Inactivation of baculovirus by quinones formed in insect-damaged plant tissues

The role of host plants in driving pathogen susceptibility in insects through chemicals, immune responses and microbiota

In this comprehensive exploration, we delve into the pivotal role of host plants in shaping the intricate interactions between herbivorous insects and their pathogens. Recent decades have seen a surge in studies that demonstrate that host plants are crucial drivers of the interactions between insects and pathogens, providing novel insights into the direct and indirect interactions that shape tri-trophic interactions. These studies have built on a wide range of pathogens, from viruses to bacteria, and from protozoans to fungi. We summarise these studies, and discuss the mechanisms of plant-mediated insect resistance to infection, ranging from the toxicity of plant chemicals to pathogens to enhancement of anti-pathogen immune responses, and modulation of the insect's microbiome. Although we provide evidence for the roles of all these mechanisms, we also point out that the majority of existing studies are phenomenological, describing patterns without addressing the underlying mechanisms. To further our understanding of these tri-trophic interactions, we therefore urge researchers to design their studies to enable them specifically to distinguish the mechanisms by which plants affect insect susceptibility to pathogens.

A novel tank-mix formulation increases the efficacy of alphabaculoviruses on different phylloplanes

Spodoptera littoralis Boisduval (Lepidoptera: Noctuidae) and Spodoptera exigua Hübner (Lepidoptera: Noctuidae) pose substantial threats to many crops, necessitating the exploration of biopesticides as potential chemical alternatives. One alternative is baculoviruses; however, their instability in the field has hindered their widespread use. Host plant phylloplane affects baculovirus activity at varying levels in different host plants. Formulation contributes significantly to optimizing the baculoviral stability on different phylloplanes against environmental conditions; however, it is expensive and difficult to make in developing or nondeveloped countries. In the current study, we developed a simple tank-mix application (MBF-Tm5) for immediate use, resembling the characteristics of a suspension concentrate formulation for Spodoptera littoralis nucleopolyhedrovirus (SpliNPV) and Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV). We examined their biological activity against 2nd instar larvae first on an artificial diet under laboratory conditions and on eggplant and pepper phylloplane in greenhouse conditions compared to plain viruses. This formulation exhibited no significant improvement in the biological activity of both viruses on an artificial diet under laboratory conditions but significantly improved the biological activity of both viruses on both plants under greenhouse conditions. The original activity remaining (OAR%) of both unformulated and formulated viruses decreased over time under greenhouse conditions; however, the OAR value of both viruses on eggplants was significantly higher than on pepper plants. Overall, the tank-mix simple formulation of baculoviruses might be a great alternative for improved stability in nature, providing better control.

Plant defense metabolites influence the interaction between an insect herbivore and an entomovirus

The tri-trophic interaction of plants, insect herbivores, and entomoviruses is an important topic in ecology and pest control. The susceptibility of insect herbivores to entomoviruses (e.g., nucleopolyhedroviruses) is influenced by host plants; however, the role of plant secondary metabolites in determining such susceptibility is poorly understood. Metabolomic analyses of Brassica oleracea, Glycine max, and Ipomoea aquatica plants, which differ in how they affect the susceptibility of Spodoptera exigua to nucleopolyhedroviruses among 14 plants, suggested that the plant secondary metabolites genistein, kaempferol, quercitrin, and coumarin play a role in influencing nucleopolyhedroviruses susceptibility. Subsequently, transcriptomic analysis of caterpillars, treated with nucleopolyhedroviruses alone or with one of these four phenolics, identified four genes (CYP340K4, CXE18, GSTe, and GSTe1) that were significantly downregulated by the phenolics. Functional characterization of these genes suggested that their downregulation significantly increased larval sensitivity to nucleopolyhedroviruses and altered aspects of the immune response. Our findings provide new insight into the role of plant defense metabolites in influencing the interactions between insect herbivores and entomopathogens and identify plant secondary metabolites as potential synergists of viral agents for the control of agricultural pests.

Dose-dependent dynamics of densovirus infection in two nymphalid butterfly species utilizing native or exotic host plants

Insects are attacked by a diverse range of microbial pathogens in the wild. In herbivorous species, larval host plants frequently play a critical role in mediating susceptibility to infection. Characterizing such plant-mediated effects on herbivore-pathogen interactions can provide insight into patterns of infection across wild populations. In this study, we investigated the effects of host plant use by two North American butterflies, Euphydryas phaeton (Nymphalidae) and Anartia jatrophae (Nymphalidae), on entomopathogen infection across a range of three doses. Both of these herbivores recently incorporated the same exotic plant, Plantago lanceolata (Plantaginaceae), into their host range and are naturally infected by the same entomopathogen, Junonia coenia densovirus (Parvoviridae), in wild populations. We performed two factorial experiments in which E. phaeton and A. jatrophae were reared on either P. lanceolata or a native host plant [Chelone glabra (Plantaginaceae) for E. phaeton; Bacopa monnieri (Plantaginaceae) for A. jatrophae] and inoculated with either a low, medium, or high dose of the virus. In E. phaeton, the outcomes of infection were highly dose-dependent, with inoculation with higher viral doses resulting in faster time to death and greater mortality. However, neither survival nor postmortem viral burdens varied depending upon the host plant that was consumed. In contrast, host plant use had a strong effect on viral burdens in A. jatrophae, with consumption of the exotic plant appearing to enhance host resistance to infection. Together, these results illustrate the variable influences of host plant use on herbivore resistance to infection, highlighting the importance of investigating plant-herbivore relationships within a tritrophic framework.

Reductive Conversion Leads to Detoxification of Salicortin-like Chemical Defenses (Salicortinoids) in Lepidopteran Specialist Herbivores (Notodontidae)

Lepidopteran specialist herbivores of the Notodontidae family have adapted to thrive on poplar and willow species (Salicaceae). Previous research showed that Cerura vinula, a member of the Notodontidae family occurring throughout Europe and Asia, uses a unique mechanism to transform salicortinoids, the host plant's defense compounds, into quinic acid-salicylate conjugates. However, how the production of this conjugates relates to the detoxification of salicortinoids and how this transformation proceeds mechanistically have remained unknown. To find the mechanisms, we conducted gut homogenate incubation experiments with C. vinula and re-examined its metabolism by analyzing the constituents of its frass. To estimate the contribution of spontaneous degradation, we examined the chemical stability of salicortinoids and found that salicortinoids were degraded very quickly by midgut homogenates and that spontaneous degradation plays only a marginal role in the metabolism. We learned how salicortinoids are transformed into salicylate after we discovered reductively transformed derivatives, which were revealed to play key roles in the metabolism. Unless they have undergone the process of reduction, salicortinoids produce toxic catechol. We also studied constituents in the frass of the Notodontidae species Cerura erminea, Clostera anachoreta, Furcula furcula, Notodonta ziczac, and Pheosia tremula, and found the same metabolites as those described for C. vinula. We conclude that the process whereby salicortinoids are reductively transformed represents an important adaption of the Notodontidae to their Salicaceae host species.

Use of an exotic host plant reduces viral burden in a native insect herbivore

Incorporation of exotic plants into the diets of native herbivores is a common phenomenon, influencing interactions with natural enemies and providing insight into the tritrophic costs and benefits of dietary expansion. We evaluated how use of an exotic plant, Plantago lanceolata, impacted immune performance, development and susceptibility to pathogen infection in the neotropical herbivore Anartia jatrophae (Lepidoptera: Nymphalidae). Caterpillars were reared on P. lanceolata or a native plant, Bacopa monnieri, and experimentally infected with a pathogenic virus, Junonia coenia densovirus. We found that virus-challenged herbivores exhibited higher survival rates and lower viral burdens when reared on P. lanceolata compared to B. monnieri, though immune performance and development time were largely similar on the two plants. These findings reveal that use of an exotic plant can impact the vulnerability of a native herbivore to pathogen infection, suggesting diet-mediated protection against disease as a potential mechanism facilitating the incorporation of novel resources.

Toxic effects of purified phenolic compounds from Acacia nilotica against common cutworm

Phenolics are the most abundant secondary metabolites of plants that are widely distributed in the plant kingdom. In the last few years, the development and identification of phenolic compounds from various plants have become a major area of the environment and health-related research. In the present study, different phenolic compounds were purified from the bark of the medicinally important plant Acacia nilotica which is rich in polyphenols and were evaluated for their insecticidal potential against a polyphagous pest, Spodoptera litura (Fab.). The compounds viz. Catechin, Chlorogenic acid, and Umbelliferone were purified from ethyl acetate-acetone (E-AE) fraction using Semi-preparative HPLC and were identified using melting point determination, Nuclear Magnetic Resonance (NMR), and Mass Spectroscopy (MS). These phenolic compounds recorded detrimental effects on the growth and development of second instar larvae (6 days old) of S. litura. The larval growth, survival, adult emergence, pupal weight, and different nutritional indices were adversely affected by the various concentrations of these purified compounds. The findings revealed the insecticidal potential of polyphenols obtained from the bark of A. nilotica, which can provide an alternative for resistance management, as these plant phytochemicals are highly effective against insecticide-resistant insect pests.

Comparative efficacy of gypsy moth (Lepidoptera: Erebidae) entomopathogens on transgenic blight-tolerant and wild-type American, Chinese, and hybrid chestnuts (Fagales: Fagaceae)

American chestnut (Castanea dentata [Marsh.] Borkh.) was once the dominant hardwood species in Eastern North America before an exotic fungal pathogen, Cryphonectria parasitica (Murrill) Barr, functionally eliminated it across its range. One promising approach toward restoring American chestnut to natural forests is development of blight-tolerant trees using genetic transformation. However, transformation and related processes can result in unexpected and unintended phenotypic changes, potentially altering ecological interactions. To assess unintended tritrophic impacts of transgenic American chestnut on plant-herbivore interactions, gypsy moth (Lymantria dispar L.) caterpillars were fed leaf disks excised from two transgenic events, Darling 54 and Darling 58, and four control American chestnut lines. Leaf disks were previously treated with an LD₅₀ dose of either the species-specific Lymantria dispar multiple nucleopolyhedrovirus (LdMNPV) or the generalist pathogen Bacillus thuringiensis subsp. kurstaki (Btk). Mortality was quantified and compared to water blank controls. Tree genotype had a strong effect on the efficacies of both pathogens. Larval mortality from Btk-treated foliage from only one transgenic event, Darling 54, differed from its isogenic progenitor, Ellis 1, but was similar to an unrelated wild-type American chestnut control. LdMNPV efficacy was unaffected by genetic transformation. Results suggest that although genetic modification of trees may affect interactions with other nontarget organisms, this may be due to insertion effects, and variation among different genotypes (whether transgenic or wild-type) imparts a greater change in response than transgene presence.

Feeding behavior, life history, and virus transmission ability of Bemisia tabaci Mediterranean species (Hemiptera: Aleyrodidae) under elevated CO2

The continuous rise of CO₂ concentrations in the atmosphere is reducing plant nutritional quality for herbivores and indirectly affects their performance. The whitefly (Bemisia tabaci, Gennadius) is a major worldwide pest of agricultural crops causing significant yield losses. This study investigated the plant-mediated indirect effects of elevated CO₂ on the feeding behavior and life history of B. tabaci Mediterranean species. Eggplants were grown under elevated and ambient CO₂ concentrations for 3 weeks after which plants were either used to monitor the feeding behavior of whiteflies using the Electrical Penetration Graph technique or to examine fecundity and fertility of whiteflies. Plant leaf carbon, nitrogen, phenols and protein contents were also analyzed for each treatment. Bemisia tabaci feeding on plants exposed to elevated CO₂ showed a longer phloem ingestion and greater fertility compared to those exposed to ambient CO₂ suggesting that B. tabaci is capable of compensating for the plant nutritional deficit. Additionally, this study looked at the transmission of the virus Tomato yellow leaf curl virus (Begomovirus) by B. tabaci exposing source and receptor tomato plants to ambient or elevated CO₂ levels before or after virus transmission tests. Results indicate that B. tabaci transmitted the virus at the same rate independent of the CO₂ levels and plant treatment. Therefore, we conclude that B. tabaci Mediterranean species prevails over the difficulties that changes in CO₂ concentrations may cause and it is predicted that under future climate change conditions, B. tabaci would continue to be considered a serious threat for agriculture worldwide.

Immunity of an insect herbivore to an entomovirus is affected by different host plants

BACKGROUND

Interactions between herbivorous insects and entomoviruses may depend on host plant, perhaps mediated through changes in herbivore innate immunity.

RESULTS

Caterpillars (Spodoptera exigua) fed Glycine max had high viral loads and low melanization rates together with low melanization enzyme [PO, DDC, TH] activities and gene expressions. Caterpillars fed Ipomoea aquatica had low viral loads and high melanization, gene activities and gene expressions while those fed Brassica oleracea or artificial diet had intermediate levels of each. Melanization rates were negatively correlated with viral loads and positively correlated with activity and expression of each of the three enzymes. Some diet effects on enzymes were constitutive because the same diets led to low (G. max) or high (I. aquatica) melanization related gene activities and expressions without infection.

CONCLUSION

Diet influences the interactions between insect herbivores and viruses by shaping the innate immune response both at the onset of infection and afterwards as viral loads accumulate over a period of days. In addition, diets that lead to low viral loads are associated with high activities and gene expressions of a variety of melanization related enzymes suggesting a common causative mechanism. © 2019 Society of Chemical Industry.

Global metabolite profiles of rice brown planthopper-resistant traits reveal potential secondary metabolites for both constitutive and inducible defenses

INTRODUCTION

Brown planthopper (BPH) is a phloem feeding insect that causes annual disease outbreaks, called hopper burn in many countries throughout Asia, resulting in severe damage to rice production. Currently, mechanistic understanding of BPH resistance in rice plant is limited, which has caused slow progression on developing effective rice varieties as well as effective farming practices against BPH infestation.

OBJECTIVE

To reveal rice metabolic responses during 8 days of BPH attack, this study examined polar metabolome extracts of BPH-susceptible (KD) and its BPH-resistant isogenic line (IL308) rice leaves.

METHODS

Ultra high performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-QToF-MS) was combined with multi-block PCA to analyze potential metabolites in response to BPH attack.

RESULTS

This multivariate statistical model revealed different metabolic response patterns between the BPH-susceptible and BPH-resistant varieties during BPH infestation. The metabolite responses of the resistant IL308 variety occurred on Day 1, which was significantly earlier than those of the susceptible KD variety which showed an induced response by Days 4 and 8. BPH infestation caused metabolic perturbations in purine, phenylpropanoid, flavonoid, and terpenoid pathways. While found in both susceptible and resistant rice varieties, schaftoside (1.8 fold), iso-schaftoside (1.7 fold), rhoifolin (3.4 fold) and apigenin 6-C-α-L-arabinoside-8-C-β-L-arabinoside levels (1.6 fold) were significantly increased in the resistant variety by Day 1 post-infestation. 20-hydroxyecdysone acetate (2.5 fold) and dicaffeoylquinic acid (4.7 fold) levels were considerably higher in the resistant rice variety than those in the susceptible variety, both before and after infestation, suggesting that these secondary metabolites play important roles in inducible and constitutive defenses against the BPH infestation.

CONCLUSIONS

These potential secondary metabolites will be useful as metabolite markers and/or bioactive compounds for effective and durable approaches to address the BPH problem.

Bottom-up trait-mediated indirect effects decrease pathogen transmission in a tritrophic system

A plant's induction of secondary defenses helps to decrease herbivore damage by changing resource quality. While these chemical or physical defenses may directly decrease herbivory, they can also have indirect consequences. In a tritrophic system consisting of a plant, an insect herbivore, and an insect pathogen, plant based trait-mediated indirect effects (TMIEs) can alter host-pathogen interactions and, thereby, indirectly affect disease transmission. In a series of field experiments, individual soybean plants (Glycine max) were sprayed with either a jasmonic acid (JA) solution to trigger induction of plant defenses or a similar control compound. Fall armyworm (Spodoptera frugiperda) larvae along with varying amounts of a lethal baculovirus were placed on the plants to measure transmission. Induction of plant defenses decreased viral transmission due to increased population heterogeneity arising from changes in individual susceptibility. The change in susceptibility via TMIEs was driven by a decrease in feeding rates and an increase viral dose needed to infect larvae. While the induction against herbivore attack may decrease herbivory, it can also decrease the efficacy of the herbivore's pathogen potentially to the plant's detriment. While TMIEs have been well-recognized for being driven by top-down forces, bottom-up interactions can dictate community dynamics and, here, epizootic severity.

Jasmonic acid-induced plant defenses delay caterpillar developmental resistance to a baculovirus: Slow-growth, high-mortality hypothesis in plant-insect-pathogen interactions

Plants damaged by herbivore feeding can induce defensive responses that reduce herbivore growth. The slow-growth, high-mortality hypothesis postulates that these non-lethal plant defenses prolong the herbivore's period of susceptibility to natural enemies, such as predators and parasitoids. While many juvenile animals increase their disease resistance as they grow, direct tests of the slow-growth, high-mortality hypothesis in the context of plant-herbivore-pathogen interactions are lacking. Caterpillars increase their resistance to lethal baculoviruses as they develop within and across instars, a phenomenon termed developmental resistance. Progression of developmental resistance can occur through age-related increases in systemic immune functioning and/or midgut-based resistance. Here, we examined the slow-growth, high-mortality hypothesis in the context of developmental resistance of caterpillars to baculoviruses. Intra-stadial (within-instar) developmental resistance of the fall armyworm, Spodoptera frugiperda, to an oral inoculum of the baculovirus SfMNPV increased more rapidly with age when larvae were fed on non-induced foliage than foliage that was induced by jasmonic acid (a phytohormone that up-regulates plant anti-herbivore defenses). The degree of developmental resistance observed was attributable to larval weight at the time of virus inoculation. Thus, slower growth on induced plants prolonged the window of larval susceptibility to the baculovirus. Developmental resistance on induced and non-induced plants was absent when budded virus was injected intrahemocoelically bypassing the midgut, suggesting that developmental resistance was gut-based. Addition of fluorescent brightener, which weakens midgut-based resistance mechanisms to oral virus challenge, abolished developmental resistance. These results highlight the impact of plant defenses on herbivore growth rate and consequences for disease risk.

Elevated atmospheric concentrations of carbon dioxide reduce monarch tolerance and increase parasite virulence by altering the medicinal properties of milkweeds

Hosts combat their parasites using mechanisms of resistance and tolerance, which together determine parasite virulence. Environmental factors, including diet, mediate the impact of parasites on hosts, with diet providing nutritional and medicinal properties. Here, we present the first evidence that ongoing environmental change decreases host tolerance and increases parasite virulence through a loss of dietary medicinal quality. Monarch butterflies use dietary toxins (cardenolides) to reduce the deleterious impacts of a protozoan parasite. We fed monarch larvae foliage from four milkweed species grown under either elevated or ambient CO₂ , and measured changes in resistance, tolerance, and virulence. The most high-cardenolide milkweed species lost its medicinal properties under elevated CO₂ ; monarch tolerance to infection decreased, and parasite virulence increased. Declines in medicinal quality were associated with declines in foliar concentrations of lipophilic cardenolides. Our results emphasize that global environmental change may influence parasite-host interactions through changes in the medicinal properties of plants.

Plant genotype and induced defenses affect the productivity of an insect-killing obligate viral pathogen

Plant-mediated variations in the outcomes of host-pathogen interactions can strongly affect epizootics and the population dynamics of numerous species, including devastating agricultural pests such as the fall armyworm. Most studies of plant-mediated effects on insect pathogens focus on host mortality, but few have measured pathogen yield, which can affect whether or not an epizootic outbreak occurs. Insects challenged with baculoviruses on different plant species and parts can vary in levels of mortality and yield of infectious stages (occlusion bodies; OBs). We previously demonstrated that soybean genotypes and induced anti-herbivore defenses influence baculovirus infectivity. Here, we used a soybean genotype that strongly reduced baculovirus infectivity when virus was ingested on induced plants (Braxton) and another that did not reduce infectivity (Gasoy), to determine how soybean genotype and induced defenses influence OB yield and speed of kill. These are key fitness measures because baculoviruses are obligate-killing pathogens. We challenged fall armyworm, Spodoptera frugiperda, with the baculovirus S. frugiperda multi-nucleocapsid nucleopolyhedrovirus (SfMNPV) during short or long-term exposure to plant treatments (i.e., induced or non-induced genotypes). Caterpillars were either fed plant treatments only during virus ingestion (short-term exposure to foliage) or from the point of virus ingestion until death (long-term exposure). We found trade-offs of increasing OB yield with slower speed of kill and decreasing virus dose. OB yield increased more with longer time to death and decreased more with increasing virus dose after short-term feeding on Braxton compared with Gasoy. OB yield increased significantly more with time to death in larvae that fed until death on non-induced foliage than induced foliage. Moreover, fewer OBs per unit of host tissue were produced when larvae were fed induced foliage than non-induced foliage. These findings highlight the potential importance of plant effects, even at the individual plant level, on entomopathogen fitness, which may impact epizootic transmission events and host population dynamics.

Tritrophic Interactions: Microbe-Mediated Plant Effects on Insect Herbivores

It is becoming abundantly clear that the microbes associated with plants and insects can profoundly influence plant-insect interactions. Here, we focus on recent findings and propose directions for future research that involve microbe-induced changes to plant defenses and nutritive quality as well as the consequences of these changes for the behavior and fitness of insect herbivores. Insect (herbivore and parasitoid)-associated microbes can favor or improve insect fitness by suppressing plant defenses and detoxifying defensive phytochemicals. Phytopathogens can influence or manipulate insect behavior and fitness by altering plant quality and defense. Plant-beneficial microbes can promote plant growth and influence plant nutritional and phytochemical composition that can positively or negatively influence insect fitness. Lastly, we suggest that entomopathogens have the potential to influence plant defenses directly as endophytes or indirectly by altering insect physiology.

Plant-mediated effects on an insect-pathogen interaction vary with intraspecific genetic variation in plant defences

Baculoviruses are food-borne microbial pathogens that are ingested by insects on contaminated foliage. Oxidation of plant-derived phenolics, activated by insect feeding, can directly interfere with infections in the gut. Since phenolic oxidation is an important component of plant resistance against insects, baculoviruses are suggested to be incompatible with plant defences. However, plants among and within species invest differently in a myriad of chemical and physical defences. Therefore, we hypothesized that among eight soybean genotypes, some genotypes would be able to maintain both high resistance against an insect pest and high efficacy of a baculovirus. Soybean constitutive (non-induced) and jasmonic acid (JA)-induced (anti-herbivore response) resistance was measured against the fall armyworm Spodoptera frugiperda (weight gain, leaf consumption and utilization). Indicators of phenolic oxidation were measured as foliar phenolic content and peroxidase activity. Levels of armyworm mortality inflicted by baculovirus (SfMNPV) did not vary among soybean genotypes when the virus was ingested with non-induced foliage. Ingestion of the virus on JA-induced foliage reduced armyworm mortality, relative to non-induced foliage, on some soybean genotypes. Baculovirus efficacy was lower when ingested with foliage that contained higher phenolic content and defensive properties that reduced armyworm weight gain and leaf utilization. However, soybean genotypes that defended the plant by reducing consumption rate and strongly deterred feeding upon JA-induction did not reduce baculovirus efficacy, indicating that these defences may be more compatible with baculoviruses to maximize plant protection. Differential compatibility of defence traits with the third trophic level highlights an important cost/trade-off associated with plant defence strategies.

Microscopy based studies on the interaction of bio-based silver nanoparticles with Bombyx mori Nuclear Polyhedrosis virus

In the present investigation, silver nanoparticles (AgNPs) interactions with Bombyx mori Nuclear Polyhedrosis virus (BmNPV) were characterized using High-Resolution Scanning Electron Microscopy (HR-SEM), Energy Dispersive X-ray Analysis (EDAX), Transmission Electron Microscopy (TEM), Atomic Force Microcopy (AFM) and Confocal Microscope (CM). HR-SEM study reveals that the biosynthesized AgNPs have interacted with BmNPV and were found on the surface. TEM micrographs of normal and viral polyhedra treated with AgNPs showed that the nanoparticles were accumulated in the membrane and it was noted that some of the AgNPs successfully penetrated the membrane by reaching the capsid of BmNPV. AFM and confocal microscopy studies reveal that the disruption in the shell membrane tends to lose its stability due to exposure of AgNPs to BmNPV.

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.

Activation of plant foliar oxidases by insect feeding reduces nutritive quality of foliage for noctuid herbivores

The foliage and fruit of the tomato plantLycopersicon esculentum contains polyphenol oxidases (PPO) and peroxidases (POD) that are compartmentally separated from orthodihydroxyphenolic substrates in situ. However, when leaf tissue is damaged by insect feeding, the enzyme and phenolic substrates come in contact, resulting in the rapid oxidation of phenolics to orthoquinones. When the tomato fruitwormHeliothis zea or the beet army-wormSpodoptera exigua feed on tomato foliage, a substantial amount of the ingested chlorogenic acid is oxidized to chlorogenoquinone by PPO in the insect gut. Additionally, the digestive enzymes of the fruitworm have the potential to further activate foliar oxidase activity in the gut. Chlorogenoquinone is a highly reactive electrophilic molecule that readily binds cova-lently to nucleophilic groups of amino acids and proteins. In particular, the -SH and -NH2 groups of amino acids are susceptible to binding or alkylation. In experiments with tomato foliage, the relative growth rate of the fruitworm was negatively correlated with PPO activity. As the tomato plant matures, foliar PPO activity may increase nearly 10-fold while the growth rate of the fruitworm is severely depressed. In tomato fruit, the levels of PPO are highest in small immature fruit but are essentially negligible in mature fruit. The growth rate of larvae on fruit was also negatively correlated with PPO activity, with the fastest larval growth rate occurring when larvae fed on mature fruit. The reduction in larval growth is proposed to result from the alkylation of amino acids/protein byo-quinones, and the subsequent reduction in the nutritive quality of foliage. This alkylation reduces the digestibility of dietary protein and the bioavailability of amino acids. We believe that this mechanism of digestibility reduction may be extrapolatable to other plant-insect systems because of the ubiquitous cooccurrence of PPO and phenolic substrates among vascular plant species.

Dietary plant phenolic improves survival of bacterial infection in Manduca sexta caterpillars

Plant phenolics are generally thought to play significant roles in plant defense against herbivores and pathogens. Many plant taxa, including Solanaceae, are rich in phenolic compounds and some insect herbivores have been shown to acquire phenolics from their hosts to use them as protection against their natural enemies. Here we demonstrate that larvae of an insect specialist on Solanaceae, the tobacco hornworm, Manduca sexta L. (Lepidoptera: Sphingidae), acquire the plant phenolic chlorogenic acid (CA), and other caffeic acid derivatives as they feed on one of their hosts, Nicotiana attenuata L. (Solanaceae), and on artificial diet supplemented with CA. We test the hypothesis that larvae fed on CA-supplemented diet would have better resistance against bacterial infection than larvae fed on a standard CA-free diet by injecting bacteria into the hemocoel of fourth instars. Larvae fed CA-supplemented diet show significantly higher survival of infection with Enterococcus faecalis (Andrewes & Horder) Schleifer & Kilpper-Bälz, but not of infection with the more virulent Pseudomonas aeruginosa (Schroeter) Migula. Larvae fed on CA-supplemented diet possess a constitutively higher number of circulating hemocytes than larvae fed on the standard diet, but we found no other evidence of increased immune system activity, nor were larvae fed on CA-supplemented diet better able to suppress bacterial proliferation early in the infection. Thus, our data suggest an additional defensive function of CA to the direct toxic inhibition of pathogen proliferation in the gut.

Limited effect of reactive oxygen species on the composition of susceptible essential amino acids in the midguts of Lymantria dispar caterpillars

The essential amino acids (EAAs) arginine, histidine, lysine, and methionine, as well as cysteine (semiessential), are believed to be susceptible to reactions with reactive oxygen species (ROS) in biological systems. The decreased availability of these EAAs could harm insect nutrition, since several of them can also be limiting for protein synthesis. However, no in vivo studies have quantified the effect of ROS in the midguts of insect herbivores on EAA composition. This study examined the association between elevated levels of ROS in the midgut fluid of Lymantria dispar caterpillars and the compositions of EAAs (protein-bound + protein-free) in their midgut fluid and frass. Contrary to expectation, the compositions of EAAs were not significantly decreased by ROS in midgut fluid ex vivo when incubated with phenolic compounds. Two in vivo comparisons of low- and high-ROS-producing leaves also showed similar results: there were no significant decreases in the compositions of EAAs in the midgut fluids and/or frass of larvae with elevated levels of ROS in their midguts. In addition, waste nitrogen excretion was not significantly increased from larvae on high-ROS treatments, as would be expected if ROS produced unbalanced EAA compositions. These results suggest that L. dispar larvae are able to tolerate elevated levels of ROS in their midguts without nutritionally significant changes in the compositions of susceptible EAAs in their food.

The adaptive significance of self-medication

Not all pharmacists are human; other species also use medicinal substances to combat pathogens and other parasites. Self-medicating behaviour is a topic of rapidly growing interest to behaviourists, parasitologists, ethnobotanists, chemical ecologists, conservationists and physicians. Although most of the pertinent literature is anecdotal, several studies have now attempted to test the adaptive function of particular self-medicating behaviours. We discuss the results of these studies in relation to simple hypotheses that can provide a framework for future tests of self-medication.

Food plant derived disease tolerance and resistance in a natural butterfly-plant-parasite interactions

Organisms can protect themselves against parasite-induced fitness costs through resistance or tolerance. Resistance includes mechanisms that prevent infection or limit parasite growth while tolerance alleviates the fitness costs from parasitism without limiting infection. Although tolerance and resistance affect host-parasite coevolution in fundamentally different ways, tolerance has often been ignored in animal-parasite systems. Where it has been studied, tolerance has been assumed to be a genetic mechanism, unaffected by the host environment. Here we studied the effects of host ecology on tolerance and resistance to infection by rearing monarch butterflies on 12 different species of milkweed food plants and infecting them with a naturally occurring protozoan parasite. Our results show that monarch butterflies experience different levels of tolerance to parasitism depending on the species of milkweed that they feed on, with some species providing over twofold greater tolerance than other milkweed species. Resistance was also affected by milkweed species, but there was no relationship between milkweed-conferred resistance and tolerance. Chemical analysis suggests that infected monarchs obtain highest fitness when reared on milkweeds with an intermediate concentration, diversity, and polarity of toxic secondary plant chemicals known as cardenolides. Our results demonstrate that environmental factors-such as interacting species in ecological food webs-are important drivers of disease tolerance.

Virulence evolution in response to anti-infection resistance: toxic food plants can select for virulent parasites of monarch butterflies

Host resistance to parasites can come in two main forms: hosts may either reduce the probability of parasite infection (anti-infection resistance) or reduce parasite growth after infection has occurred (anti-growth resistance). Both resistance mechanisms are often imperfect, meaning that they do not fully prevent or clear infections. Theoretical work has suggested that imperfect anti-growth resistance can select for higher parasite virulence by favouring faster-growing and more virulent parasites that overcome this resistance. In contrast, imperfect anti-infection resistance is thought not to select for increased parasite virulence, because it is assumed that it reduces the number of hosts that become infected, but not the fitness of parasites in successfully infected hosts. Here, we develop a theoretical model to show that anti-infection resistance can in fact select for higher virulence when such resistance reduces the effective parasite dose that enters a host. Our model is based on a monarch butterfly-parasite system in which larval food plants confer resistance to the monarch host. We carried out an experiment and showed that this environmental resistance is most likely a form of anti-infection resistance, through which toxic food plants reduce the effective dose of parasites that initiates an infection. We used these results to build a mathematical model to investigate the evolutionary consequences of food plant-induced resistance. Our model shows that when the effective infectious dose is reduced, parasites can compensate by evolving a higher per-parasite growth rate, and consequently a higher intrinsic virulence. Our results are relevant to many insect host-parasite systems, in which larval food plants often confer imperfect anti-infection resistance. Our results also suggest that - for parasites where the infectious dose affects the within-host dynamics - vaccines that reduce the effective infectious dose can select for increased parasite virulence.

Interactions among insect-resistant soybean genotypes extracts with populations of Anticarsia gemmatalis Hübner (Lepidoptera: Noctuidae) susceptible and resistant to its nucleopolyhedrovirus

Anticarsia gemmatalis nucleopolyhedrovirus (AgMNPV) is being used in Brazil as a biological insecticide. Host plant resistance of soybean to insects is been searched for and some authors have mentioned the interference of plant chemistry in virus efficiency. Interactions among soybean extracts of genotypes used as a source of resistance (PI 274454 and PI 227687) with different AgMNPV concentrations in populations of A. geatalis susceptible (S) and resistant (R) to the virus were studied at laboratory condition. Higher mortality was observed when larvae fed on diets with extracts of the soybean genotypes compared with those fed on a plain diet (control). The mean lethal concentration (LC50) was reduced about 10 ties in the S-population fed on diets containing PI 274454 extracts and different concentrations of AgMNPV, compared to control diet. Additive effect was predominantly observed when larvae fed on diets with extracts of soybean genotypes (PI 274454 and PI 227687) and AgMNPV for both larval populations. The pupal weight was negatively influenced by the extracts incorporated to the diets compared to control, for both larval populations, notably for R-population. The results suggest that, in general, leaf extracts of soybean resistant genotype did not cause any harmful effect on virus efficiency.

Plant-mediated alteration of the peritrophic matrix and baculovirus infection in lepidopteran larvae

The peritrophic matrix (PM) lines the midgut of most insects, providing protection to the midgut epithelial cells while permitting passage of nutrients and water. Herein, we provide evidence that plant-mediated alteration of the PM contributes to the well-documented inhibition of fatal infection by Autographa californica multiple nucleopolyhedrovirus (AcMNPV) of Heliothis virescens F. larvae fed cotton foliage. We examined the impact of the PM on pathogenesis using a viral construct expressing a reporter gene (AcMNPV-hsp70/lacZ) orally inoculated into larvae with either intact PMs or PMs disrupted by Trichoplusia ni granulovirus occlusion bodies containing enhancin, known to degrade insect intestinal mucin. Larvae possessing disrupted PMs displayed infection foci (lacZ signaling) earlier than those with intact PMs. We then examined PMs from larvae fed artificial diet or plant foliage using electron microscopy; foliage-fed larvae had significantly thicker PMs than diet-fed larvae. Moreover, mean PM width was inversely related to both the proportion of larvae with lacZ signaling at 18h post-inoculation and the final percentage mortality from virus. Thus, feeding on foliage altered PM structure, and these foliage-mediated changes reduced baculoviral efficacy. These data indicate that the PM is an important factor determining the success of an ingested pathogen in foliage-fed lepidopteran larvae.

Host plant species affects virulence in monarch butterfly parasites

1. Studies have considered how intrinsic host and parasite properties determine parasite virulence, but have largely ignored the role of extrinsic ecological factors in its expression. 2. We studied how parasite genotype and host plant species interact to determine virulence of the protozoan parasite Ophryocystis elektroscirrha (McLaughlin & Myers 1970) in the monarch butterfly Danaus plexippus L. We infected monarch larvae with one of four parasite genotypes and reared them on two milkweed species that differed in their levels of cardenolides: toxic chemicals involved in predator defence. 3. Parasite infection, replication and virulence were affected strongly by host plant species. While uninfected monarchs lived equally long on both plant species, infected monarchs suffered a greater reduction in their life spans (55% vs. 30%) on the low-cardenolide vs. the high-cardenolide host plant. These life span differences resulted from different levels of parasite replication in monarchs reared on the two plant species. 4. The virulence rank order of parasite genotypes was unaffected by host plant species, suggesting that host plant species affected parasite genotypes similarly, rather than through complex plant species-parasite genotype interactions. 5. Our results demonstrate that host ecology importantly affects parasite virulence, with implications for host-parasite dynamics in natural populations.

Induction of Systemic Acquired Resistance in Cotton Foliage Does Not Adversely Affect the Performance of an Entomopathogen

Baculoviral efficacy against lepidopteran larvae is substantially impacted by the host plant. Here, we characterized how baculoviral pathogenicity to cotton-fed Heliothis virescens larvae is affected by induction of systemic acquired resistance (SAR). Numerous studies have shown that SAR induced by the plant elicitor benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH) can protect against plant pathogens, but reports on the impacts of SAR on chewing herbivores or on natural enemies of herbivores are few. We found that BTH application significantly increased foliar peroxidase activity, condensed tannin levels, and total phenolic levels but did not alter dihydroxyphenolic levels. Consumption of BTH-treated foliage did not influence H. virescens pupal weight or larval mortality by the microbial control agent Autographa californica multiple nucleopolyhedrovirus any more than did consumption of untreated foliage. Thus, activation of SAR, although it did not protect the plant against a chewing herbivore, also did not reduce the effect of a natural enemy on a herbivore, indicating that SAR and microbial control agents may be compatible components of integrated pest management.

When quinones meet amino acids: chemical, physical and biological consequences

Quinones and amino acids are usually compartmentally separated in living systems, however there are several junctions in which they meet, react and influence. It occurs mainly in wounded, cut or crushed plant material during harvest, ensiling or disintegrating cells. Diffusing polyphenols are oxidized by polyphenol oxidases (PPOs) to quinonic compounds, which associate reversibly or irreversibly with amino acids and proteins. The reaction takes place with the free nucleophilic functional groups such as sulfhydryl, amine, amide, indole and imidazole substituents. It results in imine formation, in 1,4-Michael addition via nitrogen or sulphur and in Strecker degradation forming aldehydes. The formation and activity of quinone-amino acids conjugates influences the colour, taste, and aroma of foods. Physical and physiological phenomena such as browning of foods, discoloration of plants during processing, alteration of solubility and digestibility, formation of humic substances, germicidal activity, cytotoxicity and more occur when quinones from disintegrating cells meet amino acids. The mechanisms of toxicity and the pathways by which PCBs may be activated and act as a cancer initiator include oxidation to the corresponding quinones and reaction with amino acids or peptides. Sclerotization of insect cuticle is a biochemical process involving also the reaction between quinones and amino acid derivatives.

Plant-mediated effects in insect-pathogen interactions

Interactions between insect herbivores and their pathogens can be modulated by host plants. Inter- and intraspecific differences in plant chemistry and structure can alter the susceptibility of insects to infection and the production and environmental persistence of pathogens. Whether plants can manipulate insect pathogens to act as "bodyguards" and increase their own fitness remains to be shown. Reduced insect performance owing to poor plant quality can enhance the susceptibility of an insect to disease while these same phytochemicals can also reduce the effectiveness of entomopathogens in killing the host. As we discuss here, plants have an important role in the evolution of insect-pathogen relationships and a tritrophic perspective should thus be incorporated into the study of insects and their pathogens.

Herbivore-induced Plant Volatiles Trigger Sporulation in Entomopathogenic Fungi: The Case of Neozygites tanajoae Infecting the Cassava Green Mite

A large body of evidence shows that plants release volatile chemicals upon attack by herbivores. These volatiles influence the performance of natural enemies. Nearly all the evidence on the effect of plant volatiles on natural enemies of herbivores concerns predators, parasitoids, and entomophagous nematodes. However, other entomopathogens, such as fungi, have not been studied yet for the way they exploit the chemical information that the plant conveys on the presence of herbivores. We tested the hypothesis that volatiles emanating from cassava plants infested by green mites (Mononychellus tanajoa) trigger sporulation in three isolates of the acaropathogenic fungus Neozygites tanajoae. Tests were conducted under climatic conditions optimal to fungal conidiation, such that the influence of the plant volatiles could only alter the quantity of conidia produced. For two isolates (Altal.brz and Colal.brz), it was found that, compared with clean air, the presence of volatiles from clean, excised leaf discs suppressed conidia production. This suppressive effect disappeared in the presence of herbivore-damaged leaves for the isolate Colal.brz. For the third isolate, no significant effects were observed. Another experiment differing mainly in the amount of volatiles showed that two isolates produced more conidia when exposed to herbivore-damaged leaves compared with clean air. Taken together, the results show that volatiles from clean plants suppress conidiation, whereas herbivore-induced plant volatiles promote conidiation of N. tanajoae. These opposing effects suggest that the entomopathogenic fungus tunes the release of spores to herbivore-induced plant signals indicating the presence of hosts.

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.

Midgut-based resistance of Heliothis virescens to baculovirus infection mediated by phytochemicals in cotton

The decrease in susceptibility to polyhedrosis disease when Heliothis virescens larvae feed on cotton is profound, limiting the utility of baculoviruses for controlling noctuids on this important crop. We observed that the mortalities of H. virescens larvae challenged with a reporter-gene construct of Autographa californica M nucleopolyhedrovirus (AcMNPV-hsp70/lacZ) and fed either lettuce or artificial diet were approximately 2.5-fold higher than that of cotton-fed insects. This decrease in susceptibility on cotton was observed following oral but not intrahemocoelic inoculation of virus, and it was negatively correlated with levels of foliar peroxidase. The rates of development of both infected and uninfected larvae also were correlated negatively with levels of foliar peroxidase, and hence, were significantly lower for insects fed cotton. When Calcofluor White M2R, an optical brightener reported to enhance the retention of AcMNPV-infected midgut cells, was included in inoculum administered orally to larvae, mortality levels were equivalent regardless of diet. These results suggest that sloughing of infected midgut cells occurred at a higher rate in insects that fed on cotton compared to the other two diets, and that midgut cell sloughing is the mechanism whereby susceptibility to mortal infection by AcMNPV-hsp70/lacZ is decreased on cotton. This conclusion is consistent with previous reports that ingestion of cotton can generate reactive oxygen species within the midgut lumen that may damage midgut epithelial cells. As far as we know, this is the first study to link resistance intrinsic to the physiology of the insect (e.g., developmental resistance) and resistance conferred by host plant chemistry to a single mechanism, i.e., midgut cell sloughing.

Assessment of the application of baculoviruses for control of Lepidoptera

Baculoviruses, among other insect viruses, are regarded as safe and selective bioinsecticides, restricted to invertebrates. They have been used worldwide against many insect pests, mainly Lepidoptera. Their application as microbial pesticides, however, has not met their potential to control pests in crops, forests, and pastures, with the exception of the nuclear polyhedrosis virus of the soybean caterpillar (Anticarsia gemmatalis), which is used on approximately 1 million ha annually in Brazil. Problems that have limited expansion of baculovirus use include narrow host range, slow killing speed, technical and economical difficulties for in vitro commercial production, timing of application based on frequent host population monitoring, variability of field efficacy due to climatic conditions, and farmers' attitudes toward pest control, which have been based on application of fast-killing chemical insecticides. Farmer education regarding use of biological insecticides and their characteristics is considered one of the major actions necessary for increased use of baculoviruses. Strategies to counteract some of the limitations of baculoviruses, especially their slow killing activity, have been investigated and are promising. These include the use of chemical or biological substances added to virus formulations and genetic engineering of the viruses themselves to express insect toxins or hormones. Such strategies can enhance viral activity and increase speed of kill as well as reduce larval feeding activity. The use of baculoviruses against Lepidoptera is reviewed, with the utilization of the nuclear polyhedrosis virus of A. gemmatalis in Brazil serving as a case-study.

Effects of larval age and prolonged simulated acid rain on the susceptibility of European pine sawfly to virus infection

We studied the effect of simulated acid rain treatment of host trees on the susceptibility of the European pine sawfly larvae to virus, and possible differences when larvae of two different ages were infected. Older larvae were less susceptible to virus. Most larvae treated with virus 2 days after they started feeding on experimental foliage (group A) died rapidly within 10 days after the virus treatment, and survival to the end of the larval period was only 8-25%. Larvae treated with virus 1 week later (group B) were less affected by the virus and 36-49% survived. In group A the larval survival in the pH 3 treatment was higher than in other treatments; at the end of the larval period the difference was twofold. In group B there were no clear effects of acid rain on the susceptibility of larvae to virus. The study yielded the following new information: (1) the effect of prolonged acid rain treatment on reducing the efficacy of virus on young larvae was more distinct than in a previous study with shorter exposure to acid rain, and the difference was maintained to the end of larval period; (2) the susceptibility of older larvae to virus was not affected by acid rain treatments; (3) pH inside the needles did not explain the larval mortality caused by virus.