S. glaseri (VS), S. rarum (17C&E), and S. riobrave (355). To assess intraspecific variation we compared five strains of H. bacteriophora (Baine, Fl1-1, Hb, Oswego, and VS) and four strains of S. carpocapsae (All, Cxrd, DD136, and Sal), and S. riobrave (355, 38b, 7-12, and TP). S. carpocapsae exhibited the highest level of desiccation tolerance among species followed by S. feltiae and S. rarum; the heterorhabditid species exhibited the least desiccation tolerance and S. riobrave and S. glaseri were intermediate. No intraspecific variation was observed in desiccation tolerance; S. carpocapsae strains showed higher tolerance than all H. bacteriophora or S. riobrave strains yet there was no difference detected within species. In interspecies comparisons, poor freeze tolerance was observed in H. indica, and S. glaseri, S. rarum, and S. riobrave whereas H. georgiana and S. feltiae exhibited the highest freeze tolerance, particularly in the 24-h exposure period. Unlike desiccation tolerance, substantial intraspecies variation in freeze tolerance was observed among H. bacteriophora and S. riobrave strains, yet within species variation was not detected among S. carpocapsae strains. Correlation analysis did not detect a relationship between freezing and desiccation tolerance.

Biocontrol Potential of Steinernema thermophilum and Its Symbiont Xenorhabdus indica Against Lepidopteran Pests: Virulence to Egg and Larval Stages

Under laboratory conditions, the biocontrol potential of Steinernema thermophilum was tested against eggs and larval stages of two important lepidopteran insect pests, Helicoverpa armigera and Spodoptera litura (polyphagous pests), as well as Galleria mellonella (used as a model host). In terms of host susceptibility of lepidopteran larvae to S. thermophilum, based on the LC50 36 hr after treatment, G. mellonella (LC50 = 16.28 IJ/larva) was found to be more susceptible than S. litura (LC50 = 85 IJ/larva), whereas neither host was found to be significantly different from H. armigera (LC50 = 54.68 IJ/larva). In addition to virulence to the larval stages, ovicidal activity up to 84% was observed at 200 IJ/50 and 100 eggs of H. armigera and S. litura, respectively. To our knowledge this is the first report of entomopathogenic nematode pathogenicity to lepidopteran eggs. Production of infective juvenile (IJ) nematodes/insect larva was also measured and found to be positively correlated with rate of IJ for H. armigera (r = 0.990), S. litura (r = 0.892), as well as G. mellonella (r = 0.834). Both Phase I and Phase II of symbiotic bacteria Xenorhabdus indica were tested separately against neonates of H. armigera and S. litura by feeding assays and found to be virulent to the target pests; phase variation did not affect the level of virulence. Thus S. thermophilum as well as the nematode's symbiotic bacteria applied separately have the potential to be developed as biocontrol agents for key lepidopteran pests.

Aggregative group behavior in insect parasitic nematode dispersal

Movement behavior of foraging animals is critical to the determination of their spatial ecology and success in exploiting resources. Individuals sometimes gain advantages by foraging in groups to increase their efficiency in garnering these resources. Group movement behavior has been studied in various vertebrates. In this study we explored the propensity for innate group movement behavior among insect parasitic nematodes. Given that entomopathogenic nematodes benefit from group attack and infection, we hypothesised that the populations would tend to move in aggregate in the absence of extrinsic cues. Movement patterns of entomopathogenic nematodes in sand were investigated when nematodes were applied to a specific locus or when the nematodes emerged naturally from infected insect hosts; six nematode species in two genera were tested (Heterorhabditis bacteriophora, Heterorhabditis indica, Steinernema carpocapsae, Steinernema feltiae, Steinernema glaseri and Steinernema riobrave). Nematodes were applied in aqueous suspension via filter paper discs or in infected insect host cadavers (to mimic emergence in nature). We discovered that nematode dispersal resulted in an aggregated pattern rather than a random or uniform distribution; the only exception was S. glaseri when emerging directly from infected hosts. The group movement may have been continuous from the point of origin, or it may have been triggered by a propensity to aggregate after a short period of random movement. To our knowledge, this is the first report of group movement behavior in parasitic nematodes in the absence of external stimuli (e.g., without an insect or other apparent biotic or abiotic cue). These findings have implications for nematode spatial distribution and suggest that group behavior is involved in nematode foraging.

Effects of entomopathogenic fungus species, and impact of fertilizers, on biological control of pecan weevil (Coleoptera: Curculionidae)

The pecan weevil, Curculio caryae (Horn), is a key pest of pecan, Carya illinoinensis (Wangenh.) K. Koch. Prior research indicated the potential for use of Hypocreales fungi to suppress C. caryae. We compared the efficacy of two fungal spp., Beauveria bassiana (GHA strain) and Metarhizium brunneum (F52), in their ability to cause C. caryae mortality. The fungus, B. bassiana, was applied to trunks of pecan trees (a method previously shown to be effective in C. caryae suppression) and efficacy was compared with M. brunneum applied to the ground or to the trunk with or without SoyScreen Oil as an ultraviolet protecting agent. Results indicated B. bassiana to be superior to M. brunneum regardless of application method; consequently, the potential for applying B. bassiana to control C. caryae was explored further. Specifically, the impact of different fertilizer regimes (as used by pecan growers) on the persistence of B. bassiana (GHA) in soil was determined. B. bassiana was applied to soil in a pecan orchard after one of several fertilizer treatments--i.e., ammonium nitrate, crimson clover, poultry litter, clover plus poultry litter, and a no-fertilizer control. B. bassiana persistence up to 49 d in 2009 and 2010 was assessed by plating soil onto selective media and determining the number of colony forming units, and by baiting soil with a susceptible host, Galleria mellonella (L.). Fertilizer treatments did not impact B. bassiana persistence. We conclude that standard fertilizers for nitrogen management, when applied according to recommended practices, are unlikely to negatively impact survival of B. bassiana in pecan orchards when the fungus is applied for C. caryae suppression during weevil emergence. Additional research on interactions between entomopathogenic fungi and fertilizer amendments (or other tree nutrition or soil management practices) is merited.

Control of key pecan insect pests using biorational pesticides

Key pecan insect pests include the black pecan aphid, Melanocallis caryaefoliae (Davis), pecan weevil, Curculio caryae (Horn), and stink bugs (Hemiptera: Pentatomidae). Alternative control tactics are needed for management of these pests in organic and conventional systems. Our objective was to evaluate the potential utility of several alternative insecticides including three plant extract formulations, eucalyptus extract, citrus extract-8.92%, and citrus extract-19.4%, and two microbial insecticides, Chromobacterium subtsugae (Martin et al.) and Isaria fumosorosea (Wize). In the laboratory, eucalyptus extract, citrus extract-8.92%, citrus extract-19.4%, and C. subtsugae caused M. caryaefoliae mortality (mortality was reached approximately 78, 83, and 96%, respectively). In field tests, combined applications of I. fumosorosea with eucalyptus extract were synergistic and caused up to 82% mortality in M. caryaefoliae. In laboratory assays focusing on C. caryae suppression, C. subtsugae reduced feeding and oviposition damage, eucalyptus extract and citrus extract-19.4% were ineffective, and antagonism was observed when citrus extract-19.4% was combined with the entomopathogenic nematode, Steinernema carpocapsae (Weiser). In field tests, C. subtsugae reduced C. caryae damage by 55% within the first 3d, and caused 74.5% corrected mortality within 7 d posttreatment. In the laboratory, C. subtsugae and eucalyptus extract did not cause mortality in the brown stink bug, Euschistus servus (Say). Applications of C. subtsugae for suppression of C. caryae, and eucalyptus extract plus I. fumosorosea for control of M. caryaefoliae show promise as alternative insecticides and should be evaluated further.

Entomopathogenic nematode production and application technology

Production and application technology is critical for the success of entomopathogenic nematodes (EPNs) in biological control. Production approaches include in vivo, and in vitro methods (solid or liquid fermentation). For laboratory use and small scale field experiments, in vivo production of EPNs appears to be the appropriate method. In vivo production is also appropriate for niche markets and small growers where a lack of capital, scientific expertise or infrastructure cannot justify large investments into in vitro culture technology. In vitro technology is used when large scale production is needed at reasonable quality and cost. Infective juveniles of entomopathogenic nematodes are usually applied using various spray equipment and standard irrigation systems. Enhanced efficacy in EPN applications can be facilitated through improved delivery mechanisms (e.g., cadaver application) or optimization of spray equipment. Substantial progress has been made in recent years in developing EPN formulations, particularly for above ground applications, e.g., mixing EPNs with surfactants or polymers or with sprayable gels. Bait formulations and insect host cadavers can enhance EPN persistence and reduce the quantity of nematodes required per unit area. This review provides a summary and analysis of factors that affect production and application of EPNs and offers insights for their future in biological insect suppression.

Cumulative impact of a clover cover crop on the persistence and efficacy of Beauveria bassiana in suppressing the pecan weevil (Coleoptera: Curculionidae)

The pecan weevil, Curculio caryae (Horn), is a key pest of pecan. Endemic levels of the entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin can occur in pecan orchards and contribute to natural control of C. caryae. Commercial formulations of the fungus can also be applied for suppression of C. caryae. We hypothesized that a clover cover crop enhances B. bassiana efficacy and persistence (e.g., by protecting the fungus against abiotic environmental stresses). The hypothesis was tested by conducting field trials in a pecan orchard in Byron, GA, in 2009 and 2010. The study included four treatments arranged in a factorial with two levels of fungus (endemic fungus only, and application of a commercial B. bassiana product), and two levels of clover (white clover, Trifolium repens L., and no clover). Fungal persistence was measured by determining the number of CFUs per gram of soil over time (during 42 d postapplication of B. bassiana in 2009 and 29 d in 2010). Efficacy was measured by capturing naturally emerging C. caryae and subsequently determining mortality and mycosis (over 24 d in 2009 and 17 d in 2010). In 2009, greater prevalence of B. bassiana conidia was detected in plots receiving fungal applications compared with no fungus applications, and no clear effect of clover was observed in plots receiving B. bassiana applications in either year. In 2010, B. bassiana prevalence in the endemic fungus plus clover treatment was higher than fungus without clover, and was similar to plots receiving additional B. bassiana applications. Given that we observed enhanced persistence of endemic B. bassiana in 2010 but not 2009, the impact of clover appears to be a cumulative effect. Mortality of C. caryae (averaged over the sampling periods) ranged between 68-74% in plots receiving B. bassiana applications and 51-56% in plots with endemic fungus only. C. caryae mortality and mycosis data also provided evidence that endemic B. bassiana efficacy was enhanced by clover relative to plots without clover (with no clear clover effect on plots receiving fungus applications). Thus, we conclude that natural control of C. caryae can increase when clover is grown in pecan orchards with endemic populations of B. bassiana.

Self-selection of two diet components by Tenebrio molitor (Coleoptera: Tenebrionidae) larvae and its impact on fitness

We studied the ability of Tenebrio molitor L. (Coleoptera: Tenebrionidae) to self-select optimal ratios of two dietary components to approach nutritional balance and maximum fitness. Relative consumption of wheat bran and dry potato flakes was determined among larvae feeding on four different ratios of these components (10, 20, 30, and 40% potato). Groups of early instars were provided with a measured amount of food and the consumption of each diet component was measured at the end of 4 wk and again 3 wk later. Consumption of diet components by T. molitor larvae deviated significantly from expected ratios indicating nonrandom self-selection. Mean percentages of dry potato consumed were 11.98, 19.16, 19.02, and 19.27% and 11.89, 20.48, 24.67, and 25.97% during the first and second experimental periods for diets with 10, 20, 30, and 40% potato, respectively. Life table analysis was used to determine the fitness of T. molitor developing in the four diet mixtures in a no-choice experiment. The diets were compared among each other and a control diet of wheat bran only. Doubling time was significantly shorter in groups consuming 10 and 20% potato than the control and longer in groups feeding on 30 and 40% potato. The self-selected ratios of the two diet components approached 20% potato, which was the best ratio for development and second best for population growth. Our findings show dietary self-selection behavior in T. molitor larvae, and these findings may lead to new methods for optimizing dietary supplements for T. molitor.

Virulence of Entomopathogenic Nematodes to Plum Curculio, Conotrachelus nenuphar: Effects of Strain, Temperature, and Soil Type

The plum curculio, Conotrachelus nenuphar, is a major pest of stone and pome fruit (e.g., apples, pears, peaches, cherries, etc.). Entomopathogenic nematodes (Steinernema spp. and Heterorhabditis spp.) may be used to control the larval stage of C. nenuphar following fruit drop. Indeed, certain entomopathogenic nematodes species have previously been shown to be highly effective in killing C. nenuphar larvae in laboratory and field trials. In field trials conducted in the Southeastern, USA, Steinernema riobrave has thus far been shown to be the most effective species. However, due to lower soil temperatures, other entomopathogenic nematode strains or species may be more appropriate for use against C. nenuphar in the insect's northern range. Thus, the objective of this study was to conduct a broad screening of entomopathogenic nematodes. Under laboratory conditions, we determined the virulence of 13 nematode strains (comprising nine species) in two different soils (a loam and clay-loam) and three different temperatures (12°C, 18°C, and 25°C). Superior virulence was observed in S. feltiae (SN strain), S. rarum (17 C&E strain), and S. riobrave (355 strain). Promising levels of virulence were also observed in others including H. indica (HOM1 strain), H. bacteriophora (Oswego strain), S. kraussei, and S. carpocapsae (Sal strain). All nematode treatments were affected by temperature with the highest virulence observed at the highest temperature (25°C). In future research, field tests will be used to further narrow down the most suitable nematode species for C. nenuphar control.

Outcrossing and crossbreeding recovers deteriorated traits in laboratory cultured Steinernema carpocapsae nematodes

The nematode Steinernema carpocapsae infects and kills many pest insects in agro-ecosystems and is commonly used in biocontrol of these pests. Growth of the nematodes prior to distribution for biocontrol commonly results in deterioration of traits that are essential for nematode persistence in field applications. To better understand the mechanisms underlying trait deterioration of the efficacy of natural parasitism in entomopathogenic nematodes, we explored the maintenance of fitness related traits including reproductive capacity, heat tolerance, virulence to insects and 'tail standing' (formerly called nictation) among laboratory-cultured lines derived from natural, randomly mating populations of S. carpocapsae. Laboratory cultured nematode lines with fitness-related trait values below wild-type levels regained wild-type levels of reproductive and heat tolerance traits when outcrossed with a non-deteriorated line, while virulence and 'tail standing' did not deteriorate in our experiments. Crossbreeding two trait-deteriorated lines with each other also resulted in restoration of trait means to wild-type levels in most crossbred lines. Our results implicate inbreeding depression as the primary cause of trait deterioration in the laboratory cultured S. carpocapsae. We further suggest the possibility of creating inbred lines purged of deleterious alleles as founders in commercial nematode growth.

Comparative impact of artificial selection for fungicide resistance on Beauveria bassiana and Metarhizium brunneum

Hypocreales fungi such as Beauveria bassiana (Balsamo) Vuillemin and Metarhizium brunneum Petch can be negatively affected by fungicides thereby reducing their biocontrol potential. In a previous study, we demonstrated enhanced fungicide resistance in B. bassiana through artificial selection. However, it is not clear if the enhanced resistance was because of improved germination, vegetative growth, or both. Additionally, the enhanced fungicide resistance has only been demonstrated in B. bassiana, and therefore it is of interest to investigate the potential to enhance resistance in other fungi. Thus, the objectives in this study were to determine the potential to enhance fungicide resistance in M. brunneum through artificial selection, and investigate if selection is based on germination, vegetative growth, or both in B. bassiana and M. brunneum. Selection for resistance to fenbuconazole, and triphenyltin hydroxide was assessed through inhibition evaluations on solid media, and germination and mycelial growth in liquid media. Increased resistance after selection was observed for all fungicide-fungus combinations on solid and or liquid media. Selection resulted in increased resistance to fenbuconazole in both fungi in solid and liquid media; in liquid culture fungicide resistance in B. bassiana was manifested by increased germination and mycelial growth, whereas in M. brunneum fungicide resistance concerned only mycelial growth. Selection for resistance to triphenyltin hydroxide varied in the different media. For B. bassiana, triphenyltin hydroxide resistance was enhanced on solid media but not in liquid, whereas enhanced resistance of M. brunneum was detected in both media. Fungicide sensitivity and selection potential differs based on the medium and fungal species. Selection for fungicide resistance, had negative effects on other beneficial traits when fungicide pressure was removed, for example, some selected populations showed decreased germination or growth, relative to their nonselected control populations. Additionally, reduced virulence to the greater wax moth, Galleria mellonella (L.), was observed in all fungal populations that were exposed to fungicide resistance regimes. We conclude that it is possible to use genetic selection to enhance fungicide resistance in B. bassiana and M. brunneum, but before use the resulting populations should be screened for inadvertent negative impacts on beneficial traits.

Laboratory virulence and orchard efficacy of entomopathogenic nematodes against the lesser peachtree borer (Lepidoptera: Sesiidae)

The lesser peachtree borer, Synanthedon pictipes (Grote & Robinson) (Lepidoptera: Sesiidae), is indigenous to eastern North America. It is a pest of commercially grown Prunus spp., especially to southeastern peach orchards where earlier regulatory changes affected pesticide use on peach leading to increased S. pictipes damage. Pest management practices are now having a positive effect toward control of this pest, but cost-competitive biological control solutions that promote environmental stewardship are needed. Here, we tested four Steinernema species and five Heterorhabditis species of entomopathogenic nematodes against larval S. pictipes. Included were four strains of S. carpocapsae (All, DD136, Sal, and Hybrid2) and three strains of S. riobrave (3-8b, 7-12, and 355). Larvae treated with any strain of S. carpocapsae always resulted in <20% survival, whereas larval survival was always >50% when treated with any other Steinernema or Heterorhabditis spp. These differences were always significant for the Hybrid2 strain of S. carpocapsae and similarly for other tested S. carpocapsae strains except for when larvae were treated with the 3-8b strain of S. riobrave. In addition, we determined the susceptibility of different size S. pictipes larvae, because they occur simultaneously in orchards, and we found that larvae rated as "medium" and "large" were significantly more susceptible than "small" larvae. Last, we demonstrated that moisture-retaining covers (placed over S. pictipes-infested wounds on peach limbs) increased efficacy of nematode treatments against larval S. pictipes. Even when using highly virulent nematodes against S. pictipes, it is likely that an aboveground application will require an environmental modification to remain efficacious.

Effects of combining microbial and chemical insecticides on mortality of the Pecan Weevil (Coleoptera: Curculionidae)

The pecan weevil, Curculio caryae (Horn), is a key pest of pecan [Carya illinoinensis (Wangenh.) K. Koch]. Current control recommendations are based on chemical insecticide applications. Microbial control agents such as the entomopathogenic nematode, Steinernema carpocapsae (Weiser) and the fungus Beauveria bassiana (Balsamo) Vuillemin occur naturally in southeastern U.S. pecan orchards and have shown promise as alternative control agents for C. caryjae. Conceivably, the chemical and microbial agents occur simultaneously within pecan orchards or might be applied concurrently. The objective of this study was to determine the interactions between two chemical insecticides that are used in commercial C. caryae control (i.e., carbaryl and cypermethrin applied below field rates) and the microbial agents B. bassiana and S. carpocapsae. In laboratory experiments, pecan weevil larval or adult mortality was assessed after application of microbial or chemical treatments applied singly or in combination (microbial + chemical agent). The nature of interactions (antagonism, additivity, or synergy) in terms of weevil mortality was evaluated over 9 d (larvae) or 5 d (adults). Results for B. bassiana indicated synergistic activity with carbaryl and antagonism with cypermethrin in C. caryae larvae and adults. For S. carpocapsae, synergy was detected with both chemicals in C. caryae larvae, but only additive effects were detected in adult weevils. Our results indicate that the chemical-microbial combinations tested are compatible with the exception of B. bassiana and cypermethrin. In addition, combinations that exhibited synergistic interactions may provide enhanced C. caryae control in commercial field applications; thus, their potential merits further exploration.

A novel strain of Steinernema riobrave (Rhabditida: Steinernematidae) possesses superior virulence to subterranean termites (Isoptera: Rhinotermitidae)

Subterranean termites are major global pests of wood structures and wood products. Among the most economically important subterranean termite species in the US are Heterotermes aureus, Reticulitermes flavipes, and Coptotermes formosanus. In prior studies, the entomopathogenic nematode, Steinernema riobrave strain 355, exhibited a high level of virulence to H. aureus compared with other nematode species. However, S. riobrave 355 was reported to be poorly or only moderately virulent to R. flavipes and C. formosanus, respectively. We hypothesized that other strains of S. riobrave may possess a high level of virulence to all three termite species. Under laboratory conditions we compared three novel strains of S. riobrave (3-8b, 7-12, and TP) with the 355 strain for virulence to H. aureus, R. flavipes, and C. formosanus workers. H. aureus was very susceptible to all the S. riobrave strains, and termites in all nematode treatments were dead after 4 d. The TP strain of S. riobrave caused greater mortality in R. flavipes and C. formosanus compared to the other nematode strains. Specifically, the TP strain caused 75% and 91% mortality in R. flavipes and C. formosanus, respectively, which was more than 300% and 70% higher than the mortality caused by other strains. Additional studies are warranted to determine the ability of S. riobrave (TP) to control the targeted termite species under field conditions.

Transcriptional profiling of trait deterioration in the insect pathogenic nematode Heterorhabditis bacteriophora

Background

The success of a biological control agent depends on key traits, particularly reproductive potential, environmental tolerance, and ability to be cultured. These traits can deteriorate rapidly when the biological control agent is reared in culture. Trait deterioration under laboratory conditions has been widely documented in the entomopathogenic nematode (EPN) Heterorhabditis bacteriophora (Hb) but the specific mechanisms behind these genetic processes remain unclear. This research investigates the molecular mechanisms of trait deterioration of two experimental lines of Hb, an inbred line (L5M) and its original parental line (OHB). We generated transcriptional profiles of two experimental lines of Hb, identified the differentially expressed genes (DEGs) and validated their differential expression in the deteriorated line.

Results

An expression profiling study was performed between experimental lines L5M and OHB of Hb with probes for 15,220 ESTs from the Hb transcriptome. Microarray analysis showed 1,185 DEGs comprising of 469 down- and 716 up-regulated genes in trait deteriorated nematodes. Analysis of the DEGs showed that trait deterioration involves massive changes of the transcripts encoding enzymes involved in metabolism, signal transduction, virulence and longevity. We observed a pattern of reduced expression of enzymes related to primary metabolic processes and induced secondary metabolism. Expression of sixteen DEGs in trait deteriorated nematodes was validated by quantitative reverse transcription-PCR (qRT-PCR) which revealed similar expression kinetics for all the genes tested as shown by microarray.

Conclusion

As the most closely related major entomopathogen to C. elegans, Hb provides an attractive near-term application for using a model organism to better understand interspecies interactions and to enhance our understanding of the mechanisms underlying trait deterioration in biological control agents. This information could also be used to improve the beneficial traits of biological control agents and better understand fundamental aspects of nematode parasitism and mutualism.

Effects of a novel entomopathogenic nematode-infected host formulation on cadaver integrity, nematode yield, and suppression of Diaprepes abbreviatus and Aethina tumida

An alternative approach to applying entomopathogenic nematodes entails the distribution of nematodes in their infected insect hosts. Protection of the infected host from rupturing, and improving ease of handling, may be necessary to facilitate application. In this study our objective was to test the potential of a new method of formulating the infected hosts, i.e., enclosing the infected host in masking tape. Tenebrio molitor L. cadavers infected with Heterorhabditis indica Poinar, Karunakar and David or Steinernema carpocapsae (Weiser) were wrapped in tape using an automatic packaging machine; the machine was developed to reduce labor and to standardize the final product. The effects of the tape formulation on the ability to protect the cadavers from mechanical damage, nematode yield, and pest control efficacy were tested. After exposure to mechanical agitation at 7-d-post-infection, S. carpocapsae cadavers in tape were more resistant to rupture than cadavers without tape, yet H. indica cadavers 7-d-post-infection were not affected by mechanical agitation (with or without tape), nor was either nematode affected when 4-d-old cadavers were tested. Experiments indicated that infective juvenile yield was not affected by the tape formulation. Laboratory experiments were conducted measuring survival of the root weevil, Diaprepes abbreviatus (L.), or the small hive beetle, Aethina tumida Murray, after the application of two H. indica-infected hosts with or without tape per 15 cm pot (filled with soil). A greenhouse experiment was also conducted in a similar manner measuring survival of D. abbreviatus. In all experiments, both the tape and no-tape treatments caused significant reductions in insect survival relative to the control, and no differences were detected between the nematode treatments. Fifteen days post-application, the infected host treatments caused up to 78% control in A. tumida, 91% control in D. abbreviatus in the lab, and 75% in the greenhouse. These results indicate potential for using the tape-formulation approach for applying nematode infected hosts.

Evaluation of soyscreen in an oil-based formulation for UV protection of Beauveria bassiana conidia

Soyscreen oil was studied as a formulation ingredient to protect Beauveria bassiana (Balsamo) Vuillemin conidia from UV degradation. Feruloylated soy glycerides, referred to as Soyscreen oil, are biobased UV-absorbing molecules made by combining molecules of soybean oil with ferulic acid. Conidia stored in Soyscreen oil for 28 wk at 25, 30, and 35 degrees C retained viability as well as conidia stored in sunflower oil, demonstrating that Soyscreen did not adversely affect viability with prolonged storage. For samples applied to glass and exposed to simulated sunlight (xenon light), conidia in sunflower oil with or without sunscreens (Soyscreen or oxyl methoxycinnimate) had similar conidia viability after exposure. These oil formulations retained conidia viability better than conidia applied as an aqueous treatment. However, the 10% Soyscreen oil formulation applied to field grown cabbage (Brassica oleracea L.) and bean (Phaseolus vulgaris L.) plants, did not improve residual insecticidal activity compared with aqueous applications of unformulated conidia or two commercial formulations when assayed against Trichoplusia ni (Hübner) larvae. Our results suggest that the oil applications lose UV protection because the oil was absorbed by the leaf. This conclusion was supported in subsequent laboratory exposures of conidia in oil-based formulations with UV screens applied to cabbage leaves or balsa wood, which lost protection as measured by decreased viability of conidia when exposed to simulated sunlight. As a result, additional formulation techniques such as encapsulation to prevent separation of the protective oil from the conidia may be required to extend protection when oil formulations are applied in the field.

STATE-SPACE BASED MASS EVENT-HISTORY MODEL I: MANY DECISION-MAKING AGENTS WITH ONE TARGET

A dynamic decision-making system that includes a mass of indistinguishable agents could manifest impressive heterogeneity. This kind of non-homogeneity is postulated to result from macroscopic behavioral tactics employed by almost all involved agents. A State-Space Based (SSB) mass event-history model is developed here to explore the potential existence of such macroscopic behaviors. By imposing an unobserved internal state-space variable into the system, each individual's event-history is made into a composition of a common state duration and an individual specific time to action. With the common state modeling of the macroscopic behavior, parametric statistical inferences are derived under the current-status data structure and conditional independence assumptions. Identifiability and computation related problems are also addressed. From the dynamic perspectives of system-wise heterogeneity, this SSB mass event-history model is shown to be very distinct from a random effect model via the Principle Component Analysis (PCA) in a numerical experiment. Real data showing the mass invasion by two species of parasitic nematode into two species of host larvae are also analyzed. The analysis results not only are found coherent in the context of the biology of the nematode as a parasite, but also include new quantitative interpretations.