"Some [prairie remnants] are shaped so that no portion is more than a few meters away from the agricultural edge. Such reserves may have little or no area free from the negative effects of edges with agriculture," say Mark McKone of Carleton College in Northfield, Minnesota, and his co-authors in the October issue of Conservation Biology.
Before European settlement, tallgrass prairie covered more than 167 million acres of central North America. Most of the prairie has been converted to corn and other crops, leaving only scattered remnants.
McKone and his colleagues have observed large numbers of a corn rootworm beetle (Diabrotica barberi) near corn edges in prairie remnants in southeastern Minnesota. The larval stage of this beetle is one of the most damaging corn pests and in August, the adults begin moving from corn fields into prairies to feed on pollen and other parts of native flowers. The beetles seem to favor sunflowers and other yellow composite flowers.
The researchers studied corn rootworm beetles in McKnight Prairie, which is in southeastern Minnesota. The 33-acre prairie is long and narrow (about 560 feet across), and is completely surrounded by agriculture. The northern edge was planted with corn during the study, and the southern edge borders a conifer plantation. The reseachers determined the number of beetles at various points across the width of the prairie, and compared sunflower damage at two study sites: near the corn edge and near the conifer edge. These sites were about 130 and 460 feet away from the corn edge, respectively.
McKone and his colleagues found that there were more than 10 times as many beetles at the corn edge than at the conifer edge. Although the number of beetles decreased dramatically with distance from the corn edge, no part of the prairie was far enough away from the corn field to be completely beetle-free.
The researchers also found that in late August, the study site near the corn edge had about three times as many sunflowers with heavy damage (more than 75% of the petals were missing or shriveled). Petal damage can make flowers less attractive to pollinators: previous research has shown that when all the petals are removed from sunflowers, bee visitation decreases by 90%.
Preliminary evidence suggests that the beetles could reduce seed set in sunflowers: seed set was lower in sunflower heads bagged with beetles. The researchers suggest that the beetles might reduce seed set by feeding on pollen.
Because this corn rootworm beetle feeds on many types of flowers, it could also damage other prairie plants that bloom in late summer. Moreover, midwest farmers often rotate corn annually with soybeans, and soybean pests might also invade prairies to feed on native legumes such as prairie clover and leadplant.
To keep agricultural pests from threatening tallgrass prairie remnants, McKone and his colleagues recommend buffering the remnants from agricultural edges. "Areas set aside for nature reserves cannot be considered in isolation, since they are influenced by land use outside reserve boundaries," says McKone.
McKone's co-authors are: Kendra McLauchlan of the University of Minnesota in St. Paul, Edward LeBrun of the University of Utah in Salt Lake City, and Andrew McCall of the University of California at Davis; all three did this work while at Carleton College in Northfield, Minnesota.
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