Influence of Bt cotton on beneficial arthropod populations

In all areas of the world where cotton Is produced, major Insect pest problems exist. Prior to 1940, insect control in cotton was accomplished primarily through crop management practices and natural biological control. Synthetic insecticides changed the nature of crop protection and the use of insecticides became the standard for insect control. For each 10 years of insecticide use, one class of insecticides has been lost to the development of resistant pest populations (Phillips 1995). Insect resistance to all classes of insecticides represents a major obstacle to cotton producers.

Transgenic Bt cotton was developed in response to the inability of insecticides to control insecticide-resistant tobacco budworms (Heliothis virescens F.) and bollworm (Helicoverpa zea Boddie), collectively called the heliothine complex (Elzen et al. 1994; Graves et al. 1994; Plapp et al. 1994; Wall 1994). Bt cotton contains a bacterial gene that directs cotton plant cells to synthesize an insecticidal protein which is toxic to lepidopteran larvae.

Bt cotton should result in reduction of insecticide sprays to control heliothine complex, and its use will provide the opportunity for improved natural biological control of cotton pests. The effect of Bt cotton on predatory arthropods is not known. In the absence of heliothine prey, beneficial arthropods may migrate to other locations in search of food. In addition, insect pests that were inadvertently controlled by insecticide applications to control heliothines may increase in numbers sufficient to become key pests. A challenge that accompanies this technology Is how to suppress insect pests not controlled by Bt cotton (Benedict 1996). To effectively use Bt cotton in an IPM program, the impact of these introduced plants on the beneficial arthropods important for control of key and secondary pests must be determined.

To determine the influence of Bt cotton on beneficial arthropod populations, studies were conducted in 1996 on five large-field production sites and one research plot; and in 1997, one large-field production site and two research plots. Heliothine pressure and predatory arthropod numbers and species composition, and secondary pest populations were monitored in both years. Differences in heliothine pressure occurred between fields and sites. Beneficial arthropod population densities rarely differed between Bt and conventional cotton in 1996 and 1997. There were no differences in the species composition found in Bt and conventional cotton in either year. Secondary pest populations of aphids, plant bugs, and stink bugs rarely differed between the two cotton systems in either year.

In areas where the boll weevil has been eradicated, Bt cotton may play an important role in IPM programs. The reduction in insecticide applications afforded by the use of Bt cotton will benefit beneficial arthropod populations. With the advent of new selective chemistries to control secondary pests such as aphids and plant bugs, beneficial arthropods may become more important in cotton pest management.