We are what we eat: Host plant effect on saliva composition in caterpillars

Generalist herbivores feed on a wide array of host plants and are capable of defending against plant defenses with the use of general mechanisms such as suppression of induced defenses. The saliva of insects has been related to potential preingestive digestion, secretion of digestive enzymes and recently as a method of counterdefense against induced defenses in plants. In this study we hypothesize that the composition of caterpillar saliva of a generalist insect varies according to host plant and that these differences in composition play a role in allowing the insect to evade plant defenses. In order to test this hypothesis, fifth instar cabbage looper (Trichoplusia ni) larvae were allowed to feed on cabbage (good quality), tomato (poor quality) and artificial diet. After 24 hours of feeding, larvae were transferred to intact tomato plants. Plant samples were collected 48 hours post treatment and assayed for polyphenol oxidase (a defensive compound) activity. Also, fifth instar larvae were transferred from a pinto bean diet to a host plant and allowed to feed for 2, 5, 8 and 24 hours before dissecting their salivary glands. Glucose oxidase (GOX) activity was quantified and protein collected was visualized in silver stained protein gels. Results indicated that the saliva of cabbage looper caterpillars suppressed plant defenses only when feeding on a host plant but not on artificial diet. This study provides the information that can help increase our knowledge in these dynamics and potentially establish applied research for the control of polyphagous insects.