Host Plant Selection by Insects

Host plant selection by insects is often divided into „host plant finding‟ and „host plant acceptance.‟ While the two are easy to separate conceptually, in practice, they are really part of a continuum of three, rather than two, inextricably bonded links. However, the central link of host plant finding, thought previously to be governed by volatile chemicals, has, until now, proved intractable to scientific experimentation. Thus, the focus here is on host plant selection by insects associated with cruciferous plants as, since the classical work of Verschaffelt in 1910, most theoretical studies on herbaceous plants have used the interaction between insects and cruciferous plants as their test system. Such a selection is logical, as cruciferous (Cruciferae) vegetable and oilseed crops are of high economic importance and are now cultivated on large farms in most parts of the world. In addition, cruciferous plants are ideal for biological studies, as their chemistry is well understood and they support pest species from a wide range of insect orders. Many researchers have shown that the numbers of pest insects found on cruciferous crop plants are reduced considerably when the background of the crop is allowed to become weedy, when the crop is intercropped with another plant species, or when the crop is undersown with a living mulch. Obviously, if placing non-host plants in the vicinity of host plants reduces the numbers of insects that actually find their host plants, then this could provide a clue as to how insects find their host plants. It has been suggested that when the background of crop plants growing in bare soil is made more diverse by allowing other non-host plant species to grow in the inter-row spaces, that the additional diversity “disrupts” insects from selecting otherwise acceptable host plants. Such disruption is considered to be mediated through the non-host plants providing 1) physical obstruction, 2) visual camouflage, 3) masking of host plant odors, 4) repellent chemicals, or through 5) the non-host plants altering the physiology of the host plants. Two other suggestions, named by Root (1973) as the „Resource Concentration Hypothesis‟ and the „Enemies Hypothesis,‟ have also been used to explain why fewer phytophagous insects are found on host plants growing in diverse backgrounds than on similar plants growing in bare soil. A discussion of the seven hypotheses put forward to date is presented here, followed by a description of a theory based on „appropriate/inappropriate‟ landings, which the authors believe is the key, or „missing link,‟ to host plant selection by phytophagous insects. Finally, the new theory will be used 1) to discuss the type of information required to make intercropping, undersowing and companion planting more successful, 2) to suggest how insect biotypes could develop, and 3) to describe why wild host plants are not decimated by pest insects.