Beware of Flatheaded Borer Taking Bite Out of Walnut Crops

Pacific flathead borer damage to walnut tree

Damage to this tree is from a Pacific flatheaded borer.
Photo by James Solomon, USDA Forest Service

Walnut growers in recent years have reported higher infestations of the Pacific flatheaded borer, according to the University of California (UC). The pest, while not new to the state’s Central Valley, is becoming a reemerging problem, with drought likely a contributing factor, UC says. Known to cause damage to stressed, wounded, and sunburnt trees, there is a concerning trend of flatheaded borers attacking healthy trees as well.

Currently, there are no traps available for growers to assess populations and chemical control options. At this time, UC recommends walnut growers and pest control advisers be aware of the life cycle and damage that the flatheaded borer can cause and be on the lookout for emerging damage in their orchards.

Advertisement

Fall through winter is the best time to survey for the borer infestation as it is easier to detect in trees without leaves. UC advises these confirmation practices:

• Scout the orchard to detect dead and flagged branches infested by the flatheaded borer and look for external infestation signs (i.e., larval-feeding wound, fresh or old insect frass).

Top Articles
Have a Plan For Climate Change? Why Fruit Growers Need To Act Now

• Look for brown-color sap that has oozed out and spread on the bark surface of the infested part of the tree.

• Look for any visible wounds on the tree branches and limbs that are prone to sunburn, or have pruning wounds, or any other kinds of cracks and injuries.

• For young trees, check for damage on the trunk, especially on the south or west side of the tree. These sides are preferred for egg-laying by females due to higher sunlight exposure and heat. Check the graft union and pruning wounds closely.

•Use a knife to peel the bark in a suspected branch and look for feeding channels packed with frass (sawdust-like insect waste) and cream-colored larva underneath. Keep in mind that larvae tend to move into the heartwood in late fall for overwintering and are harder to find. Larval finds may be easier during June through August, when larval feeding occurs just beneath the bark in the cambium layer.

• Look for D-shaped exit holes of the beetle on infested walnut limbs.

0