Insect immune response plays a crucial role in how external threats influence overall fitness through life history traits. An understudied question is how the use of different host plants might affect the ability of herbivorous insects to resist viral pathogens. The Melissa blue butterfly (Lycaeides melissa) has colonized the exotic legume Medicago sativa as a larval host within the past 200 years. Here we investigate how novel host plant use affects the immune response of L. melissa when infected with the lepidopteran virus, Junonia coenia densovirus (JcDV). We measured immune strength in response to JcDV in two ways: 1) direct measurement of phenoloxidase activity and melanization, and 2) transcriptional sequencing of individuals exposed to different viral and host plant treatments. Viral infection caused total phenoloxidase (total PO) to increase. We detected an interaction between viral infection and host plant for total PO: for control larvae, host plant use had no effect on total PO, whereas for infected larvae, total PO was significantly higher for larvae consuming the native host. Within the exotic host plant treatment, few genes were differentially regulated due to viral infection. Approximately two times more genes were differentially regulated in response to infection for larvae eating the native or exotic host, with differential expression of few putative immune genes. These results demonstrate that consumption of a novel host plant can alter both physiological and transcriptional responses to infection, emphasizing the importance of understanding diet when studying the molecular basis of immune function.

Insect herbivory can vary from an inconsequential biotic interaction to a factor that contributes substantially to the diversity of plants and animals and overall interaction diversity. As herbivory is the result of numerous ecological and evolutionary processes, including complex population dynamics and the evolution of plant defense, it has been difficult to predict variation in herbivory across meaningful spatial scales. In the present work, we characterize patterns of herbivory on plants in a species rich and abundant tropical understory genus (Piper) across forests spanning 44° of latitude in the Neotropics. We modeled the effects of geography, climate, resource availability, and Piper species richness on the median, dispersion, and skew of generalist and specialist herbivory. By examining these multiple components of the distribution of herbivory, we were able to determine factors that increase biologically meaningful herbivory at the upper ends of the distribution. Site level variables such as latitude, seasonality, and maximum Piper richness explained variation in herbivory at the local scale (plot level) better for assemblages of Piper congeners than for a single species. Predictors that varied between local communities, such as resource availability and diversity, best explained the distribution of herbivory within sites, dampening broad patterns across latitude and climate and demonstrating why generalizations about gradients in herbivory have been elusive. The estimated population means, skew, and dispersion of herbivory responded differently to abiotic and biotic factors, illustrating the need for careful studies to explore distributions of herbivory and their effects on forest diversity. Nevertheless, we observed a roughly two-fold increase in median herbivory in humid compared to seasonal forests, and this finding aligns with the hypothesis that precipitation seasonality plays a critical role in shaping interaction diversity within tropical ecosystems.

Insect herbivory is a critical top-down force structuring plant communities, and quantifying the factors that mediate damage caused by herbivores is fundamental to understanding biodiversity. As herbivory is the result of numerous ecological and evolutionary processes, including complex population dynamics and the evolution of plant defense, it has been difficult to predict variation in herbivory across meaningful spatial scales. In the present work, we characterized patterns of herbivory on plants in a speciose and abundant tropical understory genus (Piper) across forests spanning 44° of latitude in the Neotropics. We modeled the effects of geography, climate, resource availability, species richness and top-down pressure from parasitoids on the mean, dispersion, and skew of generalist and specialist herbivory. By examining these multiple moments of the distribution of herbivory, we were able to determine factors that increase biologically meaningful herbivory at the upper ends of its distribution. The strongest pattern that emerged at a large spatial scale was a roughly two-fold increase in herbivory in humid relative to seasonal forests. Site level variables such as latitude, seasonality and maximum Piper richness explained variation in herbivory at the local scale (plot level) better for communities of Piper congeners than for a single species. Predictors that varied between local communities, such as resource availability and diversity, best explained the distribution of herbivory within sites, dampening any broad patterns across latitude and climate and demonstrating why generalizations about gradients in herbivory have been elusive. The estimated population means, skew, and dispersion of herbivory respond differently to abiotic and biotic factors, demonstrating the need for careful studies to explore the distributions of herbivory and their effects on forest diversity.

Infections by maternally inherited bacterial endosymbionts, especially Wolbachia, are common in insects and other invertebrates but infection dynamics across species ranges are largely under studied. Specifically, we lack a broad understanding of the origin of Wolbachia infections in novel hosts and the factors governing their spread. We used Genotype-by-Sequencing (GBS) data from previous population genomics studies for range-wide surveys of Wolbachia presence and genetic diversity in over 2,700 North American butterflies of the genus Lycaeides. As few as one sequence read identified by assembly to a Wolbachia pan-reference genome provided high accuracy in detecting infections as determined by confirmatory PCR tests. Using a conservative threshold of five reads, we detected Wolbachia in all but two of the 107 sampling localities spanning the continent, and with most localities having high infection frequencies (mean = 91\% infection rate). Three major lineages of Wolbachia were identified as separate strains that appear to represent three separate invasions of Lycaeides butterflies. Overall, we found extensive evidence for acquisition of Wolbachia through interspecific transfer between host lineages. Strain wLycC was confined to a single butterfly taxon, hybrid lineages derived from it, and closely adjacent populations in other taxa. While the other two strains were detected throughout the rest of the continent, strain wLycB almost always co-occurred with wLycA. Our demographic modeling suggests wLycB is a recent invasion. These results demonstrate the utility of using resequencing data from hosts to quantify Wolbachia genetic variation and provide evidence of multiple colonizations of novel hosts through hybridization between butterfly lineages and complex dynamics between Wolbachia strains.