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Insect guts often harbor an abundance of bacteria. Many of these members are commensal, but some may emerge as opportunistic pathogens when the host is under stress. In this study, we evaluated how dietary nutritional concentration mediates a shift from commensal to pathogenic, and if host species influences those interactions. We used the lepidopterans (Noctuidae) fall armyworm (Spodoptera frugiperda), beet armyworm (Spodoptera exigua), and corn earworm (Helicoverpa zea) as hosts and a Serratia strain initially isolated from healthy fall armyworm. Diet concentration was altered by bulk reduction in nutritional content with dilution using cellulose. Our experiments revealed that low nutrient diet increased mortality from Serratia for beet armyworm and corn earworm. However, for fall armyworm, little mortality was observed in any of the diet combinations. Dietary nutrition and oral inoculation with Serratia did not change the expression of two antimicrobial peptides in fall and beet armyworm, suggesting that other mechanisms that mediate mortality were involved. Our results have implications for how pathogens may persist as commensals in the digestive tract of insects. These findings also suggest that diet plays a very important role in the switch from commensal to pathogen. Finally, our data indicate that the host response to changing conditions is critical in determining if a pathogen may overtake its host and that these three lepidopteran species have different responses to opportunistic enteric pathogens., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Published by Elsevier Inc.)

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., (Copyright © 2018 Elsevier Inc. All rights reserved.)

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., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Enhancins are metalloproteases found in many betabaculoviruses and several alphabaculoviruses, which enhance alphabaculovirus potency by degrading a protein component of the peritrophic matrix (PM), facilitating passage of virions through this structure. Earlier studies on betabaculovirus enhancins within heterologous systems suggested that enhancins facilitate virion binding to midgut cells. We compared the potency of wild-type Lymantria dispar multiple nucleopolyhedrovirus (LdMNPV) with that of single and double enhancin deletion viruses in L. dispar in the presence and absence of an intact PM. Compared to wild-type virus, the double enhancin deletion virus was 6-fold and 14-fold less potent, respectively, indicating that within this homologous system the LdMNPV enhancin genes have a function beyond PM degradation.

Recombinant insect nucleopolyhedroviruses lacking the egt gene generally kill their hosts faster than wild-type strains, but the response of insects to mixtures of virus genotypes is less well known. Here, we compared the survival time, lethal dose and occlusion body yield in third instar larvae of Helicoverpa armigera (Hübner) after challenge with wild-type H. armigera SNPV (HaSNPV-wt), a strain with a deletion of the egt gene, HaSNPV-LM2, and a 1:1 mixture of these two virus strains. A range of doses was used to determine whether the total number of OBs influenced the response to challenge with a mixture of virus strains versus single strains. At high virus doses, HaSNPV-LM2 killed H. armigera larvae significantly faster (ca. 20 h) than HaSNPV-wt, but at low doses, there was no significant difference in survival time between the viruses. The survival time after challenge with mixed virus inoculum was significantly different from and intermediate between that of the single viruses at high doses, and not different from that of the single viruses at low doses. No differences in lethal dose were found between single and mixed infections or between virus genotypes. The number of occlusion bodies produced per larva increased with time to death and decreased with virus dose, but no significant differences among virus types were found., ((c) 2010 Elsevier Inc. All rights reserved.)

Addition of the antibiotic streptomycin to two artificial diets routinely used in bioassays of neonate lar vae of Heliothis virescens (tobacco budworm) infected with Autographa californica M nucleopolyhedrovirus (AcMNPV) increased lethal times of the virus. After storage of diets for 3 weeks at 4 degrees C, lethal times of infected larvae were significantly slower compared to those for larvae bioassayed using diets stored for 2 weeks or less. The effect of diet-age on rate of mortality was not the result of a change in total protein content or pH of the diet, but was apparently the result of some other alteration in the quality of the diet (e.g. microbial spoilage, palatibility, and/or nutritional value unrelated to total protein). Although we did not determine why lethal times were slower in response to streptomycin concentration or diet-age, we did find that slower lethal times were correlated with slower relative growth rates (RGR) of infected larvae. In addition, RGR of infected larvae decreased as a function of increasing streptomycin concentration, diet-age, and the interaction of the two factors. These results demonstrate that it is difficult to obtain consistent and comparable bioassay results if antibiotic composition and diet-age are not controlled. We suggest a standardized diet or highly standardized procedures for a given diet be developed that permits comparison of bioassays among and within laboratories.

Both wild-type and recombinant baculoviruses are becoming more attractive for the control of insect pests. Thus, there is an increased incentive to address and resolve logistical problems associated with large-scale production of these viruses. In this study, we have compared the potential of two insect cell lines, Tn5B1-4 and Sf21, for the production of polyhedra and compared the efficacy of both cell culture-derived and host-derived viruses by bioassay. The efficacy of both wild-type AcMNPV and AcAaIT, a recombinant baculovirus expressing an insect-specific scorpion toxin, were compared. Yields of polyhedra from Tn5B1-4 were sixfold higher than those from the cell line Sf21. Morphological analysis of polyhedra derived from cell culture showed greater variability in size relative to host-derived polyhedra. The maximum size of cell culture-derived polyhedra was over 1.5 times larger than that of insect-derived polyhedra. The efficacy of AcMNPV and AcAaIT derived from cell culture, or from amplification in larvae of Trichoplusia ni or Heliothis virescens, was compared by bioassay in H. virescens. There was a significant difference between the slopes for lethal time data for host-derived and cell culture-derived wild-type virus. Mortality occurred at a faster rate following infection with host-derived virus. No significant difference was seen for the recombinant virus AcAaIT. Lethal doses of cell- and host-derived polyhedra were not significantly different. The reasons for and implications of this for pest control are discussed.(ABSTRACT TRUNCATED AT 250 WORDS)