Insects use flight to evade predators, locate mates, and colonize new habitat; thus, improved flight capability has the potential to increase an adult insect’s survival, reproductive success, and geographic distribution. Plant tissues, consumed by larvae (caterpillars), will later provide nutrients the adult insects ultimately need to develop their flight muscles. Most studies investigating the influence of host plants on insect herbivores only look at effects on caterpillars. However, this approach overlooks nutritional effects on the adults and the important contributions the adults make to the size and distribution of the insect’s population. Here the authors examine how differences in the quality of horsenettle (Solanum carolinense) host plants affect flight muscle development and flight muscle function of one of its natural herbivores, tobacco hornworm moths (Manduca sexta).
The study used inbreeding as a mechanism to produce variation in host plant quality. Inbreeding in horsenettle is known to reduce the plant’s ability to defend itself against herbivores and pathogens. In both field and laboratory conditions, tobacco hornworm caterpillars prefer to feed on inbred plants compared to outbred plants, suggesting fitness advantages from eating weakly defended inbred plants as opposed to better defended outbred plants. The authors found caterpillars that ate inbred plants grew faster and developed into larger pupae (chrysalises) compared to caterpillars that ate outbred plants. Growth differences in the caterpillars also impacted the adult stage (moth) of the insect. In free-flight tests, moths that fed on inbred plants as caterpillars exhibited improved flight muscle metabolic function. Moreover, it was found molecular evidence showing higher muscle metabolic outputs correlated with changes to the amino acid composition of a key regulatory protein in their flight muscles.
The study shows that host plant inbreeding can create effects that cascade through larval and pupal development to affect flight muscle function of the adult stage. Hence, host plant inbreeding can influence important life history traits of insect herbivores, such as mating success, survival, and dispersal. The findings reveal that changes to the genetics of a population at one trophic level can affect the development and physiology of an animal at a higher trophic level.