TCD is a potential threat to California’s commercial English walnut industry.

California farmers grow 99 percent of the nation’s English walnut crop. According to the California Department of Food and Agriculture, the 2009 value of the walnut industry totaled $738 million. The top walnut-producing counties (in order) include San Joaquin, Butte, Tulare, Stanislaus, Tulare, and Sutter. Growers harvested about 227,000 acres of walnuts in 2009.

TCD is found in native black walnut trees from Los Angeles in the south to Sutter and Lake counties in the north. Outside of California, TCD is found in black walnuts in Arizona, Oregon, Washington, Utah, Idaho, Colorado, New Mexico, and Tennessee.

Janine Hasey, UC Cooperative Extension tree crop farm advisor for Sutter and Yuba counties, first found TCD in English walnuts in Sutter County in August 2009 in the Howard variety.

“The disease can kill a tree in two to three years,” Hasey said.

Symptomatic bleeding from the beetle holes can resemble symptoms from other walnut ailments including shallow bark canker and Phytophthora root rot.

Hasey and Steve Seybold collaborated in TCD research in 2009 including yellow sticky traps placed on a dying black walnut tree in Sutter County to monitor WTB flight and landing rates. Beetle captures occurred from early mid-April to late October. The highest catches occurred in late June and August.

Hasey and Seybold found WTB adults and larvae in the dying tree trunk in early April which suggests the insect overwintered in the tree.

This year, the researchers are conducting a chemical insecticide and fungicide trial on black walnut. The trial is designed to prevent the WTB from entering the bark and increase tree resistance to fungal infection prior to the beetle entering the tree. They are also testing an experimental pheromone trap.

Tested chemicals include the longer residual insecticides Brigade and Sevin. Brigade is a longer residual pyrethroid. Sevin is commonly used to treat bark beetles in forests.

“We are looking mainly at prevention which would require several sprays,” Hasey said. “Once the beetle enters the bark, fungal control is difficult due to canker formation by the fungus.”

Hasey has since found the disease in more Howards and Franquette varieties. The disease was found only in declining trees from age or other problems.