Picking sides: feeding on the abaxial leaf surface is costly for caterpillars

The study provides us with the evidence that caterpillars tend to feed on the abaxial leaf surface despite the damage caused to them because of higher trichome density. To defend against herbivory, plants have evolved physical and chemical defense mechanisms, including trichomes (hair like appendages on leaves and stem) being one of them. Caterpillars, a major group of insect herbivores are generally found to occupy the abaxial (underside) leaf surface, considered as an avoidance mechanism from biotic and abiotic stresses. Since trichomes are a first line of defense, we examined the correlation between abaxial vs adaxial (above side) trichomes and caterpillar feeding, behavior, and growth. A combination of field, lab and microscopy experiments were performed using tobacco hornworm, Manduca sexta (Lepidoptera: Sphingidae), a Solanaceae specialist caterpillar, and multiple host species. We found that M. sexta caterpillars overwhelmingly preferred to stay and feed on the abaxial leaf surface, but the abaxial leaf surface also had significantly more trichomes, and consequently, caterpillars took significantly longer to commence feeding. In addition, lab-based diet experiment containing shaved trichomes showed that feeding on the abaxial leaf surface with more trichomes also affected caterpillar growth. Taken together, our study shows that although caterpillars prefer to feed on the abaxial leaf surface, they accrue feeding delays and developmental constraints, indicating tradeoffs affecting performance, and exposure to predation and abiotic stressors.