Removal of Infested Branches Helps to Minimize Flatheaded Borer Damage in Walnuts
Jhalendra Rijal, PhD, UCCE Integrated Pest Management Advisor, Stanislaus, Merced, and San Joaquin Counties
Pest status and biology
In the last four-to-five years, growers have been reporting a higher level of flatheaded borer infestation in walnut orchards, particularly in the northern San Joaquin Valley. The name ‘flatheaded’ comes from the enlarged and flattened shape just behind the head of the larvae (Fig. 1) of the metallic wood-boring beetles (Insect family - Buprestidae). The insect species attacking walnuts is the Pacific flatheaded borer (Chrysobothris mali). This pest is not new to the Central Valley, but has become a problem in newer ways in many walnut orchards. The flatheaded borers are known to cause damage to stressed, wounded, and sunburnt trees, and these trees are still more likely to be at- tacked by flatheaded borers. However, there is a concerning trend of flatheaded borers attacking healthy trees and not limiting the damage to wounded and sunburn-damaged branches. The observed damage ranged from small pencil-sized twigs to 2-4 inch diameter branches, limbs, and tree trunks.
The general biological life cycle of the Pacific flatheaded borer commences when females depos- it eggs singly in the potentially weaker portions of wood (i.e., sunburnt, freshly pruned, etc.), bark crevices or depressions. Freshly hatched larva bore into the bark, initially feeding on the cambium layer (especially the smaller branches), but can eventually reach to the heartwood by the fall. The borers remain there throughout the winter before pupating and emerging as adults the next summer. There is only one generation per year but there are reports in the literature that a life cycle may be longer (1-3 years) in cooler areas.
We previously did not have much information about the seasonal phenology of the Pacific flat- headed borer in California’s walnut orchards so the California Walnut Board funded a recent study to look at adults' emergence pattern from infested walnut branches. In this study, we collected infested branches in the winter, and beetles were reared out from that collection. Based on this, we found most beetles were emerged in mid-June, with the emergence occurring over two months, May through June. The emergence timing can vary from year-to-year based on potential factors such as variety, temperature, locations, drought conditions, etc. More research is needed to understand various factors influencing seasonal emergence and borer incidence in orchards. Also, there are no traps available for growers to use, and we are continuing to do more research on that front; and in the next couple of years, we will be exploring cultural to chemical control options.
At this time, we recommend walnut growers be aware of damage of the flatheaded borer and find out whether flatheaded borer is causing subtle background damage given the fact that damage may not be obvious in the beginning years be- cause other factors may produce similar types of external symptoms (i.e., flagged branches, dead twig, etc.).
Symptoms of flatheaded borer infestation
Winter is the best time to survey infestation as it is easier to detect in trees without leaves. The following are the symptoms used to confirm flat- headed borer infestation in walnut orchards.
1. 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, Fig. 2).
2. For young trees, check the damage on the trunk, especially on the south or west side of the tree. These sides are the preferred sides for egg-laying by females due to higher sunlight expo- sure and heat. Check the graft union and pruning wounds closely.
3. 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 (Fig. 3). Keep in mind, larvae tend to move into the heartwood in late fall for overwintering and would be harder to find. Larval finds may be easier during June through August when feeding occurs just beneath the bark in the cambium layer.
4. Look for any visible wounds on the tree branch- es and limbs that are prone to sunburn, or have pruning marks, or any other kinds of cracks and injuries (Fig. 4).
5. Look for the brown-color sap which has oozed out and spread on the bark surface of the infested part of the tree. (Fig. 5).
6. Look for D-shaped exit holes of the beetle on infested walnut limbs (Fig 6).
Managing flatheaded borer infestation - winter tasks
Pacific flatheaded borer infestation may be reduced by adopting cultural practices that encourage vigorous, healthy plants; although as previous- ly mentioned, the borer seems to have the ability to attack healthy trees as well.
1. Young trees should be protected from sunburn by applying white latex paint (1:1 paint and water ratio), or using mechanical covers over the trunk (e.g., trunk guard) as sunburnt tissue is more susceptible to borer attack.
2. Orchard sanitation consisting of the removal of weakened, injured, dead, and flagged branches is highly recommended during late fall and winter as when the mature larvae overwinter in the infested wood. Chipping the infested branches kills those larvae.