Different Host Plants Affect Immune Response in a Generalist Caterpillar

Generalist herbivorous insects are widespread and often occur in a variety of environments. Across their geographic range, herbivorous insects can 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. Host plant quality can affect larval growth and survival in that larvae feeding on low-quality plants often suffer reduced fitness. However, different host plants are also subject to different levels of parasitism. High-quality plants confer stronger larvae performance (survival, number of offspring), but larvae may also face higher parasitism. In some herbivore systems, diet mediates larvae immune function. The generalist insect herbivore fall webworm (Hyphantria cunea, hereafter FW) is a moth found in Colorado, and its larvae have considerable variance in their performance when reared on different host plants. We investigated if the FW immune system is affected by larvae feeding on good and bad quality host plants. We measured immune function by melanization of a nylon filament. We found significant differences in immune response across host plants, meaning diet mediates immune function in fall webworm larvae. Our study helps elucidate the factors that cause variation in immune response in a generalist herbivore.