Ag Alert. August 16, 2023

Borer Continued from Page 7

which said Pacific flatheaded borers didn’t target apparently healthy trees. In addi- tion, Rijal has had reports of infestations in Yolo and Tulare counties. Adult Pacific flatheaded borers are ½- to ¾-inch, rather nondescript brownish beetles with gray markings on the wing covers. Adult females lay single eggs in weakened or damaged woody parts of trees. Sites include sunburned trunks or branches, fresh pruning wounds, bark crevices or depressions. A single female may lay up to 100 eggs, but it’s the larvae that cause the actual tree damage. The pest gets its name from what ap- pears to be the flat “head” of the larvae, although it’s actually the thorax. The immature insects bore into trees shortly after hatching and live most of their lives inside tree branches or trunks. They feed on the cambium, a layer of actively growing cells responsible for new bark and tissue production. The larvae, which can reach 1 inch long, eventually reach the heartwood at the center of the tree or branch. After six to eight months, the larvae pu- pate and emerge as adults the following May through July, leaving behind telltale D-shaped exit holes in trees. Based on observations, Rijal said emer- gence peaks in mid-June in the northern San Joaquin Valley. Currently, no insecticides are regis- tered for the pest. But he and his team are looking at potential chemical con- trols and application timings. He said he hopes to have recommendations by the start of next season. In the meantime, Rijal said protecting young trees from sunburn is key. In addition, he said maintaining healthy trees overall helps reduce sun-ex- posed branches, disease and other issues

The flatheaded borer larvae, below, once damaged only young, sunburned walnut trees. The insect now also attacks older, healthy trees, leaving larval feeding wounds with insect frass and brown- colored sap oozing out, such as on the walnut branch at left.

Photo/University of California IPM

that may stress trees and attract flathead- ed borers. In late fall or early winter, he recom- mended sanitation—including remov- ing weakened, injured, dead or flagged branches—to target mature larvae that overwinter in infected wood. Chipping the infested material kills the larvae. Infestations can best be detected by scouting trees in the fall after leaf fall when dead or flagged branches infest- ed by flatheaded borer are easier to see. Visual symptoms include larval feed- ing wounds, fresh or old insect frass, or brown-colored sap oozing and spread- ing on the bark surface.

Rijal also recommended checking for visible wounds on tree branches or limbs prone to sunburn or that have pruning wounds, cracks, flagged branches or oth- er injuries. In young orchards, growers and pest control advisors should check for dam- age on the trunk, particularly on the south or west side of trees where females prefer to lay eggs. In addition, they should check young tree graft unions and prun- ing wounds. If growers or PCAs suspect an infesta- tion, they should peel back the bark and look for feeding channels packed with frass—insect excrement that resembles

sawdust. The channels may be easier to detect in the summer when the larvae are feeding on the cambium just under the bark than in the fall, when they’ve moved deeper into the heartwood. No traps or lures are currently avail- able to monitor for flatheaded borers, ei- ther, but developing those tools are part of the Rijal’s research goals. His work was initially funded by the California Walnut Board and is now being un- derwritten by the U.S. Department of Agriculture National Institute of Food and Agriculture. (Vicky Boyd is a reporter in Modesto. She may be contacted at vlboyd@att.net.)

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8 Ag Alert August 16, 2023