Ecological Principles of Habitat Management for Weed and Insect Biological Control
Beneficial insects are important in reducing populations of insect and weed pests. Predators and parasites can limit the abundance and damage caused by pests. Insects that feed on weed seeds may play a role in managing soil seed banks. While such biocontrols often function well in less disturbed natural ecosystems, the intensity of annual cropping systems often limits their effectiveness in agricultural landscapes. By modifying production practices and managing habitats at field edges and in surrounding landscapes, farmers may provide critical resources for natural enemies. Understanding ecological principles behind habitat management to enhance effectiveness of beneficial insects is the focus of this project.
Ground beetles, also known as carabid beetles, are numerous and ubiquitous predators in agricultural landscapes. Most species feed on other insects, while others prefer to feed on weed seeds. In Michigan corn fields, ground beetles and other invertebrates can remove more than 40 weed seeds per square foot per day. We have also shown that agricultural practices focused on providing habitat for ground beetles can be effective in increasing their numbers and the number of weed seeds removed. We studied the interaction of refuge strips and cover crops in this regard. Refuge strips comprised of orchard grass, clovers and perennial flowering plants consistently harbored more ground beetles than a control area. A cover crop of red clover frost-seeded into oats also increased ground beetle abundance during some sample periods.
To explore practical applications of this knowledge, we studied ground beetle communities of on-farm filterstrips comprised of switchgrass or mixed legumes and grasses, versus the center of an adjacent soybean field. Switchgrass harbored the greatest beetle diversity (38 species) followed by legume-grass filterstrip (29) and crop field (25). Weed seed removal (giant foxtail) was also highest in the switchgrass (84 percent) followed by the legume-grass filterstrip (42 percent) and the crop (17 percent). In other studies we examined the role of landscape structure in affecting populations of beneficial wasps. Armyworms placed in corn fields in a complex ag landscape, comprised of a mixture of habitats: crop fields, woodlots, fencerows and hedgerows, had three times greater parasitism than a nearby landscape consisting of crop fields. Other studies show that the wasps were likely dependent on alternate hosts, common in the woods and hedgerows of the complex landscape.
The principles of habitat management to enhance beneficial insects which have emerged from these and other studies must be interpreted cautiously. However, it appears that the presence of stable, undisturbed habitats may enhance the numbers and effectiveness of several beneficial insects. Coupled with practices which reduce disturbance in crop fields, these practices may serve to increase biological pest control and reduce reliance on chemical pesticides. North Central Region SARE 1997 Annual Report.