Epsilon Archive for Student Projects

Abstract

The selection of a host plant for egg-laying is an important element in insect-plant relationships, the survival of the offspring often depends on the choices made by gravid females. Plants are diverse and not evenly distributed in space or time and insect females search for host plants at gradually finer spatial scales. To choose a host plant they use a combination of olfaction, vision, contact chemoreception and tactile senses and the resolution of those senses are important. Cruciferous crops are of economic importance and have a well known chemistry. Consequently the insects associated with them have received particular attention over the years. One such insect is the small white butterfly, Pieris rapae, a well known and worldwide pest of cruciferous crops. The objective of this MSc thesis was to gain a greater insight in the short range host selection by females of the small white butterfly and their use of senses at different spatial scales. This was done in the laboratory using three genotypes of Brassica oleracea, leaf models sprayed with extracts from those genotypes, and leaf models with different shape and colour. Three experiments were performed: one with plants, one with models differing in form sprayed with the same extract and one with identical models sprayed with different extracts. The genotypes or different models were distributed either evenly between three patches or concentrated in a single patch keeping the density constant. I found that the distribution of eggs on plants was not significantly affected by how the host plants were arranged but the egg distribution on the leaf models was affected. With the models differing in form the contrast increased with proximity and with the models differing in chemistry the contrast decreased with proximity. It is clear from this study that females of the small white butterfly are able to distinguish between different host plants based on both visual appearance and chemistry, but that their relative importance varies with scale. Chemistry has more impact on a slightly larger scale while the visual aspects of the leaves have the most impact at a very fine scale when the contrasting leaves can be perceived simultaneously.