Seven hypotheses, including the 'Resource Concentration Hypothesis' and the 'Enemies Hypothesis', have been put forward to explain why fewer specialist insects are found on host plants growing in diverse backgrounds than on similar plants growing in bare soil. All seven hypotheses are discussed and discounted, primarily because no one has used any of them to produce a general theory of host plant selection, they still remain as hypotheses. However, we have developed a general theory based on detailed observations of insect behaviour. Our theory is based on the fact that during host plant finding the searching insects land indiscriminately on green objects such as the leaves of host plants (appropriate landings) and non-host plants (inappropriate landings), but avoid landing on brown surfaces, such as soil. The complete 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-volatile plant chemicals. The previously 'missing' central link, which is based on what we have described as 'appropriate/inappropriate landings', is governed by visual stimuli. Our theory explains why attempts to show that olfaction is the crucial component in the central link of host plant selection proved intractable. The 'appropriate/inappropriate landings' theory is discussed 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 to suggest how insect biotypes could develop and to describe why pest insects do not decimate wild host plants growing in 'natural' situations.

Companion plants grown as 'trap crops' or 'intercrops' can be used to reduce insect infestations in field crops. The ways in which such reductions are achieved are being described currently using either a chemical approach, based on the 'push-pull strategy', or a biological approach, based on the 'appropriate ⁄ inappropriate landing theory'. The chemical approach suggests that insect numbers are reduced by chemicals from the intercrop 'repelling' insects from the main crop, and by chemicals from the trap-crop 'attracting' insects away from the main crop. This approach is based on the assumptions that (1) plants release detectable amounts of volatile chemicals, and (2) insects 'respond' while still some distance away from the emitting plant. We discuss whether the above assumptions can be justified using the 'appropriate ⁄ inappropriate landing theory'. Our tenet is that specialist insects respond only to the volatile chemicals released by their host plants and that these are released in such small quantities that, even with a heightened response to such chemicals, specialist insects can only detect them when a few metres from the emitting plant. We can find no robust evidence in the literature that plant chemicals 'attract' insects from more than 5 m and believe that 'trap crops' function simply as 'interception barriers'. We can also find no evidence that insects are 'repelled' from landing on non-host plants. Instead, we believe that 'intercrops' disrupt host-plant finding by providing insects with a choice of host (appropriate) and non-host (inappropriate) plant leaves on which to land, as our research has shown that, for intercropping to be effective, insects must land on the non-host plants. Work is needed to determine whether non-host plants are repellent (chemical approach) or 'non-stimulating' (biological approach) to insects.

S U M M A R YSeveral factors influencing the efficiency of water-traps in capturing cabbage root flies were studied at Wellesbourne in 1971 and 1972. In both the laboratory and field, approximately twice as many flies were caught in fluorescent as in non-fluorescent yellow traps. Depending upon trap density, addition of a source of the attractant allylisothiocyanate (ANCS) increased the numbers of females captured by approximately twofold in fluorescent traps and from two-to sevenfold in non-fluorescent traps. Traps were equally efficient irrespective of whether the ANCS was renewed every 2, 3, 4 or 5 days. On the first day of trapping, the number of flies caught per unit area was linearly related to the square root of the number of traps in that area. On the following days the rate was probably in equilibrium with the combined effect of immigration and the rate of development of responsive flies in the trapping zone. Most males were caught 30cm above the soil surface and most females at soil level. Traps 120 cm above the soil surface caught few flies.Populations of marked flies were released into large field cages containing both a section of hedgerow and a plot of cauliflowers. Even after a week, only 81 yo of the males and 55 yo of the females had been recaptured from the most responsive of these captive populations. Furthermore, only 30 % of females were recaptured when they were more than 8 days old, the age at which most probably enter the new host-crop.

The behaviour of female cabbage root flies during host plant selection was studied in the laboratory using brassica plants growing in backgrounds of bare soil, clover, grass, peas and four non‐living materials. Gravid females landed about twice as often on brassica plants growing in bare soil than on comparable plants growing amongst non‐host plants. Once a receptive female landed on a brassica plant, the female made, on average, four ‘spiral flights’ and two jumps on and off the plant before laying alongside the plant. Surrounding a brassica plant with a diverse background altered the behaviour of the flies, so that the spiral flights around the host plant were replaced by short hops between nearby vertical objects. The loss of contact and recontact with the host plant then prevented the females from accumulating sufficient contacts with the host plant to be stimulated to lay. Spiral flights around host plants appear to determine whether or not flies will lay alongside host plants. Flies in mixed plantings have a reduced rate of settling on the host plant, and a higher rate of locomotion, because they land frequently on non‐host plants. Hence, visual stimuli appeared to have greater effects than chemical or mechanical barriers in deterring flies from laying alongside brassica plants in diverse backgrounds. In ‘choice’ situations, backgrounds of real plants reduced oviposition alongside brassica plants by at least 50%. In ‘no‐choice’ situations, flies laid similar numbers of eggs alongside all brassica plants irrespective of plant background or plant size. If numbers of fly eggs are to be reduced on commercial brassica crops by undersowing the crops with clover, plants growing in bare soil may also have to be included to provide the flies with sites preferred for oviposition.

Using mimics of real plants, we found that female cabbage root flies, Delia radicum (L.), detected plants within host patches at least partly on the basis of leaf colour and area, with leaf pattern playing an insignificant role in the case of multi‐leaved plants. Composition of the background, whether bare soil or green grass, did not affect female ability to distinguish between leaf mimics of different colour. The contribution of visual stimuli in eliciting female landings on individual plants within a patch decreased as distance between the plants was increased. RÉSUMÉ Repérage visuel des plantes‐hǒtes par Delia radicum En utilisant les leurres de véritables plantes, nous nous sommes aperçus que les femelles de Delia radicum L. détectaient des plantes dans un lot tout au moins en partie d'après la couleur et la taille des feuilles, tandis que la forme des feuilles ne jouait qu'un rǒle insignifiant chez les plantes à nombreuses feuilles. La composition du fond, du sol nu ou de l'herbe verte, ne modifiait pas l'aptitude des femelles à distinguer entre les couleurs des leurres des feuilles. La contribution des stimuli visuels dans l'induction de l'atterrissage des femelles sur des plantes déterminées dans un lot diminuait en fonction de la distance entre les plantes.