EFFECTS OF HOST PLANT AND DEVELOPMENTAL STAGE ON CATERPILLAR IMMUNE RESPONSE

Herbivorous insects are exposed to many different natural enemies throughout their development; however, they can defend themselves using a variety of strategies. Once an insect is attacked, it can utilize chemical defenses that they sequester from their diet to fight off predators or their immune response will target smaller natural enemies, like parasites, parasitoids, and pathogens. Despite many studies showing that chemical defenses are effective, they can also be costly in terms of resources and energy, which can affect immune function. Junonia coenia (Nymphalidae), the buckeye, feeds on five different families that contain iridoid glycosides, making it a specialist and has been observed feeding on an introduced exotic weed, Plantago lanceolata (Plantaginaceae). It also feeds on a native host plant, Mimulus guttatus (Phrymaceae), which does not contain iridoid glycosides. In this study, we performed two immune assays, encapsulation and a hemocyte count, on buckeyes feeding on both the introduced and native host plant over 3rd, 4th, and 5th instar. Encapsulation consists of using small nylon monofilaments to challenge immune function, while a hemocyte count is a total estimate of the hemocytes present to participate in the immune response. The total number of hemocytes increased with instar and was higher in caterpillars fed on P. lanceolata; however, melanization increased with instar, but was not significantly affected by host plant. Gas chromatography was used to quantify iridoid glycosides in P. lanceolata, the immune challenged caterpillars, and a set of intact caterpillars to compare amounts of iridoid glycosides. The immune challenged caterpillars contained higher percent dry weight iridoid glycosides than the intact caterpillars. These results suggest that the incorporation of an introduced exotic weed into the diet of the buckeye larvae, in addition to developmental stage, can have important consequences for the immune function, thus affecting the larvae’s protection against natural enemies