Understanding variation in resource specialization is important for progress on issues that include coevolution, community assembly, ecosystem processes, and the latitudinal gradient of species richness. Herbivorous insects are useful models for studying resource specialization, and the interaction between plants and herbivorous insects is one of the most common and consequential ecological associations on the planet. However, uncertainty persists regarding fundamental features of herbivore diet breadth, including its relationship to latitude and plant species richness. Here, we use a global dataset to investigate host range for over 7,500 insect herbivore species covering a wide taxonomic breadth and interacting with more than 2,000 species of plants in 165 families. We ask whether relatively specialized and generalized herbivores represent a dichotomy rather than a continuum from few to many host families and species attacked and whether diet breadth changes with increasing plant species richness toward the tropics. Across geographic regions and taxonomic subsets of the data, we find that the distribution of diet breadth is fit well by a discrete, truncated Pareto power law characterized by the predominance of specialized herbivores and a long, thin tail of more generalized species. Both the taxonomic and phylogenetic distributions of diet breadth shift globally with latitude, consistent with a higher frequency of specialized insects in tropical regions. We also find that more diverse lineages of plants support assemblages of relatively more specialized herbivores and that the global distribution of plant diversity contributes to but does not fully explain the latitudinal gradient in insect herbivore specialization.

The genetic and ecological factors that shape the evolution of animal diets remain poorly understood. For herbivorous insects, the expectation has been that trade-offs exist, such that adaptation to one host plant reduces performance on other potential hosts. We investigated the genetic architecture of alternative host use by rearing individual Lycaeides melissa butterflies from two wild populations in a crossed design on two hosts (one native and one introduced) and analysing the genetic basis of differences in performance using genomic approaches. Survival during the experiment was highest when butterfly larvae were reared on their natal host plant, consistent with local adaptation. However, cross-host correlations in performance among families (within populations) were not different from zero. We found that L. melissa populations possess genetic variation for larval performance and variation in performance had a polygenic basis. We documented very few genetic variants with trade-offs that would inherently constrain diet breadth by preventing the optimization of performance across hosts. Instead, most genetic variants that affected performance on one host had little to no effect on the other host. In total, these results suggest that genetic trade-offs are not the primary cause of dietary specialization in L. melissa butterflies.

Summary Chemically mediated plant–herbivore interactions contribute to the diversity of terrestrial communities and the diversification of plants and insects. While our understanding of the processes affecting community structure and evolutionary diversification has grown, few studies have investigated how trait variation shapes genetic and species diversity simultaneously in a tropical ecosystem.We investigated secondary metabolite variation among subpopulations of a single plant species, Piper kelleyi (Piperaceae), using high‐performance liquid chromatography (HPLC), to understand associations between plant phytochemistry and host‐specialized caterpillars in the genus Eois (Geometridae: Larentiinae) and associated parasitoid wasps and flies. In addition, we used a genotyping‐by‐sequencing approach to examine the genetic structure of one abundant caterpillar species, Eois encina, in relation to host phytochemical variation.We found substantive concentration differences among three major secondary metabolites, and these differences in chemistry predicted caterpillar and parasitoid community structure among host plant populations. Furthermore, E. encina populations located at high elevations were genetically different from other populations. They fed on plants containing high concentrations of prenylated benzoic acid.Thus, phytochemistry potentially shapes caterpillar and wasp community composition and geographic variation in species interactions, both of which can contribute to diversification of plants and insects.

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Color mimicry is often celebrated as one of the most straightforward examples of evolution by natural selection, as striking morphological similarity between species evolves in response to a shared predation pressure. Recently, a large North American mimetic complex was described that included 65 species of Dasymutilla velvet ants (Hymenoptera: Mutillidae). Beyond those 65 species, little is known about how many species participate in this unique Müllerian complex, though several other arthropods are thought to be involved as Müllerian mimics (spider wasps) and Batesian mimics (beetles, antlions, and spiders; see references in). Müllerian mimicry is similarity in appearance or phenotype among harmful species, while Batesian mimicry is similarity in which not all species are harmful. Here, we investigate the extent of the velvet ant mimicry complex beyond Dasymutilla by examining distributional and color pattern similarities in all of the 21 North American diurnal velvet ant genera, including 302 of the 361 named species (nearly 84%), as well as 16 polymorphic color forms and an additional 33 undescribed species. Of the 351 species and color forms that were analyzed (including undescribed species), 336 exhibit some morphological similarities and we hypothesize that they form eight distinct mimicry rings (Figure 1A; Supplemental Information). Two of these eight mimicry rings, red-headed Timulla and black-headed Timulla, were not documented in earlier assessments of mimicry in velvet ants, and are newly described here. These findings identify one of the largest known Müllerian mimicry systems worldwide and provide a novel system to test hypotheses about aposematism and mimicry, especially those regarding the evolution of imperfect mimicry.