During late August 2018, a few walnut growers and PCAs reported the infestation of a pest which is almost unheard of before by walnut growers. The insect is called the Pacific flatheaded borer – the larval stage of this beetle insect has greatly enlarged and flattened thorax (not the head!).
Between August and October, we visited more than ten walnut orchards with this problem in several locations particularly in the east side of San Joaquin and northeast side of Stanislaus Counties. We observed the infestation from young (2 years) to mature (15-20 years) orchards irrespective of the walnut variety — be it Howard, Chandler or Tulare.
Although I received 3 to 4 phone calls in the past years about this insect, all of those calls were about flatheaded borer infestation in young trees, both walnut,and cherry. But this year’s finds are different as the infestation appears to be much more widespread and severe across the bigger geographic area.
The flatheaded borers are known to cause damage to weaker, wounded and sunburn-susceptible parts of the tree. However, in our observation, the damage was not limited to wounded and sunburn-damaged branches, and this is concerning for walnut growers. The damage observed was more or less random throughout the tree ranging from small twigs (pencil-size), branches (2-4 inches diameter), limbs and the trunks.
Mode of Operation
The insect belongs to beetle family ‘Buprestidae,’ the members of which are wood borers. Adults are ½ to ¾ inch long small-sized, with brown with gray markings on the wing covers, and have an oval head with the wedge-shaped body. We don’t have much information about the seasonal phenology and life cycle of this insect in walnuts in California. Based on the literature from other parts of the Country, adults emerge from April through August, with the majority of them emerges during June-July.
Female beetles deposit about 100 eggs singly in the potentially weaker portion of the wood (i.e., sunburnt, freshly pruned, etc.), bark crevices or depressions through which freshly hatched larva bore into the bark, feed on cambium layer of the wood initially, but can reach to the xylem eventually.
The larvae are cream colored and legless. They construct pupal chambers and molt into the final instar (i.e., prepupae stage) for overwintering. Pupation occurs in the spring and early summer, and the adult emerges. There is one generation per year, but the life cycle may be longer (1-3 years) in cooler areas as reported in some literature.
At least 70 forest and other tree species of 21 plant families have been reported as hosts that include alder, birch, ash, ceanothus, oak, boxelder, mahogany, maple, poplar, sycamore, willow, apple, pear, beech, elm, cotoneaster, peach, plum, avocado, loquat, cherry, currant, fig, apricot, walnut.
Symptoms and Monitoring
Although literature suggested that adults may be seen on sunny sides of the tree trunk during the summer, we never spotted adult beetles in our field visits and observations. There have been precedents of using different colored sticky traps to capture adult beetle of a similar kind but not specific to the Pacific flateheaded borer. Since finding the optimal trap type is critical to study the emergence pattern of this beetle, we plan to conduct these studies in next season.
At this point, the best way to find whether the flatheaded borer has infested the orchard or not is by doing the visual examination of the tree and look for the following signs:
- Brown color sap oozing from under the bark on the trunk, limbs, and lower branches.
- Presence of visual wounds on the tree branches and limbs that are prone to sunburn. Locate flagged branches and look for infestation signs on those.
- 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 the bark (Fig 5).
- Look for D-shaped exit holes of the beetle on the bark.
Okay, What To Do?
The infestation may be reduced by adopting cultural practices that encourage vigorous, healthy plants, although the borer seems to attack healthy trees as well. Young trees should be protected by applying the white latex paint or using mechanical covers over the trunk (e.g., trunk guard).
Orchard sanitation-the removal of the weakened, injured, dead and flagged branches, is highly recommended during the late fall and winter as the mature larvae overwinter on the infested wood. To my knowledge, there is no insecticide registered to target this pest in walnuts, in California and we will begin investigating potential insecticides next year.