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Agricultural lands upset predatory insects.

In many parts of the world, farms have split up vast meadows and other natural ecosystems. This fragmentation appears to hurt the predators of plant-eating insects more than the herbivores themselves, a new study finds. That's good news for plant eaters but bad news for farmers.

Herbivorous insects seem less finicky about where they settle than their predators, or parasitoids, reveals the study in the June 10 SCIENCE. "Habitat isolation affects species diversity, having the greatest impact on parasitoids, thereby releasing pest insects from parasitism," report German researchers Andreas Kruess of University Fridericiana in Karlsruhe and Teja Tscharntke of Georg August University in Gottingen.

The new report "says what people have been predicting for a long time," states Deborah Letourneau of the University of California, Santa Cruz.

Kruess and Tscharntke planted patches of clover measuring 1.2 square meters either in meadows or in areas 100 to 500 meters away from a meadow. In the meadow patches, researchers identified 8 species of herbivores and 13 species of parasitoids in one growing season. All but one predator species ate Apions, the most abundant herbivorous pest. Spintherus dubius, a wasp, was the most common parasitoid, accounting for almost 80 percent of the specimens captured.

The investigators then compared the plots in meadows with the clover islands. The patches farthest from a meadow had the fewest Apions and the least species of parasitoids -- only two to four. They also had the lowest ratio of predators to herbivores. Plant-eating bugs in the clover islands located nearer a meadow suffered a much higher death rate from enemy insect attacks, Kruess and Tscharntke report.

In a separate, ongoing study, the researchers are finding that meadow size influences the relative proportion of insect predators and the vetch-eating herbivores on which they munch. In meadows 800,000 square meters in size, this ratio was more than double the ratio in meadows measuring just 300 square meters. Plant density did not account for the difference.

The data from this new study "really help substantiate what they're finding [in the clover-patch experiments]," says Letourneau. Meter-size plots can provide misleading results because not all insects come equally equipped to locate such small feeding grounds, she says.

Kruess and his colleagues are looking into why parasitoids appear to be so sensitive to fragmentation of natural ecosystems. Either fewer predators than herbivores try to colonize the isolated patches, or as many try but have less success than their prey, Kruess believes. Parasitoids may migrate from large meadows less often than herbivores do, he adds. Also, because their numbers start out smaller and fluctuate with the availability of herbivores, predator populations are more fragile.

Farmers could give predatory insects a boost by setting aside whole fields or strips surrounding fields, Kruess suggests.
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Title Annotation:fragmentation disrupts predators of plant-eating insects
Author:Adler, Tina
Publication:Science News
Article Type:Brief Article
Date:Jun 11, 1994
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