Sensory cues in host selection for oviposition by the cabbage butterfly, Pieris rapae

The behavioural events leading up to oviposition by Pieris rapae and the sensory modalities involved in host selection by these butterflies are reviewed. The process of host finding and acceptance is divided into three phases: searching, landing and contact evaluation. Little is known about sensory aspect of searching, but visual stimuli play some role. Landing appears to be mediated primarily by visual cues, of which colour is the most important. Choice experiments using artificial leaves of identical spectral and chemical properties revealed little or no preference for shape or size of oviposition substrate. Contact evaluation depends on positive or negative chemical stimuli at the leaf surface. Oviposition stimulant was extracted from cabbage leaves and partially isolated by solvent extraction and a series of chromatographic separations. Chemical constituents other than glucosinolates appear to be responsible for the discriminatory behaviour of ovipositing butterflies. These stimulatory compounds are water-soluble but many are less polar than the glucosinolates, and may be active alone or in combinations. Identification of these compounds would greatly improve our understanding of the mechanism of host acceptance by cabbage butterflies.