First record of entomopathogenic nematodes from Yucatán State, México and their infectivity capacity against Aedes aegypti

Background

Biological control using entomopathogenic nematodes (EPN) has demonstrated good potential to contribute to the integral control of mosquito larvae, which as adults are vectors of diseases such as Dengue fever, Zika and Chikungunya. However, until now there are no records of the presence of EPN or their killing capacity in Yucatán state, southern México. The objectives of the current study were: (1) to report the entomopathogenic nematodes present in Yucatán soils and (2) to determine the killing capacity of the most frequent and abundant EPN against Aedes aegypti mosquito larvae and the microbial community developed by Ae. Aegypti exposed to this EPN.

Methods

The nematodes were collected by the insect trap technique using the great wax moth Galleria mellonella. Internal transcribed spacer (ITS), 28S gene of ribosomal DNA and phylogenetic analyses were performed to identify the EPN. For the bioassay, four concentrations of the most frequent and abundant EPN were tested: 1,260:1 infective juveniles (IJs) per mosquito larvae, 2,520 IJs:1, 3,780 IJs:1 and 5,040 IJs:1. High-throughput sequencing of the 16S rRNA gene was used to identify bacterial amplicon sequences in the mosquito larvae infected with EPN.

Results

Six isolates of Heterorhabditis were recovered from 144 soil samples. Heterorhabditis indica (four isolates) was the most frequent and abundant EPN, followed by Heterorhabditis n. sp. (two isolates). Both nematodes are reported for the first time for Yucatán state, Mexico. The concentration of 2,520 IJs:1 produced 80% of mosquito larvae mortality in 48 h. Representative members of Photorhabdus genus were numerically dominant (74%) in mosquito larvae infected by H. indica. It is most likely that these bacteria produce secondary toxic metabolites that enhance the mortality of these mosquito larvae.

Performance of entomopathogenic nematodes on the mealybug, Dysmicoccus brevipes (Hemiptera: Pseudococcidae) and the compatibility of control agents with nematodes

The performance of nine isolates of Heterorhabditis amazonensis and one of Heterorhabditis indica on the mealybug Dysmicoccus brevipes, (Hemiptera: Pseudococcidae), were evaluated. The most virulent isolates were evaluated for nematode vertical and horizontal dispersal, and for efficiency at concentrations of 0 (control), 25, 50, 75, and 100 infective juveniles (IJs)/cm² on adult females of the insect. A compatibility assessment was also carried out with commercial products, registered or in the process of registration, for use in the cassava culture. The isolates that caused the highest mortality rate of D. brevipes were NEPET11 (93.8% ± 4.1) and IBCB-n40 (84.0% ± 8.1), both isolates of Heterorhabditis amazonensis, while the isolate NEPET11 was more virulent than IBCB-n40 at all concentrations evaluated. In the dispersal test, the NEPET11 isolate caused mortality in the mealybug at a depth of up to 20 cm and a horizontal displacement of 7.25 cm. In the compatibility test, the NEPET11 isolate exhibited reduced viability due to the products Poquer, Tiguer 100 EC, Actara 250 WG, and Gaucho FS. The insecticide Curyom 550 EC was the only one that reduced infectivity (reduction of 92%) and is the only product classified as moderately toxic, while all the others were classified as compatible based on E%.

Susceptibility of Various Developmental Stages of the Fall Armyworm, Spodoptera frugiperda, to Entomopathogenic Nematodes

Simple Summary

The fall armyworm, Spodoptera frugiperda, native to Central and South America, has recently invaded Africa and Asia, causing serious economic damage to various crops. The chemical control of fall armyworm is not effective due to the development of pesticide resistance as well as environmental contamination. Alternatively, biological control using natural enemies can be used successfully in a sustainable way. Entomopathogenic nematodes are soil-dwelling natural enemies for many underground pest insects including lepidopteran caterpillars. This study evaluates the virulence of several entomopathogenic nematodes on different developmental stages of fall armyworm. We found that all the larval and pupal stages of fall armyworm were highly susceptible to the entomopathogenic nematodes. Our study provides important information of entomopathogenic nematodes for the practical application of biological control of fall armyworm.

Abstract

The fall armyworm, Spodoptera frugiperda, which is native to Central and South America, has recently invaded Africa and Asia, causing serious damage to various crops. Although management to date has been largely unsuccessful, entomopathogenic nematodes (EPNs) are a potential biological control agent that could be used to control the late larval and pupal stages of S. frugiperda that dwell under the ground. Here, we compared the virulence of seven EPNs against larval and pupal stages of S. frugiperda. In a Petri dish assay, both Heterorhabditis indica and Steinernema carpocapsae were highly virulent against younger larvae, whereas S. arenarium and S. longicaudum were highly virulent against older larvae. In contrast, H. bacteriophora, Heterorhabditis sp., and S. kushidai showed low virulence against all larval stages. In soil column and pot assays, H. indica, S. carpocapsae, and S. longicaudum were highly virulent against late larval and pupal stages compared with the other EPN species. Thus, H. indica, S. carpocapsae, and S. longicaudum are recommended for the biological control of S. frugiperda. Our study provides important information of EPNs for the practical application of biological control of fall armyworm.

Efficacy of entomopathogenic nematodes in insect cadaver formulation against engorged females of Rhipicephalus microplus (Acari: Ixodidae) in semi-field conditions

The present study evaluated, in the laboratory, the virulence of Heterorhabditis bacteriophora LPP30 against engorged females of Rhipicephalus microplus; in addition, we evaluated, in semi-field tests, the effects of four isolates of Heterorhabditis spp. (i.e., H. bacteriophora HP88, Heterorhabditis baujardi LPP7, Heterorhabditis indica LPP1 and H. bacteriophora LPP30) in insect cadaver formulation against the non-parasitic phase of R. microplus. In the first experiment (in vitro), engorged females were exposed, in Petri dish, to H. bacteriophora LPP30 at 75, 150, 300, 600, 1200 and 2400 nematodes/tick (10 ticks per concentration tested). In the second experiment (semi-field), five engorged females and four cadavers of Galleria mellonella infected with nematodes were placed in pots with soil and grown Brachiaria decumbens; the pots were hold outdoor, exposed to natural environment conditions during the tests. In the third experiment (semi-field), 65 days after the cadavers had been placed in the pots for the second experiment, new engorged females (five per pot) were placed in the pots of the groups treated with H. bacteriophora HP88 or H. baujardi LPP7, to assess their persistence in the soil. In the first test, the percent control was higher than 95% in all treatment groups. In the second experiment, at day 22, the mean mortality rate was 78% in the groups treated with H. bacteriophora LPP30 or H. indica LPP1, and reached 100% and 98% when treated with H. bacteriophora HP88 and H. baujardi LPP7, respectively. In this experiment, the egg-laying inhibition index was higher than 90% in the groups treated with H. bacteriophora HP88 (97.2%) or H. baujardi LPP7 (91.9%). In the third experiment with H. bacteriophora HP88 and H. baujardi LPP7, the egg-laying inhibition index was 59.1% and 43.1%, respectively. We concluded that the isolate LPP30 was highly virulent under laboratory conditions, whereas in semi-field tests, HP88 and LPP7 were the most effective isolates. Moreover, HP88 and LPP7 remained active against engorged females of R. microplus in the soil for 65 days after application of EPN-infected cadavers of G. mellonella.

Integrated Management of Chive Gnats (Bradysia odoriphaga Yang & Zhang) in Chives Using Entomopathogenic Nematodes and Low-Toxicity Insecticides

Bradysia odoriphaga is a major pest that causes damage to chive production, and which has developed resistance to highly toxic chemical insecticides. Entomopathogenic nematodes (EPN) show a high potential for B. odoriphaga control. This study aimed to develop an effective management method against B. odoriphaga larvae, using EPN with low-toxicity insecticides. Fourteen selected insecticides had no significant effects on the survival and infectivity of Steinernema feltiae SN and Heterorhabditis indica LN2. Synergistic interactions were observed for imidacloprid and osthole with S. feltiae SN against B. odoriphaga larvae. Steinernema feltiae SN was more effective than H. indica LN2 against B. odoriphaga at 15 and 20 °C, and the addition of imidacloprid at 1/10 recommended concentration (RC) significantly increased the efficacy of S. feltiae SN. The year-round occurrence of the B. odoriphaga larvae in chive fields treated by EPN and imidacloprid at 1/10 RC was studied. Results showed that the application of EPN with imidacloprid at 1/10 RC successfully suppressed larval populations of B. odoriphaga in chive fields, thus significantly increasing the yield of chive. The practical method of applying EPN–imidacloprid combinations provided a cost-effective and environmental safety strategy for controlling B. odoriphaga larvae in chive production, which can reduce the usage of toxic chemical insecticides.

The population structure of Ochrobactrum isolated from entomopathogenic nematodes indicates interactions with the symbiotic system

Entomopathogenic nematodes (EPNs) form specific mutualistic associations with bioluminescent enterobacteria. In Heterorhabditidis indica, Ochrobactrum spp. was identified beside the symbiont Photorhabdus luminescens but its involvement in the symbiotic association in the EPNs remains unclear. This study describe the population structure and the diversity in Ochrobactrum natural populations isolated from EPNs in the Caribbean basin in order to question the existence of EPN-specialized clones and to gain a better insight into Ochrobactrum-EPNs relationships. EPN-associated Ochrobactrum and Photorhabdus strains were characterized by multi-locus sequence typing, Pulsed-Field Gel Electrophoresis fingerprinting and phenotypic traits. Population study showed the absence of EPN-specialized clones in O. intermedium and O. anthropi but suggested the success of some particular lineages. A low level of genetic and genomic diversification of Ochrobactrum isolated from the natural population of Caribbean nematodes was observed comparatively to the diversity of human-associated Ochrobactrum strains. Correspondences between Ochrobactrum and P. luminescens PFGE clusters have been observed, particularly in the case of nematodes from Dominican Republic and Puerto Rico. O. intermedium and O. anthropi associated to EPNs formed less biofilm than human-associated strains. These results evoke interactions between Ochrobactrum and the EPN symbiotic system rather than transient contamination. The main hypothesis to investigate is a toxic/antitoxic relationship because of the ability of Ochrobactrum to resist to antimicrobial and toxic compounds produced by Photorhabdus.

Assessing the Pathogenicity of Two Bacteria Isolated from the Entomopathogenic Nematode Heterorhabditis indica against Galleria mellonella and Some Pest Insects

The entomopathogenic nematodes Heterorhabditis are parasites of insects and are associated with mutualist symbiosis enterobacteria of the genus Photorhabdus; these bacteria are lethal to their host insects. Heterorhabditis indica MOR03 was isolated from sugarcane soil in Morelos state, Mexico. The molecular identification of the nematode was confirmed using sequences of the ITS1-5.8S-ITS2 region and the D2/D3 expansion segment of the 28S rRNA gene. In addition, two bacteria HIM3 and NA04 strains were isolated from the entomopathogenic nematode. The genomes of both bacteria were sequenced and assembled de novo. Phylogenetic analysis was confirmed by concatenated gene sequence datasets as Photorhabdus luminescens HIM3 (16S rRNA, 23S rRNA, dnaN, gyrA, and gyrB genes) and Pseudomonas aeruginosa NA04 (16S rRNA, 23S rRNA and gyrB genes). H. indica MOR03 infects Galleria mellonella, Tenebrio molitor, Heliothis subflexa, and Diatraea magnifactella larvae with LC50 values of 1.4, 23.5, 13.7, and 21.7 IJs/cm², respectively, at 48 h. These bacteria are pathogenic to various insects and have high injectable insecticide activity at 24 h.

Molecular Identification of Entomopathogenic Nematode Isolates from the Philippines and their Biological Control Potential Against Lepidopteran Pests of Corn

In search for local entomopathogenic nematode (EPN) species as a biological control agent of lepidopterous insect pests of corn, a survey for EPN in the major islands in the Philippines was conducted. Seven EPN populations from 279 soil samples were isolated using Ostrinia furnacalis, the key target insect pest of corn in the country, as bait. Analysis of the ITS1-5.8S-ITS2 ribosomal DNA sequence revealed the presence of Steinernema abbasi, Steinernema minutum , Steinernema tami , and Heterorhabditis indica . The pathogenicity of these EPN was tested in Ostrinia furnacalis , Spodoptera litura , and Helicoverpa armigera larvae under laboratory conditions. All the EPN isolates were pathogenic to the lepidopteran species with, H. indica PBCB and S. abbasi MBLB exhibiting the highest virulence (88%-99.33% and 90%-100% mortality, respectively) at 48 hr post infection (HPI) and thus, further studies were done on these two EPN. The highest penetration rate at 48 HPI was observed in H. armigera infected with S. abbasi MBLB (28.15%), while the lowest was in O. furnacalis infected with H. indica PBCB (14.25%). Nonetheless, based on LC 50 at 48 HPI, H. indica PBCB was most virulent to S. litura (8.89 IJ per larva), but not significantly different from O. furnacalis (10.52 IJ per larva). Steinernema abbasi MBLB was most virulent to O. furnacalis (10.98 IJ per larva), but not significantly different to S. litura (17.08 IJ per larva). LT 50 estimates showed that O. furnacalis was significantly the most susceptible to H. indica PBCB (21.90 hr) and S. abbasi (21.18 hr). Our results suggest that H. indica PBCB and S. abbasi MBLB are good candidates as biological control agents against these insect pests of corn. Moreover, O. furnacalis as alternative bait for EPN was discussed. To date, this is the most extensive research on Philippine EPN, comprised of wide sampling coverage, molecular identification and bioefficacy assays.

Effect of entomopathogenic nematode of Heterorhabditis indica infection on immune and antioxidant system in lepidopteran pest Spodoptera litura (Lepidoptera: Noctuidae)

Entomopathogenic nematodes form excellent tools to study insect immunity in response to during infection. Insects activate as several defense mechanisms, namely Phenoloxidase, haemocytes, detoxification and antioxidant enzymes. However little mechanistic information is available about the sublethal effects of entomopathogenic nematodes infection on detoxification and immune mechanisms in lepidopteran insects. In the present study, the effects of infection on antioxidant, detoxification and immune systems of Spodoptera litura larvae were studied. Results show a significant reduction in Total Haemocyte Count observed after 3 h of infection. A significant increase Superoxide dismutase, Catalase, Glutathione S-transferase, Glutathione Peroxidase and Acid phosphatase were observed 6 h after infection and, progressive decrease in Peroxidase, Alkaline phosphatase and Lipid peroxidation was also observed. This study shows that increased detoxification enzyme levels in response to nematode infection are a protective mechanism in insects. Nematode infection suppresses insect immune response, which is evident from low haemocyte count and Phenoloxidase levels to ultimately cause larval mortality.

Photorhabdus luminescens LN2 requires rpoS for nematicidal activity and nematode development

Photorhabdus (Enterobacteriaceae) bacteria are pathogenic to insects and mutualistic with entomopathogenic Heterorhabditis nematodes. Photorhabdus luminescens subsp. akhurstii LN2, associated with Heterorhabditis indica LN2, shows nematicidal activity against H. bacteriophora H06 infective juveniles (IJs). In the present study, an rpoS mutant of P. luminescens LN2 was generated through allelic exchange to examine the effects of rpoS deletion on the nematicidal activity and nematode development. The results showed that P. luminescens LN2 required rpoS for nematicidal activity against H06 nematodes, normal IJ recovery and development of H. indica LN2, however, not for the bacterial colonization in LN2 and H06 IJs. This provides cues for further understanding the role of rpoS in the mutualistic association between entomopathogenic nematodes and their symbionts.

Assessing the Role of Environmental Conditions on Efficacy Rates of Heterorhabditis indica (Nematoda: Heterorhabditidae) for Controlling Aethina tumida (Coleoptera: Nitidulidae) in Honey Bee (Hymenoptera: Apidae) Colonies: a Citizen Science Approach

Certain species of entomopathogenic nematodes, such as Heterorhabditis indica Poinar, Karunakar & David, have the potential to be effective controls for Aethina tumida (Murray), or small hive beetles, when applied to the soil surrounding honey bee (Apis mellifera L.) hives. Despite the efficacy of H. indica, beekeepers have struggled to use them successfully as a biocontrol. It is believed that the sensitivity of H. indica to certain environmental conditions is the primary reason for this lack of success. Although research has been conducted to explore the impact of specific environmental conditions--such as soil moisture or soil temperature-on entomopathogenic nematode infectivity, no study to date has taken a comprehensive approach that considers the impact of multiple environmental conditions simultaneously. In exploring this, a multivariate logistic regression model was used to determine what environmental conditions resulted in reductions of A. tumida populations in honey bee colonies. To obtain the sample sizes necessary to run a multivariate logistic regression, this study utilized citizen scientist beekeepers and their hives from across the mid-Atlantic region of the United States. Results suggest that soil moisture, soil temperatures, sunlight exposure, and groundcover contribute to the efficacy of H. indica in reducing A. tumida populations in A. mellifera colonies. The results of this study offer direction for future research on the environmental preferences of H. indica and can be used to educate beekeepers about methods for better utilizing H. indica as a biological control.

Effects of storage temperature on survival and infectivity of three indigenous entomopathogenic nematodes strains (Steinernematidae and Heterorhabditidae) from Meghalaya, India

Three locally isolated strains of entomopathogenic nematodes (EPNs), viz. Heterorhabditis indica, Steinernema thermophilum and Steinernema glaseri, from Meghalaya, India were characterized in terms of storage temperature and survival and infectivity of their infective juveniles (IJs). The survival and infectivity of nematode IJs was studied at, 5 ± 2 and 25 ± 2 °C, for a period of 120 days, using deionized water as storage medium. The viability of nematode IJs was checked by mobility criterion at different storage periods, while the infectivity of nematode IJs was ascertained on the basis of establishment of IJs, using Galleria mellonella larva mortality tests in petridishes. The results of this study revealed that storage temperature markedly affects the survival as well as the establishment of nematode IJs of the three EPN species. At 5 °C, comparatively higher rate of IJ's survival (i.e. 74-86 %) was observed for 15 days of storage, but the same reduced drastically to 28-32 % after 30 days of storage for H. indica and S. thermophilum. On the other hand, at 25 °C, the survival of nematode IJs was observed till 120 days for all the three studied EPNs. In case of S. thermophilum and S. glaseri, higher rate of IJs survival (>75 %) was observed respectively at 15 and 30 days of observation. The study also showed that the establishment of IJs of the three EPN species declines with increase in storage periods, at both the test temperatures. In general, the nematodes stored at 25 °C showed comparatively better establishment than those stored at 5 °C. Among the three EPN studied, the establishment of S. glaseri was comparatively better than the rest of the species at both the temperatures and for different storage durations. In conclusion, our study adds further valuable information about the effect of storage temperature on survival and infectivity of three indigenous EPN species of Meghalaya, India which appears to be promising biocontrol agents of local insect pests.

Efficacy of indigenous entomopathogenic nematodes from Meghalaya, India against the larvae of taro leaf beetle, Aplosonyx chalybaeus (Hope)

The efficacy of three entomopathogenic nematode (EPN) species, Heterorhabditis indica, Steinernema thermophilum, and S. glaseri, from Meghalaya, India was studied against the larvae of taro leaf beetle, Aplosonyx chalybaeus (Hope) (Coleoptera: Chrysomelidae), under the laboratory conditions. The beetle larvae (grubs) were exposed to 25, 50, 75, 100 and 200 infective juveniles (IJs) of each nematode species for different time periods and they were found to be susceptible to all the EPNs tested. However, the susceptibility of grubs to nematode infection varied according to the dosages of IJs and their exposure periods. Appreciably good performance was achieved by S. glaseri, which showed 100 % mortality of insect larvae in 48 h exposure time. At 48 h of incubation, its LC50 value was 90.3 IJs/larva, which was lower than that of S. thermophilum (115.0 IJs/larva) and H. indica (186.0 IJs/larva), at the same exposure time. All the tested nematode species were also found to reproduce within the host and produced infective juveniles. H. indica, however, showed comparatively more production of IJs per cadaver of infected host (168.9 × 10(3) IJs/larva), as compared to the other two tested nematode species. The production of IJs per cadaver of infected host by S. thermophilum was recorded to be 82.0 × 10(3) IJs/larva. In case of S. glaseri, while production of IJs increased initially to 18.9 × 10(3) IJs/larva at concentration of 100 IJs/larva, it declined thereafter to 14.7 × 10(3) IJs/larva at the dose of 200 IJs/larva. In conclusion, the evidence obtained in this study suggests that all the three indigenous EPN species are virulent enough to produce 100 % mortality in the last instar larvae of A. chalybaeus. These EPN species thus have potential scope for the management of A. chalybaeus in taro crops.

Evaluation of the efficacy of three indigenous strains of entomopathogenic nematodes from Meghalaya, India against mustard sawfly, Athalia lugens proxima Klug (Hymenoptera: Tenthredinidae)

The objective of this study was to evaluate the efficacy of three indigenous strains of entomopathogenic nematodes (EPN) from Meghalaya, India, namely Heterorhabditis indica Poinar, Karunakar and David, Steinernema thermophilum Ganguly and Singh, and Steinernema glaseri (Steiner) against the last instar larva of mustard sawfly, Athalia lugens proxima Klug, a serious pest of mustard and radish in India. The larvae of A. lugens proxima were exposed to 10, 25, 50, 75 and 100 infective juveniles (IJs) concentration of each nematode species in Petri dishes. Percentage larval mortality and nematode reproduction in insect larvae was studied. The sawfly larvae were found to be susceptible to all the three EPNs tested, but the degree of susceptibility to infection varied from among nematode species. Based on LC50 value, H. indica was the most pathogenic species. Nevertheless, S. thermophilum and S. glaseri also showed a high insect mortality. This study also revealed that all the three test nematodes are also able to propagate in the host cadaver and produce first generation infective juveniles. However, H. indica produced significantly more number of IJs per insect larva than the other two nematode species. The progeny production was recorded to be the least in case of S. glaseri. In conclusion, our findings suggest that of the three indigenous EPNs studied, H. indica and S. thermophilum have good potential as biological control agents against mustard sawfly, A. lugens proxima.

Soil moisture effects on the activity of three entomopathogenic nematodes (Steinernematidae and Heterorhabditidae) isolated from Meghalaya, India

Entomopathogenic nematodes (EPNs) are obligate parasites of insects that are widely distributed in soils throughout the world. They have great potential for use as biological control agents for insect pests. It is known that strains of Steinernema and Heterorhabditis isolated from different geographical regions exhibit differences in their ecological traits, such as infectivity, establishment, survival, reproduction, etc. A precise knowledge of these factors is therefore an essential pre-requisite for devising successful strategies to use these nematodes in biological control programmes. The present study investigated the effect of soil moisture on the activity (as measured by number of nematodes established in hosts) of three entomopathogenic nematode species (Heterorhabditis indica Poinar, Karunakar & David; Steinernema thermophilum Ganguly & Singh; Steinernema glaseri Steiner), isolated from forest soils in Meghalaya, India, under laboratory conditions. The experiments for EPNs were conducted at 25 ± 2°C (30 ± 2°C for S. thermophilum) in a sandy loam soil (85% sand, 12% silt and 3% clay, pH 6.54). Last instar larvae of wax moth, Galleria mellonella served as the experimental insect host. The soil moistures tested were 3, 4, 5, 6, 8, 10, 12, 14, 16, 18, 20, 22 and 25% (w/w). The study revealed that soil moisture has marked influences on establishment of infective juveniles of different nematode species in insect host. While, S. thermophilum showed establishment at 4% and above soil moistures, H. indica and S. glaseri showed establishment at 5% and above soil moistures. The optimum soil moisture for different nematode species were noted as: H. indica 8-18%, S. thermophilum 6-20%, and S. glaseri 8-25%. Further, a minimum of 6% soil moisture was noted to be essential for achieving 100% host mortality for all the three nematode species.