Host-plant selection by insects - a theory based on 'appropriate/inappropriate landings' by pest insects of cruciferous plants

Seven hypotheses, including the 'Resource Concentration Hypothesis' and the 'Enemies Hypothesis', have been put forward to explain why fewer specialis t insects are found on host plants growing in diverse backgrounds than on s imilar plants growing in bare soil. All seven hypotheses are discussed and discounted, primarily because no one has used any of them to produce a gene ral theory of host plant selection, they still remain as hypotheses. Howeve r, we have developed a general theory based on detailed observations of ins ect behaviour. Our theory is based on the fact that during host plant findi ng the searching insects land indiscriminately on green objects such as the leaves of host plants (appropriate landings) and non-host plants (inapprop riate landings), but avoid landing on brown surfaces, such as soil. The com plete system of host plant selection involves a three-link chain of events in which the first link is governed by cues from volatile plant chemicals, the central link by visual stimuli, and the final link by cues from non-vol atile plant chemicals. The previously 'missing' central link, which is base d on what we have described as 'appropriate/inappropriate landings', is gov erned by visual stimuli. Our theory explains why attempts to show that olfa ction is the crucial component in the central link of host plant selection proved intractable. The 'appropriate/inappropriate landings' theory is disc ussed to indicate the type of work needed in future studies to improve our understanding of how intercropping, undersowing and companion planting can be used to optimum effect in crop protection. The new theory is used also t o suggest how insect biotypes could develop and to describe why pest insect s do not decimate wild host plants growing in 'natural' situations.