Continuous years of drought is likely a contributing factor in the re-emergence of the Pacific flatheaded borer as a serious pest of walnut. Typically, flatheaded borers attack stressed or wounded trees, but recently the damage caused by this insect has been observed even on healthy walnut trees. As their name suggests, the newly hatched larvae bore into wood and feed inside the tree. The boring damage causes nut-bearing branches to break. In young trees, flatheaded borer infestation could cause the tree to die. In some cases, the infestation reached over 90% of the trees and growers were forced to replant the entire orchard. There is an absence of basic information about flatheaded borer infesting walnut orchards.
Area IPM Advisor Jhalendra Rijal and Associate Specialist Sudan Gyawaly studied the lifecycle, seasonal phenology, and the extent of the damage caused. Their research was funded by the California Walnut Board and the U.S. Department of Agriculture’s National Institute of Food and Agriculture. They collected walnut branches infested by borers in the winter and held them until the adults emerged. They observed that the adults emerged from May through June, with most emerging in mid-June. This pattern held true when they caged infested branches in walnut trees in the field and observed adult emergence. Although only an observation right now and needing more years of surveying, the degree of infestation seemed to be affected by multiple factors such as location, the tree’s susceptibility to sunburn, and other environmental conditions such as temperature and drought conditions that likely impose stress on the trees.
Unfortunately, there are no traps or lures to monitor for flatheaded borer activities in the orchard. Preliminary trap research with green prism and purple funnel traps had some positive results, but the capture rate was low. Rijal and Gyawaly tell growers and pest control advisers to monitor for infestations during fall through winter when trees have no leaves. It is easier to see dead or flagged branches and signs of infestation such as wounds, oozing brown sap, beetle frass, and D-shaped exit holes. During the summer, signs of larval activity may be easier to find. Peeling off the bark where an infestation is suspected uncovers feeding channels full of frass and sometimes cream-colored, flat-headed larvae.
Rijal and Gyawaly believe the first step to protecting walnut orchards is to adopt cultural practices that lead to healthy trees. Some cultural practices suggested are to protect trees from sunburn and freeze damage, plus orchard sanitation. Rijal and Gyawaly plan to continue learning more about the biology of the flatheaded borer and host susceptibility. Their results can inform the development of monitoring and management practices for a Pacific flatheaded borer IPM program in walnut.
Growers and pest control advisers are now aware that Pacific flatheaded borer can damage healthy walnut trees. They are also able to diagnose the borer infestations, which were previously misdiagnosed as a canker disease. It is anticipated that more awareness will increase monitoring and lead to the detection of infestations. Learning how to monitor for this pest, cultural methods to prevent infestation and damage, knowing population abundance, and understanding the characteristics of a susceptible plant host are essential for developing an IPM program. IPM programs focus on the long-term prevention of pests and their damage through a combination of management practices that minimize the risk of pests or pest management practices to humans, other organisms, and the environment.
Rijal and Gyawaly’s article: Fall Walnut IPM—Looking Back and Looking Forward. Sacramento Valley Orchard Source. http://www.sacvalleyorchards.com/walnuts/insects-mites-walnuts/fall-walnut-ipm-looking-back-and-looking-forward/
Rijal’s article: Pacific flatheaded borer as a resurgent pest of walnuts in California: background, questions, and future research needs. CAPCA Adviser vol. 22, no. 6. https://capca.com/publication/capca-adviser-magazine-december-2019/
UC IPM Pest Management Guidelines: Walnut—Pacific Flatheaded Borer. ipm.ucanr.edu/agriculture/walnut/Pacific-Flatheaded-Borer
