EIGENBRODE, S. D.: Attachment by Predatory Insects to Waxy Plant Surfaces: Mechanisms and Ecological Implications

A manifestation of biological adhesion is attachment by phytophagous insects and their natural enemies to plant surfaces. Plant surfaces are covered with epicuticular waxes (EW), comprised of mixtures of long chain hydrocarbons and related compounds. EW morphology ranges from amorphous films to dense vestures of microscopic crystals. Attachment to EW by predacious insects influences their behavior and effectiveness, which influences the population densities of their phytophagous prey in the field. To examine EW effects on insect predator attachment we measured attachment by the coccinellid beetle, Hippodamia convergens, to natural plant surfaces, extracted EW, and pure compounds presented as amorphous films or beds of crystals. Crystallization of waxes reduces attachment, apparently because the wax crystals are dislodged from the plant surface. Presumably, H. convergens employs wet adhesion, as evidenced by the liquid �footprints� produced by this species and other coccinellids. However, wet adhesion produced by the adhesive organ of another predator Chrysoperla plorabunda, overcomes the problem of friable crystalline waxes and even allows greater attachment forces to such surfaces than to amorphous ones. Attachment by H. convergens to amorphous surfaces formed of natural EW extracted from genotypes of Brassica oleracea differs with composition, tending to decrease as the proportion of n-alkan-1-ols increases. In tests with amorphous surfaces of pure n-alkan-1-ols and n-alkanoic acids, H. convergens attachment decreases with the chain length of the compound tested. Thus, composition and structure of EW influence insect attachment, with implications for the evolution of plant defense and habitat specialization by insect herbivores and carnivores.