Effect of light energy on alkali-released virions from Anagrapha falcifera nucleopolyhedrovirus

A novel formulation technology for baculoviruses protects biopesticide from degradation by ultraviolet radiation

Biopesticides are biological pest control agents that are viewed as safer alternatives to the synthetic chemicals that dominate the global insecticide market. A major constraint on the wider adoption of biopesticides is their susceptibility to the ultraviolet (UV: 290-400 nm) radiation in sunlight, which limits their persistence and efficacy. Here, we describe a novel formulation technology for biopesticides in which the active ingredient (baculovirus) is micro-encapsulated in an ENTOSTAT wax combined with a UV absorbant (titanium dioxide, TiO2). Importantly, this capsule protects the sensitive viral DNA from degrading in sunlight, but dissolves in the alkaline insect gut to release the virus, which then infects and kills the pest. We show, using simulated sunlight, in both laboratory bioassays and trials on cabbage and tomato plants, that this can extend the efficacy of the biopesticide well beyond the few hours of existing virus formulations, potentially increasing the spray interval and/or reducing the need for high application rates. The new formulation has a shelf-life at 30 °C of at least 6 months, which is comparable to standard commercial biopesticides and has no phytotoxic effect on the host plants. Taken together, these findings suggest that the new formulation technology could reduce the costs and increase the efficacy of baculovirus biopesticides, with the potential to make them commercially competitive alternatives to synthetic chemicals.

Biopesticide synergy when combining plant flavonoids and entomopathogenic baculovirus

Four crop plants known to be hosts for the lepidopteran Trichoplusia ni (soybean, green bean, cotton, and cabbage) were treated with the biopesticide AfMNPV baculovirus in a dosage response assay. Treated soybean had, on average, a 6-fold increase in virus activity compared with the other crops. Leaf trichomes on soybeans were not found to be responsible for the observed increase of insecticidal activity. Three flavonoid compounds (daidzein, genistein, and kaempferol) were uniquely found only in the soybean crop, and were not detected in cotton, cabbage, or green bean plant matter. The individual flavonoid compounds did not cause T ni. mortality in no-virus assays when incorporated into artificial insect diet. The combination of the three flavonoid compounds at leaf level concentrations significantly increased baculovirus activity in diet incorporation assays. When the daidzein, genistein, and kaempferol were added to artificial diet, at 3.5–6.5 × leaf level concentrations, virus activity increased 1.5, 2.3, and 4.2-fold for each respective flavonoid. The soybean flavonoid compounds were found to synergistically improve baculovirus activity against T. ni.

In Vivo Production of Agrotis ipsilon Nucleopolyhedrovirus for Quantity and Quality

The black cutworm, Agrotis ipsilon (Hüfnagel) (Lepidoptera: Noctuidae), is a pest causing damage to a variety of plants including turf and row crops. A recently discovered baculovirus has the potential to be developed as a microbial-based biological pesticide to provide targeted control of this insect pest. In an effort to develop this baculovirus as a biological pesticide, experiments were conducted to determine parameters necessary to maximize in vivo production using cutworm larvae. Treatment combinations including three larval diets, larval age at infection (6- to 10-d old), and dosage of virus exposure (1 × 105 to 1 × 108 occlusion bodies [OBs]/ml) were evaluated. Production quantity and quality were measured as number of OBs produced and insecticidal activity of the virus, respectively. Generally speaking, insect diets that maximized larval growth resulted in a greater quantity of virus OBs. Less virus was produced when younger (small) larvae were exposed to higher dosages of virus resulting in rapid mortality and when older (large) larvae were exposed to low dosages of virus resulting in low levels of infection. Virus quality was measured as insecticidal activity (low LC50 representing high activity) and was highest for larger larvae exposed to minimal virus concentrations needed to initiate infections. When considering both quantity and quality measurements, maximum production was achieved for 8- to 9-d-old larvae fed a general purpose lepidoptera diet. These results will support the development of this baculovirus as an additional tool for the integrated control of the black cutworm.

Comparison of tolerance to sunlight between spatially distant and genetically different strains of Lymantria dispar nucleopolyhedrovirus

Baculoviruses are a family of insect-specific pathogenic viruses can persist outside for long periods through the formation of occlusion bodies. In spite of this ability, the UV of sunlight is an essential factor that limits the survival of baculoviruses outside the host. In the current study, we compared the UV tolerance of two strains of Lymantria dispar multiple nucleopolyhedrovirus (LdMNPV), which were isolated in spatially different regions (LdMNPV-27/0 in Western Siberia (Russia) and LdMNPV-45/0 in North America (USA)) and dramatically differ in their potency. We exposed the studied strains to sunlight in an open area for 0.25, 0.5, 1, and 2 hours and later perorally inoculated host larvae with the same doses of virus (5x105) and with doses leading to same effect (LD90). We observed that strain LdMNPV-45/0, which previously showed high virulence against L. dispar larvae, was more sensitive to UV irradiation (estimated as the relative rate of inactivation (r, h -1) and as the half-life of the virus (τ1/2, h)) compared to LdMNPV-27/0. Exposure to sunlight induced a significant delay of LdMNPV-45/0-induced pathogenesis already after 0.25 h of sunlight exposure, while for LdMNPV-27/0 this delay was occurred only after 2 h exposure in spite of used concentrations. We also compared the sequences of the main structural proteins of the studied strains as UV light contributes not only to genome damage in viruses but also to structural protein damage. The most prominent genetic difference between the structural proteins of the strains was related to the loss of the virus enhancin factor-1 (vef-1) gene in the LdMNPV-27/0 strain. Thus initially highly potent viral strain (such as LdMNPV-45/0) is not recommend to use in the regions (or forest stand density) with high UV load. The role of virus enhancin factor-1 in baculovirus tolerance to UV needs for following studies.

Detection of genes encoding antimicrobial peptides in Mexican strains of Trichoplusia ni (Hübner) exposed to Bacillus thuringiensis

The systemic immune response of Trichoplusia ni after Bacillus thuringiensis (Bt) exposure was evaluated by comparing the expression of genes encoding antimicrobial peptides (AMPs) in Bt-susceptible and -resistant T. ni strains that were either exposed or not to XenTari (Bt-XT). AMP genes were detected by RT-PCR using primers for attacin, gloverin, lebocin, lysozyme, and peptidoglycan recognition peptide (PGRP). In general, AMP genes were detected more frequently in Mexican field strains previously exposed to Bt (SALX and GTOX) than in a Mexican laboratory strain (NL), but expression was similar to the AMP expression in USA laboratory strains (US and USX). Among the AMPs, transcripts for lebocin were the least detected (11.7%) and those for lysozyme were the most detected (84.8%) in all samples. Lebocin was detected only in 2nd instar and pupa. All untreated controls expressed attacin. Attacin and gloverin were not detected in any midgut sample, and their highest detection was in pupa. Lysozyme was rarely detected in 2nd instar larvae from any strain or treatment but was detected in almost all midgut and hemolymph samples. Overall, AMPs were found more in T. ni strains previously exposed to Bt-XT, especially lebocin and globerin (1.8-fold increase) and PGRP (3.8-fold increase). The data suggest that the expression of AMPs in T. ni correlates to previous Bt exposure.

Efficacy of spiracular infection of Helicoverpa armigera with its nucleopolyhedrovirus and its role in virus production

Baculoviruses are important microbial control agents of insects, with per os mode of infectivity. However, recently the spiracular infection of this virus group was suggested as an optimum method for virus production in grown up larvae. In this regard, a detailed evaluation of the spiracular infection with intact polyhedral inclusion bodies (PIB), alkali liberated virions and alkali liberated filtered virions of Helicoverpa armigera (Hubner) nucleopolyhedrovirus at 1 x 10(8), 1 x 10(7) and 2 x 10(6)PIB/ml concentrations was undertaken and compared with the standard diet surface treatment method. All the spiracle treatments resulted in larval death due to virus infection with alkali liberated virions causing higher mortality of larvae than alkali liberated filtered virions and intact PIB. Diet surface treatment method resulted in very high mortality as compared to spiracle treatment and among the different inoculum tested the intact PIB resulted in higher larval mortality. The PIB yield/larva in spiracle treatment was comparable with the diet surface treatment method, but due to very low larval mortality it resulted in low virus yield/100 inoculated larvae. Diet surface treatment with 5 x 10(7)PIB/ml concentration of virus resulted in the maximum yield of PIB/100 inoculated larvae. Low mortality, higher labour requirement and low amenability for mechanization for spiracle treatment method make it unviable for mass production of the virus in large scale compared to the standard diet surface treatment method.

Field activity and storage stability of Anagrapha falcifera nucleopolyhedrovirus (AfMNPV) in spray-dried lignin-based formulations

A multiple-embedded nucleopolyhedrovirus isolated from Anagrapha falcifera (Kirby) (AfMNPV) has potential to be developed into a microbial bioinsecticide because the host range includes several economic pests. We tested spray-dried AfMNPV formulations after storage for insecticidal activity based on bioassays with neonate Trichoplusia ni (Hübner). Eight experimental lignin-based spray-dried formulations, a glycerin-based formulation, and an unformulated sample were made with virus stock from three commercial production lots. Samples of these formulations were stored at 30 degrees C in individually sealed sample containers for destructive sampling after 1, 3, and 6 mo whereas the remaining product was stored in glass jars under refrigeration for up to 30 mo. Spray drying did not significantly reduce the initial LC50s of AfMNPV in experimental formulations compared with unformulated virus that was not spray dried. Refrigerated storage for 6 mo did not significantly lower virus activity of formulated samples compared with the unformulated AfMNPV stored frozen, while samples stored for 30 mo had higher LC50 values determined by both droplet and leaf feeding assays. When stored at 30 degrees C, most formulations (22 of 24) maintained insecticidal activity for 3 mo, but most (21 of 24) lost significant activity after 6 mo of storage. The glycerin-based formulation also lost activity within 6 mo of storage at 30 degrees C when compared with frozen unformulated virus, but did not lose activity when stored refrigerated for up to 30 mo. These formulations were evaluated after 7 mo at 4 degrees C for residual insecticidal activity when applied to field grown cabbage. Insecticidal activity was determined against T. ni neonates for treated leaf samples collected at 3, 7, 27, and 51 h after application of 2.5 x 10(12) obs/ha. Field tests showed no differences in activity among samples of stored formulations and one freshly made formulation. Spray-dried formulations had significantly higher insecticidal activity (67.5% mortality) compared with the unformulated treatment (30% mortality) sampled 3 h after application. At 3, 7, and 27 h after application, the spray-dried formulations had higher residual activity (67%, 59%, and 42% mortality, respectively), compared with the commercial glycerin-based formulation (61%, 38%, and 23% mortality, respectively). These experiments demonstrated that AfMNPV in lignin-based spray-dried formulations had a shelf-life of up to 3 mo at 30 degrees C and up to 30 mo at 4 degrees C, and with longer residual insecticidal activity in the field compared with unformulated or a glycerin formulation.

Effect of transgenic plants expressing high levels of a tobacco anionic peroxidase on the toxicity of Anagrapha falcifera nucleopolyhedrovirus to Helicoverpa zea (Lepidoptera: Noctuidae)

Wild type and corresponding transgenic tomato (Lycopersicon esculentum Miller) and two tobacco (Nicotiana spp.) plants that express high levels of a tobacco anionic peroxidase were used to determine what type of interactions occurred between peroxidase altered plant chemistry and the baculovirus Anagrapha falcifera nucleopolyhedrovirus (AfMNPV) for control of neonate corn earworms, Helicoverpa zea (Boddie). Transgenic plants expressed approximately five to 400 times higher peroxidase activity than corresponding tissues of wild type plants. The H. zea larvae typically fed 1.5 times less on transgenic compared with wild type leaf disks. There was only one experiment (of three with tomato leaves) where the larvae that fed on transgenic leaves were less susceptible to the virus based on nonoverlapping 95% confidence intervals for LC50 values. When the exposure dose was corrected for reduced feeding on the transgenic leaf disks, the insecticidal activity of the virus was not significantly different for larvae fed on transgenic versus wild type plants. Eight other experiments (with tomato and two species of tobacco) indicated either no significant effect or enhanced susceptibility (when corrected for feeding rates) to the virus of larvae fed on the transgenic leaves. These results indicate enhanced insect resistance in plants expressing high levels of a specific anionic peroxidase may be compatible with applications of AfMNPV. Potential reasons for this compatibility are discussed.

Storage stability of Anagrapha falcifera nucleopolyhedrovirus in spray-dried formulations

A multiply embedded nucleopolyhedrovirus isolated from Anagrapha falcifera (Kirby) (AfMNPV) can lose insecticidal activity during months of dry storage in ambient room conditions. We tested the spray-dried AfMNPV formulations after storage for up to 1 year at room temperatures for insecticidal activity against neonate Trichoplusia ni (Hübner). Experimental formulations were made using combinations of corn flours, lignin, and sucrose, and were selected based on previous work which demonstrated that these formulations resisted solar degradation in field experiments. Twelve experimental formulations (organized in three groups of four formulations) compared the effect of (1) the ratio of formulation ingredients (lignin and corn flour) to virus concentration, (2) different sources of lignin, or (3) different corn flours and sugar. Based on a single-dose plant assay with these 12 formulations, none of the formulations lost significant activity due to the drying process, when compared with the unformulated wet AfMNPV. Samples of the 12 dried formulations were stored at room (22+/-3 degrees C) and refrigerated (4 degrees C) temperatures. Insecticidal activity (LC(50)) was determined with a dosage-response assay for neonates fed on treated cotton-leaf disks. After 6 (or 9) and 12 months storage, refrigerated samples maintained insecticidal activity better than corresponding samples stored at room temperatures with LC(50)s that averaged 2.0 x 10(6) polyhedral inclusion bodies per milliliter (pibs/ml) for refrigerated samples and 5.4 x 10(6) pibs/ml for samples stored at room temperatures. Compared with unformulated stock virus stored frozen, six formulations stored at room temperature and 10 formulations stored in the refrigerator did not lose significant insecticidal activity after 1 year based on overlapping 90% confidence intervals. Changing the ratio of virus to formulation ingredients did not provide a clear trend over the range of concentrations tested, and may be less important for shelf-life of virus activity compared with formulations made with different ingredients. Two of the four formulations made with different lignins were about 15 times less active after 1 year at room temperature compared with refrigerated samples, indicating that specific formulation ingredients can affect storage stability. Formulations that contained sugar generally maintained activity during storage better than formulations without sugar. Unformulated virus stock maintained insecticidal activity (ranged from 0.20 to 2.5 x 10(6) pibs/ml) better during storage than dried formulations with LC(50)s that ranged from 0.39 to 27 x 10(6) pibs/ml. Unformulated virus stock, which is essentially a suspension of virus occlusion bodies in homogenized insect cadavers, did not lose activity when stored at refrigerated or room temperature. We believe that stability of AfMNPV insecticidal activity during storage as dry formulations is related to the general composition of the formulation and that sugar may play a critical role in maintaining insecticidal activity.

Comparative field stability of selected entomopathogenic virus formulations

Nucleopolyhedroviruses originally isolated from Anagrapha falcifera (Kirby) and Autographa californica (Speyer) were formulated with various ingredients using a spray dry method and tested for residual field activity in Illinois and Mississippi. In Mississippi, field tests were conducted on cotton in 1997, whereas in Illinois tests were conducted on cabbage in 1997 and 1998. Within 24 h, significant differences were observed among formulations in all tests. Unformulated virus had significantly less insecticidal activity than formulated virus and formulations containing lignin retained activity significantly longer than other formulations. Relatively small amounts of Blankophor BBH, when encapsulated within the formulation, did not greatly enhance (>10x) insecticidal activity based on LC50 determinations nor prolong insecticidal activity based on field evaluations. In most tests, >50% activity remained in formulations containing lignin, whereas unformulated virus retained <50% activity within 24 h after application.