Birds help reduce codling moth larvae in orchards

Issue Date: September 9, 2020
By Kathy Coatney
Cages surround bags of codling moth larvae affixed to walnut trees, above. The cages protected the larvae from birds, so researchers could compare predation of the protected larvae against larvae left vulnerable to birds.
Photo/Sacha Heath
A University of California study showed birds such as the white-breasted nuthatch photographed by a field camera to be effective predators of the larvae.
Photo/Sacha Heath
UC Davis researcher Sacha Heath checks bags of codling moth larvae affixed to a walnut tree for signs of predation. A UC study monitored how many larvae beneficial birds removed from an orchard.
Photo/Sara Kross

Codling moth in walnuts can cause significant yield and quality losses if left uncontrolled. During the wintertime, its larvae are more vulnerable to predators, offering opportunities for encouraging biocontrol by natural enemies, including insectivorous birds.

Rachael Long, University of California Cooperative Extension farm advisor in Yolo County, trapped bats in walnut orchards and found bats eat adult codling moths and essentially prevent egg laying.

Since that research, Long and her colleague Sacha Heath of UC Davis and the Living Earth Collaborative at Washington University in St. Louis started a project to discover the potential for bug-eating birds to help control codling moths in walnuts. The research was completed last year.

Long and Heath monitored bird activity in 20 different walnut orchards in the Sacramento Valley during the wintertime, with a focus on birds that search for insect prey in tree bark. The most abundant insectivorous birds with this feeding behavior included woodpeckers, flickers, bushtits, oak titmice and nuthatches.

To measure the impact of bird predation on codling moth control, the researchers obtained larvae from the U.S. Department of Agriculture lab in Parlier, where a colony is maintained for research purposes. The larvae were coaxed to spin cocoons in pieces of corrugated cardboard and to go dormant as the researchers manipulated light and temperature in growth chambers where the larvae were reared.

Long and Heath glued bags of the hibernating larvae onto tree trunks. Some larvae were available to predators; others had cages over them to prevent bird predation—serving as the control group—and field cameras were set up to identify avian predators.

About three months later, they checked for predation and found 46% of the sentinel codling moth larvae exposed to the birds were eaten, while only 11% of the caged larvae were eaten, showing that birds were major predators. Videos of nuthatches and Nuttall's woodpeckers suggested these two species were responsible for much of the pest control in walnut orchards. The 11% of predated larvae in cages were killed by insect predators and parasitoid wasps, the researchers said.

Reducing larvae in the winter months with natural enemies should help reduce the codling moth springtime flight and reduce pest pressure during the growing season, the researchers said—but Long acknowledges concern birds will damage the crop.

"In walnuts, there's just very minimal issues with birds, except maybe crows that could come in and cause some damage to developing walnuts. However, overall, birds are either going to be harmless or actually even beneficial, particularly in the wintertime," she said.

"For walnuts, and even almonds, almost all birds are beneficial because they feed on insects," Long continued, noting again the exception of crows.

Using birds in field crops—particularly seed crops—is a learning process that could mean watching birds to understand which are pests, when they're pests, when to manage them and learning to appreciate when they're beneficial, she said.

Seed crops such as sunflowers, for example, could have birds that are beneficial early in the season, such as gold or purple finches that feed on insects to feed their young. Later in the season, those same birds switch to seeds, Long said.

"It is a little bit of a learning curve with understanding birds and their biology, but once you get it, birds can provide a huge amount of beneficial pest control services," she said, adding that providing natural habitat on the farm plays a very significant role in encouraging more natural enemies, including birds.

"When you remove habitat all around the farm, there's nowhere for this wildlife to go—birds and such—except for on crops. Then you're more likely to get wildlife and crop-production conflicts with birds feeding on crops," she said.

There is always the fear that habitat will draw in pest birds, but Long said pest birds are present in crops regardless of field-edge habitat.

"Large flocking pest birds like starlings, blackbirds and crows seem most interested in the crop, or sometimes structures like corrals," Long said.

"We found that if you have hedgerows, you're increasing the beneficial bird numbers by tenfold and diversity by fivefold," she said. "The more we work with Mother Nature to encourage natural enemies on farms, the more natural pest control we'll have."

Long said their research found birds to be beneficial for controlling pests, and that they controlled at least 35% of the codling moth pests in walnuts. If bats are added, along with beneficial insects, she said there would be "pretty good natural control" of the pests.

Bat and owl houses represent a good investment, Long said, as the research shows that encouraging these natural enemies really does help to control pests.

Using birds becomes even more important with fewer chemicals being registered in California, she said, noting that in alfalfa, for example, "It's just really worrisome that there's just no new insecticides coming down the pipeline for weevil control."

Using birds for pest control for walnuts poses minimal food-safety risk, Long said, because of the husk around the walnut shell and the enclosed nut. She said the benefits from pest control by bug-eating birds far outweighs any risks.

"Clearly, in this case where we are losing so many insecticides due to resistance or due to regulations, we really need to look for ways to control pests naturally. One way is to encourage natural enemies like birds and beneficial insects, and bats even, to come in and help us out," Long said.

(Kathy Coatney is a reporter in Bend, Oregon. She may be contacted at kacoatney@gmail.com.)

Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item.




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