Caterpillars lack a resident gut microbiome.

Many animals are inhabited by microbial symbionts that influence their hosts' development, physiology, ecological interactions, and evolutionary diversification. However, firm evidence for the existence and functional importance of resident microbiomes in larval Lepidoptera (caterpillars) is lacking, despite the fact that these insects are enormously diverse, major agricultural pests, and dominant herbivores in many ecosystems. Using 16S rRNA gene sequencing and quantitative PCR, we characterized the gut microbiomes of wild leaf-feeding caterpillars in the United States and Costa Rica, representing 124 species from 15 families. Compared with other insects and vertebrates assayed using the same methods, the microbes that we detected in caterpillar guts were unusually low-density and variable among individuals. Furthermore, the abundance and composition of leaf-associated microbes were reflected in the feces of caterpillars consuming the same plants. Thus, microbes ingested with food are present (although possibly dead or dormant) in the caterpillar gut, but host-specific, resident symbionts are largely absent. To test whether transient microbes might still contribute to feeding and development, we conducted an experiment on field-collected caterpillars of the model species Manduca sexta Antibiotic suppression of gut bacterial activity did not significantly affect caterpillar weight gain, development, or survival. The high pH, simple gut structure, and fast transit times that typify caterpillar digestive physiology may prevent microbial colonization. Moreover, host-encoded digestive and detoxification mechanisms likely render microbes unnecessary for caterpillar herbivory. Caterpillars illustrate the potential ecological and evolutionary benefits of independence from symbionts, a lifestyle that may be widespread among animals.

Specialist Herbivore Performance on Introduced Plants During Native Host Decline.

Ash (Fraxinus spp.) is in rapid decline across the northeastern USA due to the invasive emerald ash borer (Agrilus planipennis Fairmaire). Three recently co-occurring confamilial species may serve as alternative larval host plants for ash-reliant Lepidoptera. These prospective hosts are nonnative shrubs often planted in managed suburban landscapes and are sometimes invasive or naturalized in North America. Given the imminent decline of ash trees, we considered potential downstream effects on insect herbivores historically specialized on ash foliage. We measured the performance of three ash-specialist hawkmoths (Lepidoptera: Sphingidae) on native white ash (Fraxinus americana L.) and alternative host plants: common lilac (Syringa vulgaris L.), weeping forsythia [Forsythia suspensa (Thunb.) Vahl], and European privet (Ligustrum vulgare L.). We found the nonnative host plants provided varied support for larval survival to pupation, with biomass and growth rate affected differently by both plant and insect identity. Nearly all caterpillars reared on one alternative host, European privet, exhibited distinct malformations of the wing buds at pupation. Given caterpillar presence on privet in the field, privet may constitute an ecological trap (i.e., when female moths select a sub-optimal host, offspring survival and fitness are reduced). This work demonstrates how performance testing can reveal species-specific effects of host plant loss on mono- or oligophagous insects. For some ash specialists, alternative nonnative host plants may be suboptimal, but some cultivated host plants may be able to support certain specialist insects during native host decline. We suggest that landscaping decisions can be tailored to support threatened insect species.