Habituation to a Deterrent Plant Alkaloid Develops Faster in the Specialist Herbivore Helicoverpa assulta Than in Its Generalist Congener Helicoverpa armigera and Coincides with Taste Neuron Desensitisation

The two closely related moth species, Helicoverpa armigera and H. assulta differ strongly in their degree of host-plant specialism. In dual-choice leaf disk assays, caterpillars of the two species that had been reared on standard artificial diet were strongly deterred by the plant-derived alkaloid strychnine. However, caterpillars of both species reared on artificial diet containing strychnine from neonate to the 5th instar were insensitive to this compound. Fifth instar caterpillars of H. assulta and 4th or 5th instars of H. armigera not exposed to strychnine before were subjected to strychnine-containing diet for 24 h, 36 h, 48 h, or 72 h. Whereas H. assulta displayed habituation to strychnine after 48 h, it took until 72 h for H. armigera to become habituated. Electrophysiological tests revealed that a deterrent-sensitive neuron in the medial sensillum styloconicum of both species displayed significantly reduced sensitivity to strychnine that correlated with the onset of habituation. We conclude that the specialist H. assulta habituated faster to strychnine than the generalist H. armigera and hypothesis that desensitization of deterrent-sensitive neurons contributed to habituation.

Elucidation of Triadic Relationship Among Light Environment, Host Plant Quality, and Feeding Behavior of Zizeeria maha (Kollar) (Lepidoptera: Lycaenidae) IN Oxalis corniculata L. (Oxalidales: Oxalidaceae)

In the dynamics of light-plant-insect interaction, the light affects plant metabolisms which may directly influence the production of defensive secondary metabolites and may consequently alter the feeding behavior of herbivores. The present study aimed to investigate the triadic interactions by using Oxalis corniculata L. (Oxalidales: Oxalidaceae) and its specialist herbivore, Zizeeria maha (Kollar) (Lepidoptera: Lycaenidae), in relation to the light intensity of plant habitats and physicochemical properties of the plants which would affect the larval feeding behavior of Z. maha. Firstly, leaves of O. corniculate in the field with seven different light conditions were collected. A part of which was subjected to chemical analyses, and the rest was fed to Z. maha larvae to evaluate growth and feeding activity; larval period, death rate, weight, amount of consumption, and amount of frass were measured to calculate the relative growth rate, approximate digestion rate, and relative consumption rate. Secondly, light/shade mock environment test tests were conducted with laboratory-grown O. corniculata. The results under both field and laboratory conditions showed positive effects of light intensity on the production of the defensive compound, oxalic acid, in the plants. Furthermore, the larval feeding activity was higher when fed with leaves in higher light intensities. These results relate to our previous study that demonstrated oxalic acid stimulates the feeding of Z. maha larvae. Thus, the triadic interaction among light, O. corniculata, and Z. maha larvae could be explained by the light-driven up-regulated production of oxalic acid positively influenced the larval feeding.

Effect of Nitrogen Fertilization on the Density of the Corn Leafhopper and Its Impact on Both Disease Incidence and Natural Parasitism

The corn leafhopper Dalbulus maidis (Hemiptera: Cicadellidae) is a specialist herbivore that attacks maize in the tropical and subtropical regions of the Americas. It is vector of three relevant plant pathogens being responsible for severe yield losses. Modern agriculture is dependent on the addition of fertilizers, especially nitrogen, which may influence the nutritional quality of the plants possibly with a subsequent increment of herbivorous insect populations. Through a field experiment, using a randomized design with four treatments with different levels of fertilization, we evaluated the effects of nitrogen fertilization in corn on the population levels of the vector D. maidis, on the incidence of the diseases transmitted by it, and on the levels of parasitism of the vectors’ eggs. The amount of nitrogen fertilizer used significantly influenced the density of the corn leafhopper and the parasitism by egg parasitoids, but not the incidence of the diseases transmitted by it. Two weeks after fertilization, the vector density was significantly higher in the highly fertilized treatment. The disease incidence was not directly linked with the level of fertilization, however, the symptoms of the diseases were much less evident in plants that received higher fertilization. Parasitism levels by egg parasitoids increased accordingly to the level of D. maidis populations.

A sink host allows a specialist herbivore to persist in a seasonal source

In seasonal environments, sinks that are more persistent than sources may serve as temporal stepping stones for specialists. However, this possibility has to our knowledge, not been demonstrated to date, as such environments are thought to select for generalists, and the role of sinks, both in the field and in the laboratory, is difficult to document. Here, we used laboratory experiments to show that herbivorous arthropods associated with seasonally absent main (source) habitats can endure on a suboptimal (sink) host for several generations, albeit with a negative growth rate. Additionally, they dispersed towards this host less often than towards the main host and accepted it less often than the main host. Finally, repeated experimental evolution attempts revealed no adaptation to the suboptimal host. Nevertheless, field observations showed that arthropods are found in suboptimal habitats when the main habitat is unavailable. Together, these results show that evolutionary rescue in the suboptimal habitat is not possible. Instead, the sink habitat functions as a temporal stepping stone, allowing for the persistence of a specialist when the source habitat is gone.

Intraspecific interaction of host plant influences local distribution of specialist herbivores through metabolic alterations in leaves

Recent studies suggest that changes in leaf traits due to interactions between plants affect the resource utilisation and distribution of herbivores. However, this has not yet been confirmed experimentally. Here, we investigated the effects of phenotypic plasticity in leaf traits of Rumex obtusifolius (host plant) in response to the intra and interspecific interaction on distribution of two leaf beetles, Gastrophysa atrocyanea (specialist herbivore) and Galerucella grisescens (generalist herbivore). We investigated the local population density of R. obtusifolius plants and the presence of leaf beetles on the plants at five study sites. Leaf chemicals (condensed tannins and total phenolics) were compared between aggregated and solitary R. obtusifolius plants. To clarify the effects of the interaction environment of R. obtusifolius plants on their leaf traits and resource utilisation by leaf beetles, we conducted cultivation and preference experiments. Leaf chemicals (chlorophylls, organic acids, primary metabolites, condensed tannins and total phenolics) and preferences of adult leaf beetles were compared between intraspecific, interspecific plant interaction, or no-interaction treatments. Finally, we evaluated the effects of interaction between R. obtusifolius on leaf beetle distribution in mesocosm experiments. In the field, the presence of the specialist leaf beetle, G. atrocyanea, was positively correlated with the local population density (rosette overlap ratio) of R. obtusifolius plants; however, no correlation was observed in the case of the generalist leaf beetle, G. grisescens. In the cultivation experiment, plants in the intraspecific interaction treatment increased their leaf contents of condensed tannins and total phenolics, and G. atrocyanea consumed more of these leaves than leaves in other treatments. Similar results were observed in the field. In the mesocosm experiment, larger numbers of G. atrocyanea were distributed on R. obtusifolius plants exposed to below-ground intraspecific interaction than on plants not exposed to intraspecific interaction. Our results provide experimental evidence that leaf trait changes in response to intraspecific interaction between host plants influence specialist herbivore distribution. This highlights the need to integrate plant plant interactions into our understanding of plant animal interactions.

The preeminent role of directional selection in generating extreme morphological change in Glyptodonts (Cingulata; Xenarthra)

The prevalence of stasis on paleontological and comparative data has been classically taken as evidence of the strong role of stabilizing selection in shaping morphological evolution. When confronted against biologically informed predictions, empirical rates of evolution tend to fall below what is expected under genetic drift, suggesting that the signal for directional selection is erased at longer time scales. However, empirical tests of these claims are few and tend to focus on univariate traits, ignoring the potential roles of trait covariances in constraining evolution. Here we investigated the multivariate rates of morphological evolution in a fossil lineage that underwent extreme morphological modification, the glyptodonts. Contrary to what was expected, biologically informed models of evolution suggest a preeminent role of directional selection on the divergence of glyptodonts from living armadillos. Furthermore, the reconstruction of selection patterns shows that traits selected to generate a glytodont morphology are markedly different from those necessary to explain the extant armadillos' morphological diversity. Changes in both direction and magnitude of selection are probably tied to the invasion of a specialist-herbivore adaptive zone by glyptodonts. These results suggest that directional selection might have played a more important role in the evolution of extreme morphologies than previously imagined.

Sugar transporters enable a leaf beetle to accumulate plant defense compounds

Many herbivorous insects selectively accumulate plant toxins for defense against predators; however, little is known about the transport processes that enable insects to absorb and store defense compounds in the body. Here, we investigate how a specialist herbivore, the horseradish flea beetle, accumulates glucosinolate defense compounds from Brassicaceae in the hemolymph. Using phylogenetic analyses of coleopteran major facilitator superfamily transporters, we identify a clade of glucosinolate-specific transporters (PaGTRs) belonging to the sugar porter family. PaGTRs are predominantly expressed in the excretory system, the Malpighian tubules. Silencing of PaGTRs leads to elevated glucosinolate excretion, significantly reducing the levels of sequestered glucosinolates in beetles. This suggests that PaGTRs reabsorb glucosinolates from the Malpighian tubule lumen to prevent their loss by excretion. Ramsay assays corroborated the selective retention of glucosinolates by Malpighian tubules of P. armoraciae in situ. Thus, the selective accumulation of plant defense compounds in herbivorous insects can depend on the ability to prevent excretion.

Aphid Species Specializing on Milkweed Have Distinct Bacterial Symbiont Communities

Host plant range is arguably one of the most important factors shaping microbial communities associated with insect herbivores. However, it is unclear whether host plant specialization limits microbial community diversity or to what extent herbivores sharing a common host plant evolve distinct microbiomes. To investigate whether variation in host plant specialization influences the composition of herbivore symbiont populations we compared bacterial diversity across three milkweed aphid species (Aphis nerii, Aphis asclepiadis, Myzocallis asclepiadis) feeding on a common host plant (Asclepias syriaca) using 16S rRNA metabarcoding. Overall, bacterial species richness did not vary with degree of host plant specialization. However, aphid species harbored distinct bacterial communities that varied in composition and relative abundance of key symbionts. Differences in aphid microbiomes were primarily due to strain variation in the obligate symbiont Buchnera and facultative symbiont Arsenophonus, as most of the low-abundant taxa were found in all three species. Interestingly, A. asclepiadis harbored a greater diversity of unique strains of Buchnera and significantly higher Arsenophonus relative abundances compared to the other two aphid species. Although many low abundance microbes were shared across all milkweed aphids, key differences exist in symbiotic partnerships that could influence additional ecological variation, including variation in ant tending observed across milkweed aphid species via microbial induced changes to honeydew or defensive chemical profiles. This study suggests generalist and specialist herbivore microbiomes are similar when feeding on a common host plant and highlights an intriguing potential role for strain level variation of key aphid symbionts in host-plant interactions.

Sugar transporters enable a leaf beetle to accumulate plant defense compounds

Many herbivorous insects selectively accumulate plant toxins for defense against predators; however, little is known about the transport processes that enable insects to absorb and store defense compounds in the body. Here, we investigate how a specialist herbivore, the horseradish flea beetle, accumulates high amounts of glucosinolate defense compounds in the hemolymph. Using phylogenetic analyses of coleopteran membrane transporters of the major facilitator superfamily, we identified a clade of glucosinolate-specific transporters (PaGTRs) belonging to the sugar porter family.PaGTRexpression was predominantly detected in the excretory system, the Malpighian tubules. Silencing ofPaGTRs led to elevated glucosinolate excretion, significantly reducing the levels of sequestered glucosinolates in beetles. This suggests thatPaGTRs reabsorb glucosinolates from the Malpighian tubule lumen to prevent their loss by excretion. Ramsay assays performed with dissected Malpighian tubules confirmed a selective retention of glucosinolates. Thus, the selective accumulation of plant defense compounds in herbivorous insects can depend on the ability to prevent excretion.