A SPECIAL GROWERS’ REPORT OF AG ALERT ® CALIFORNIA Trees & Vines ®
To protect walnut trees from the flatheaded borer, researchers say farmers should remove weakened, injured, dead or flagged branches to target mature larvae that overwinter in infected wood.
Borer insect’s appetite grows, damaging older trees By Vicky Boyd
borer moot on a lot of his acres. Nevertheless, Vogel said it was another pest to watch for in his remaining trees, es- pecially younger replants. “There’s always something,” he said. “I always carry a pocket knife so if I see some- thing, I can investigate.” Four flatheaded borer species have been reported in the West. Of those, Rijal said the Pacific flatheaded borer is the predominant one causing problems in California walnuts. “In the fall of 2018, we had several (walnut) growers from the northern San Joaquin Valley—especially in San Joaquin and Stanislaus counties—report some sort of damage they had never seen before,” he said. Since then, Rijal has found the pest in many of the region’s walnut orchards at varying levels. “Pretty much all orchards have infestations, although certain areas of the counties have a lot more than others,” he said. “I think it all goes back to that stress factor, and trees on heavy soils are more stressed. Because of that, I’ve seen more damage in those areas than on lighter soils.” In the worst cases, more than 90% of young walnut trees were infested, prompting grow- ers to replant entire orchards. The recent damage also has gone against historical reports, See BORER, Page 8
A native insect pest, the flatheaded borer, has long been only an occasional problem for young, sunburned walnut trees. Now the insect is adding healthy orchards, including mature trees, to its menu. Why the flatheaded borer has begun damaging healthy walnut trees is unknown. But University of California area integrated pest management advisor Jhalendra Rijal said he suspects a number of stress factors, including the multiyear drought and orchard expansion onto marginal soils. “We’re trying to get a better understanding of the pest,” said Rijal, who serves San Joaquin, Stanislaus and Merced counties. That includes the flatheaded borer’s life cycle, phenology or how the cycles relate to seasonal changes, damage potential, monitoring traps and possible management strategies. While many of the research results are still preliminary, he said protecting trunks of 1- and 2-year-old walnut trees with white latex paint, diluted 1-to-1 with water, mini- mizes sunburn and significantly reduces flatheaded borer infestations. This season, Rijal also is conducting a trial with entomopathogenic, or beneficial, nema- todes to see if spraying the microscopic worms on infested trees may provide some control. Ken Vogel, who farms cherries and walnuts east of Stockton, said he has cut down his Vina and Hartley walnuts and plans to keep only blocks of Chandlers, making the
August 16, 2023 Ag Alert 7
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