Parental diet drives offspring performance: Specialized parental effects in a generalist insect herbivore

Adaptive transgenerational plasticity is predicted to evolve when parental environments are predictive of offspring environments, and when reliable cues are available to induce adaptive offspring phenotypes. In plant-feeding insects, parental diet may have pronounced effects on offspring performance, as specific host plants may provide induction cues as well as affect the nutritional and chemical environment in the egg. In this study, we investigate the effects of parental diet on offspring performance using the broadly polyphagous grazing caterpillar, Grammia incorrupta. We fed the parental caterpillar generation either a plant that contained pyrrolizidine alkaloids (PAs), a plant that contained iridoid glycosides (IGs), a mixture of the two plants, or nutritious synthetic diet lacking defensive chemicals. We then split broods from parents reared on each diet among similar experimental diets and weighted them on the sixth day of larval development. The rate of growth and development during early larval stadia was dependent on parental diet, and its interaction with offspring diet. Our results suggest that offspring performance depends partially on the quality of parental food. However, the quality of parental diets only predicted offspring performance to the extent that it predicted offspring size at hatching, and this was true to a greater degree for diets that contained PAs. In contrast, parents attained larger body sizes on the IG plant than on the PA plant, but had significantly smaller offspring. Our results further illustrate that PA plants, to which G. incorrupta is physiologically adapted, and which have been shown to provide them defense against natural enemies, also enhance offspring performance when found in parental diets.