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1. Variation in immune response in the generalist herbivore fall webworm across four common host plants

   https://doi.org/10.1111/eea.13502  

   Mitchell, Audrey M; Vyas, Dhaval K; Murphy, Shannon M (November 2024, Entomologia Experimentalis et Applicata)

   Abstract Dietary generalist herbivorous insects are widespread and often occur in a variety of environments. Across their geographic range, herbivorous insects may encounter variable plant traits as they feed on high‐quality or low‐quality plants. Herbivorous insect larvae experience both bottom‐up (host plant) and top‐down (parasitoid) factors that affect survival. Host plant quality may affect larval growth and survival in that larvae feeding on low‐quality plants often suffer reduced fitness. However, herbivores on different host plants are also subject to different levels of parasitism. High‐quality plants confer stronger larval performance (higher survival, more offspring), but larvae may also face higher parasitism. In some herbivore species, diet mediates larval immune response. The generalist insect herbivore fall webworm (FW),Hyphantria cuneaDrury (Lepidoptera: Erebidae), is a moth native to North America, and its larvae have considerable variance in their performance when reared on different host plants. We investigated whether diet affects the immune response in FW larvae when they are reared on different host plant species known to vary in food quality. We measured immune response by melanization of a nylon filament. We found significant differences in immune response across host plants, indicating that diet mediates immune response in FW larvae. Our study helps elucidate the factors that cause variation in immune response in a generalist herbivore. 

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2. Clonal versus non-clonal milkweeds ( Asclepias spp.) respond differently to stem damage, affecting oviposition by monarch butterflies

   https://doi.org/10.7717/peerj.10296  

   He, Elise; Agrawal, Anurag A. (January 2020, PeerJ)

   null (Ed.)

   Background Oviposition decisions are critical to the fitness of herbivorous insects and are often impacted by the availability and condition of host plants. Monarch butterflies ( Danaus plexippus ) rely on milkweeds ( Asclepias spp.) for egg-laying and as food for larvae. Previous work has shown that monarchs prefer to oviposit on recently regrown plant tissues (after removal of above-ground biomass) while larvae grow poorly on plants previously damaged by insects. We hypothesized that these effects may depend on the life-history strategy of plants, as clonal and non-clonal milkweed species differ in resource allocation and defense strategies. Methodology/Principal Findings We first confirmed butterfly preference for regrown tissue in a field survey of paired mowed and unmowed plots of the common milkweed A. syriaca . We then experimentally studied the effects of plant damage (comparing undamaged controls to plants clipped and regrown, or damaged by insects) on oviposition choice, larval performance, and leaf quality of two closely related clonal and non-clonal species pairs: (1) A. syriaca and A. tuberosa , and (2) A. verticillata and A. incarnata . Clonal and non-clonal species displayed different responses to plant damage, impacting the proportions of eggs laid on plants. Clonal species had similar mean proportions of eggs on regrown and control plants (≈35–40% each), but fewer on insect-damaged plants (≈20%). Meanwhile non-clonal species had similar oviposition on insect-damaged and control plants (20–30% each) but more eggs on regrown plants (40–60%). Trait analyses showed reduced defenses in regrown plants and we found some evidence, although variable, for negative effects of insect damage on subsequent larval performance. Conclusions/Significance Overall, non-clonal species are more susceptible and preferred by monarch butterflies following clipping, while clonal species show tolerance to clipping and induced defense to insect herbivory. These results have implications for monarch conservation strategies that involve milkweed habitat management by mowing. More generally, plant life-history may mediate growth and defense strategies, explaining species-level variation in responses to different types of damage. 

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3. Host plant specificity of the monarch butterfly Danaus plexippus: A systematic review and meta-analysis

   https://doi.org/10.1371/journal.pone.0269701  

   Greenstein, Lewis; Steele, Christen; Taylor, Caz M. (June 2022, PLOS ONE)

   Mansour, Ramzi (Ed.)

   The preference-performance hypothesis explains host specificity in phytophagous insects, positing that host plants chosen by adults confer the greatest larval fitness. However, adults sometimes oviposit on plants supporting low larval success because the components of host specificity (adult preference, plant palatability, and larval survival) are non-binary and not necessarily correlated. Palatability (willingness to eat) is governed by chemical cues and physical barriers such as trichomes, while survival (ability to complete development) depends upon nutrition and toxicity. Absence of a correlation between the components of host specificity results in low-performance hosts supporting limited larval development. Most studies of specificity focus on oviposition behavior leaving the importance and basis of palatability and survival under-explored. We conducted a comprehensive review of 127 plant species that have been claimed or tested to be hosts for the monarch butterfly Danaus plexippus to classify them as non-hosts, low performance, or high performance. We performed a meta-analysis to test if performance status could be explained by properties of neurotoxic cardenolides or trichome density. We also conducted a no-choice larval feeding experiment to identify causes of low performance. We identified 34 high performance, 42 low performance, 33 non-hosts, and 18 species with unsubstantiated claims. Mean cardenolide concentration was greater in high- than low-performance hosts and a significant predictor of host status, suggesting possible evolutionary trade-offs in monarch specialization. Other cardenolide properties and trichome density were not significant predictors of host status. In the experiment, we found, of the 62% of larvae that attempted to eat low-performance hosts, only 3.5% survived to adult compared to 85% of those on the high-performance host, demonstrating that multiple factors affect larval host plant specificity. Our study is the first to classify all known host plants for monarchs and has conservation implications for this threatened species. 

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4. Plant water limitation and its impact on the oviposition preferences of the monarch butterfly (Lepidoptera: Nymphalidae)

   https://doi.org/10.1093/jisesa/iead075  

   Diethelm, Aramee C.; Kost, Konnor E.; Pringle, Elizabeth G.; Hesler, ed., Louis (August 2023, Journal of Insect Science)

   Abstract Intensifying drought conditions across the western United States due to global climate change are altering plant–insect interactions. Specialist herbivores must find their host plants within a matrix of nonhosts, and thus often rely upon specific plant secondary chemistry for host location and oviposition cues. Climate-induced alterations to plant chemistry could thus affect female selection of larval food plants. Here, we investigated whether host-plant water limitation influenced oviposition preference in a threatened invertebrate: the monarch butterfly (Danaus plexippus). We found that females deposited more eggs on reduced-water than on well-watered narrowleaf milkweed plants (Asclepias fascicularis), but we could not attribute this change to any specific change in plant chemistry. Specialist herbivores, such as the monarch butterfly, which are tightly linked to specific plant cues, may experience shift in preferences under global-change conditions. Understanding oviposition preferences will be important to directing ongoing habitat restoration activities for this declining insect. 

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5. Phylogenomics reveals the evolutionary timing and pattern of butterflies and moths

   https://doi.org/10.1073/pnas.1907847116  

   Kawahara, Akito Y.; Plotkin, David; Espeland, Marianne; Meusemann, Karen; Toussaint, Emmanuel F.; Donath, Alexander; Gimnich, France; Frandsen, Paul B.; Zwick, Andreas; dos Reis, Mario; et al (November 2019, Proceedings of the National Academy of Sciences)

   Butterflies and moths (Lepidoptera) are one of the major superradiations of insects, comprising nearly 160,000 described extant species. As herbivores, pollinators, and prey, Lepidoptera play a fundamental role in almost every terrestrial ecosystem. Lepidoptera are also indicators of environmental change and serve as models for research on mimicry and genetics. They have been central to the development of coevolutionary hypotheses, such as butterflies with flowering plants and moths’ evolutionary arms race with echolocating bats. However, these hypotheses have not been rigorously tested, because a robust lepidopteran phylogeny and timing of evolutionary novelties are lacking. To address these issues, we inferred a comprehensive phylogeny of Lepidoptera, using the largest dataset assembled for the order (2,098 orthologous protein-coding genes from transcriptomes of 186 species, representing nearly all superfamilies), and dated it with carefully evaluated synapomorphy-based fossils. The oldest members of the Lepidoptera crown group appeared in the Late Carboniferous (∼300 Ma) and fed on nonvascular land plants. Lepidoptera evolved the tube-like proboscis in the Middle Triassic (∼241 Ma), which allowed them to acquire nectar from flowering plants. This morphological innovation, along with other traits, likely promoted the extraordinary diversification of superfamily-level lepidopteran crown groups. The ancestor of butterflies was likely nocturnal, and our results indicate that butterflies became day-flying in the Late Cretaceous (∼98 Ma). Moth hearing organs arose multiple times before the evolutionary arms race between moths and bats, perhaps initially detecting a wide range of sound frequencies before being co-opted to specifically detect bat sonar. Our study provides an essential framework for future comparative studies on butterfly and moth evolution. 

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