How do flies and butterflies recognize brassicaceous host-plants?

We live in a green world where the organisms that dominate our view are plants. Virtually everywhere there is a wild or cultivated plant there will be insects. Hence, insect-plant relationships and how insects recognize their host plants are fundamental to agricultural science. Most insects are specialist feeders, and so can survive only by finding specific secondary plant substances, such as the glucosinolates that characterize brassicaceous plants. Before laying eggs on such plants the gustatory (taste) receptors of the adapted insect have to come into direct contact with the brassicaceous chemicals. For beetles (Coleoptera) and bugs (Hemiptera) this is not a problem, as both larval and adult stages have mouthparts capable of chewing (mandibles) or piercing into (stylets) plant tissues. Problems arise with the adult Diptera and Lepidoptera, however, as their mouthparts, described in common terms as a sponge (labellum of Diptera) or drinking straw (proboscis of Lepidoptera), have evolved for imbibing liquids and appear unsuitable for puncturing plant surfaces.

This is not considered a problem by most authors (see Finch & Collier, 2011) as there is a general belief, supported by hundreds of papers and several text books (see Schoonhoven, van Loon & Dicke, 2005) that the chemicals that characterize the host-plants of insects are freely-available in the waxes found on plant leaves. This conclusion was arrived at by dipping intact plant leaves into alcoholic solutions and assuming that the chemicals extracted were in the wax. In 2005, however, Reifenfrath et al. showed that wax scraped mechanically from plant leaves did not contain plant chemicals. The conundrum, therefore, is “If host-plant chemicals are not present on the leaf surface, how do flies and butterflies manage to damage plant tissues to sample plant chemicals?”

The project will look in considerable detail at the behaviour of the insects and in microscopic detail at the interface between the insect and the plant. With their highly sensitive gustatory receptors the amount of damage done to the leaf surface by the insect may be slight, but for our tenet to hold the insect must make direct contact with the chemicals to check if the plant is a suitable host for its offspring. The insects used in the project will be the cabbage aphid (Brevicoryne brassicae) (Homoptera), the cabbage root fly (Delia radicum) and the small white butterfly (Pieris rapae), as cultures are available at Wellesbourne and both supervisors have considerable expertise on all three species.

Key experimental skills involved:
Host-plant recognition, insect behaviour, insect morphology, insect sense organs, electrophysiology, plant cytology, glucosinolate chemistry, gas-liquid chromatography, cine-photography, electron microscopy, high-power light microscopy, experimental design, experimental analysis, writing scientific papers, and hopefully helping to influence future text-books.