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Does attracting natural enemies to an orchard by planting a cover crop translate to better biological control of pests in the trees? Dr. David Horton, entomologist with the U.S. Department of Agriculture in Yakima, Washington, hopes to find out.

Horton is conducting trials in a research orchard and three commercial organic pear orchards, where he has planted an alfalfa cover crop. He is studying the movement of beneficial insects and their impact on the key pest, pear psylla.

When the alfalfa is planted, there tends to be a big build-up of predators, such as damsel bugs, lady beetles, green lacewings, and big-eyed bugs, he reported, but it’s not clear to what extent the insects trickle up into the trees and ultimately control pests.

“We not only have to get them to move up into the trees—they have to switch habitat and diet,” he said.

Horton is collaborating with Dr. Vince Jones, entomologist with Washington State University in Wenatchee, who has devised a way to mark the cover crop with an egg-white protein applied through a weed sprayer. The protein attaches easily to insects that are in the cover crop. Insects can be brought back to the lab, where the protein can be detected on them with an ELISA test.

“We can tell by collecting insects on the tree whether they’ve been in the cover crop or not,” Horton explained. “We can tell whether we’ve had habitat switching.”

Horton also needs to know if beneficial insects that have moved up into the trees are attacking the target pest. Dr. Tom Unruh, geneticist with the USDA in Yakima, Washington, has developed an ELISA test that can show if an insect has consumed pear psylla. The insects are ground up to find out if they have psylla proteins in their guts.

This is the third year of the project at the experimental site, and the first year in the commercial orchards. Horton said that though he’s finding large populations of insects in the cover crop, they seem reluctant to move up into the trees. Next year, he will explore if it’s possible to force them up into the trees by timed mowing of the cover crop. However, Horton said his biggest challenge in conducting trials in commercial orchards is that mowing is extremely disruptive of the natural enemies, with many of them getting ground up by the mower. “It’s a huge perturbation,” he said.

The project is funded in part by the WSU’s Center for Sustaining Agriculture and Natural Resources, the Fresh Pear Committee, and Western Sustainable Agriculture Research and Education.