A SPECIAL GROWERS’ REPORT OF AG ALERT ® C A L I F O R N I A Trees & Vines ®
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. A University of California study showed birds such as the white-breasted nuthatch photographed by a field camera, right, to be effective predators of the larvae.
Birds help reduce codling moth larvae in orchards ByKathyCoatney
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 avianpredators. About threemonths later, they checked for predation and found 46% of the sentinel codlingmoth larvae exposed to the birdswere eaten, while only 11%of the caged larvae were eaten, showing that birdsweremajor predators. Videos of nuthatches andNuttall’s woodpeckers suggested these two specieswere responsible formuchof the pest control inwalnut 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 winter- time,” she said. “For walnuts, and even almonds, almost all birds are beneficial because they feed
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 SachaHeath of UCDavis and the Living Earth Collaborative at Washington University in St. Louis started a project to discover thepotential for bug-eatingbirds tohelp control codlingmoths inwalnuts. 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 ismaintained 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
See BIRDS, Page 8
September 9, 2020 Ag Alert 7
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