Turn on the heat: The impact of temperature on the performance and insect defense of a specialist and a generalist lepidopteran

Temperature can be a major determinant of insect performance and their ecological success, impacting a range of biological processes, including growth, immune function, and chemical sequestration. For herbivorous insects, changes in thermal environments will also affect their interactions with their host plants. Herbivorous insects not only acquire their nutrition from plants, but in some cases also acquire chemical defenses from their host plants. Those sequestered plant compounds may, in turn, affect insect interactions with other trophic levels, such as predators, parasitoids, and pathogens, via effects on larval performance, immunocompetence, and chemical defense levels. Changes in thermal environments, predicted from global change scenarios, will likely influence these multitrophic interactions. To investigate the consequences of thermal environment on larval performance, chemical defense and immunocompetence, we compared the growth, chemical sequestration and immune response of two nymphalid lepidopteran species: a generalist, Vanessa cardui, and a specialist, Junonia coenia, on a shared host plant Plantago lanceolata (Plantaginaceae). Both species were reared individually in one of three temperature regimes: 20C, 25C, and 30C. Specifically, we compared: 1) larval performance 2) immune response, and 3) chemical sequestration of two chemical compounds, the iridoid glycosides aucubin and catalpol, which are found in P. lanceolata. Results showed that 1) larval growth was significantly affected by temperature only in the painted lady, 2) there was no effect of temperature on immune response in either species, and 3) that the two species differed significantly in their ability to sequester iridoid glycosides, but there was no effect of temperature on sequestration.