Genetic manipulation of crops for insect resistance

Crop protection plays a vital and integral role in modern-day agricultural production. The ever increasing demands on yield and the intensification of farming practice, particularly in the Developed world, has increased the problem of pest damage and hence control. It has long been recognized that extensive cultivation of certain crops, to the exclusion of other plants, i.e. monoculture, may favour drastic increases in the populations of insects which feed upon these crops; the number of herbivores exploiting a given host species increasing with the area occupied by that host (Strong, 1979). Not only does this situation occur in the field, but also during storage of food crops, where accumulation of large quantities of seed means that any insect which can utilize them as a food source is almost certain to undergo a population explosion, thus resulting in significant damage and loss. In West Africa up to 100% of the stored cowpea crop may be damaged within 5 months due to attack by the bruchid beetle Callosobruchus maculatus. Although the build-up of pests to a specific host plant is generally reduced in diverse plant communities (Cromartie, 1981), agricultural practices such as combining crops where each acts as host for a particular pest (e.g. cotton and maize, both of which are attacked by the corn earworm Heliothis zea), can have the opposite effect, thereby exacerbating the situation (Pimental et al., 1977).