Assistant Professor University of Texas Rio Grande Valley Edinburg, Texas
Plants have evolved an array of physical and chemical defense mechanisms against herbivores with trichomes (epidermal hairs on leaves and stem) being one of them. Caterpillars, a major group of insect herbivores are generally found to stay more on abaxial (underside) leaf surface, which is considered as an avoidance mechanism against biotic and abiotic stresses. Since trichomes act as the first line of defense that caterpillars must encounter before feeding, we examined the correlation between abaxial vs adaxial (above side) trichome density and caterpillar feeding behaviour, and growth. A combination of field, lab and microscopy experiments were performed using a Solanaceae specialist caterpillar, tobacco hornworm, Manduca sexta (Lepidoptera: Sphingidae), and multiple host species. We found that although abaxial leaf surface had significantly higher trichomes, M. sexta caterpillars overwhelmingly preferred to stay on abaxial leaf surface. Consequently, caterpillars took significantly longer to commence feeding on abaxial leaf surface. In addition to this, manipulative lab-based diet experiments showed that staying on abaxial leaf surface with higher trichomes also affected caterpillar growth. Taken together, our study shows that whilst caterpillars prefer to feed on abaxial leaf surface, they accrue feeding delays and developmental constraints, and thereby potentially have a reduced impact on their host plants.
