Can Targeted Genetic Manipulation Assist Biocontrol?
Crop plants and insect pests are part of a complex agroecosystem that involves interactions between many trophic levels often referred to as multitrophic interactions. Consequently, modern pest management needs to be viewed from a holistic viewpoint. Because of a lack of harmonization between plant breeders and biocontrol specialists, breeders have continued to strive for total resistance to pests, while biocontrol specialists have often ignored the role of the plant in enhancing successful natural enemy foraging behavior. Although some have highlighted the need to consider both the bottom-up (plant defense) and top-down (biological control) management of insect pests, there have been too few serious attempts at combining these approaches. As we understand more about the function of direct defenses (e.g. allelochemicals, trichomes) and indirect defenses (e.g. recruitment of natural enemies), the potential now exists for breeding plants that can combine both strategies.
The principal advantage of using plants to manage insects is quite clearly the potential for reduced broad-spectrum insecticide inputs. The introduction of B.t. varieties has already dramatically reduced the amount of chemical pesticides applied to cotton. In 1998, U.S.A. cotton farmers used 450 fewer tons of pesticides on B.t. cotton than they would have used on conventional varieties. Reduction in the use of insecticides has immediate benefits for biocontrol by allowing greater numbers of parasitoids and predators to survive.
However, it is probable that conventional breeding of many of our crop varieties in the past has bred out traits that could be beneficial to natural enemies, whether indirectly or directly. An example of this is manipulation of host-plant chemistry so that it could be possible to chemically attract predators and parasitoids to their prey and hosts. Cotton is an often cited example of this phenomenon in which attack by herbivorous insects induces plant signals for their corresponding parasitoids. Naturalized (wild) cotton plants produce more signal volatiles than commercial varieties, but traditionally breeding wild cotton with modern cultivars has proved problematic. In the future, it may be possible to genetically engineer this pathway into modern cotton cultivars.
Adult parasitoids and some predators tend to rely on an alternative food source other than the larval diet, and this is usually in the form of floral nectar, extrafloral nectar, pollen, and honeydew. Genetic modification of crop plants could be utilized to increase the production of resources for natural enemies to enhance biological control. A classic example from cotton, in which the resource requirements of parasitoids were ignored, occurred when crop varieties were bred without extrafloral nectaries. This was done to prevent sooty mold formation on the developing bolls. This nectarless cotton had increased bollworm damage due to a lack of parasitoids in the field. The adult parasitoids had previously been utilizing the nectar as a food resource. (Information Systems for Biotechnology via Agnet, 12/2/04).
