Chemical signals on leaf surfaces: Keys to recognition by ovipositing insects

Ovipositing insects generally make final host-selection decisions on the basis of compounds present on the leaf surface. Little is known about the chemistry of leaf surfaces and how variation in surface-chemical profiles is related to host selection by insects.; The goal of this dissertation was to examine the leaf-surface chemistry of a single plant species, Daucus carota, with particular reference to compounds already identified as oviposition stimulants for one of its specialist insect herbivores (Papilio polyxenes). The project included comparisons of the surface-chemical profiles of plants differing in acceptability as oviposition substrates; comparisons of larval performance on plant genotypes differing in oviposition acceptability; and comparisons of the surface- and tissue-chemical profiles of leaves differing in form, life history and genotype.; Plant surfaces were extracted with a combination of chloroform and near-boiling water, and the resulting extracts fractionated and analyzed using high performance liquid chromatography. My work confirmed the presence of trans-chlorogenic acid (CA) and luteolin-7-O-(6″-O-malonyl)-PD-glucoside (L7MG), polar oviposition stimulants for P. polyxenes, in leaf-surface extracts of D. carota.; P. polyxenes females discriminate among domesticated carrot plants, consistently preferring certain host genotypes, such as cultivars Netherlands and Sytan, over other genotypes like cultivars China 1 and USSR 1. This intraspecific host discrimination takes place both prior to and after leaf contact. Pre-alighting preferences of females were negatively correlated with concentrations of higher boiling-point volatiles. Volatile concentrations, in turn, were negatively correlated with plant nitrogen. Consequently, the pre-alighting preferences of P. polyxenes were positively correlated with plant nitrogen content. Post-alighting preferences of females were positively correlated with the combined concentrations of luteolin-7-glucoside and L7MG on the leaf surface. Flavonoid concentrations were positively correlated with plant water content.; Host-plant genotype significantly affected the time of development to the third instar for black swallowtail larvae. The post-alighting oviposition preferences of P. polyxenes for different carrot genotypes were correlated with the performance of larvae on those genotypes: females preferred to lay eggs on those genotypes that supported the fastest development of larvae.; This research revealed that chemical signals on leaf surfaces potentially serve as keys for recognition of quality host sites by ovipositing female butterflies.