Hazelnut-Pacific flatheaded borer

Chrysobothris mali

Pest description and crop damage The Pacific flatheaded borer is a pest of many different trees and shrubs, including fruit trees and hazelnuts. In hazelnuts, flatheaded borer has been a problem in young orchards, where small trees are attacked and often killed. Adults are metallic reddish bronze beetles with copper-color spots on wing covers, and about 0.25 to 0.5 inch long. The female is considerably larger than the male. The adult beetle is rarely observed and it is not destructive although some minor feeding on leaf margins may occur. The females lay their eggs on the trunk of young hazelnut trees, and the larvae enter the wood, boring out the cambium as they feed. Larvae are whitish to pale yellow and about 0.5 inch long when fully developed. The head is enlarged and flattened giving the "flat-headed" appearance. Larval feeding beneath the bark can result in partial or complete girdling and subsequent tree death. It can take time for the tree to completely die. The feeding site obstructs the flow of water and nutrients from the roots to the leaves and branches. This can worsen drought stress on the tree during hot and dry weather because the tree cannot replenish canopy moisture effectively. A symptom of borer-infested trees is excessive wilting, yellowing and other signs of stress in individual trees during hot periods. The borers have already done most of their damage by the late season and the symptoms of girdling will be most apparent in the leaves and appearance of the tree at that time. Trees that are exhibiting these symptoms should be examined for borer damage starting at the soil line and up the trunk to height of approximately 2.5 feet. If the stresses of a partial girdle do not kill the tree, then there are serious risks that the tree will ultimately snap off as it grows top heavy and begins to put on a nut crop.

Biology and life history Adults begin to emerge from wood in late May or early June and continue emerging into August. They fly and mate and females seek out host trees to attack. The female lays the egg in imperfections on the bark, and the larvae hatches from the egg and bores into the tree, mining mostly the cambium layer. The larvae have done most of the damage by the end of the growing season and they move to the middle of the trunk to overwinter in the larval stage. In the spring, the larvae pupate and the adult chews its way out of the host.

Pest monitoring Stunted leaves and wilting in the upper canopy of young trees while vigorous suckers or watersprouts are growing are a good indicator of girdling of the main trunk by Pacific flatheaded borer. Catkins may form but never distend. Sawdust frass may be apparent on the soil around the base of the tree, especially when trunk guards are removed. Watch for depressions in the bark or cracks through which frass may be seen. The thin bark of hazelnuts will eventually peel back to reveal the feeding sites. The damage may resemble mechanical or rodent damage, but inspect for shallow galleries where the larvae were feeding and evidence of frass. Dead sticks can be flexed and the wood will typically break at the weak point where the larvae were feeding.

Management-biological control

Birds peck the larvae from under the bark with their beaks. Some wasp parasites attack the borer by drilling the ovipositor through the bark to lay the egg on the flatheaded borer larvae. Carpenter ants eat both larvae and pupae from the wood. Generalist predatory mites can enter galleries and feed on overwintering larvae and pupae.

Management-cultural control

Young, recently planted trees are most susceptible. Trees that are stressed because of drought or other causes are especially vulnerable. Irrigate new plantings. Minimize drought stress on young trees with irrigation. Avoid planting in very dry soils. Control weeds and avoid intercrop competition for moisture. These borers are attracted to weakened, sunburned, or injured parts of trunks and lay eggs in cracks on bark exposed to the sun. Plastic trunk guards and paint do not prevent attack on trunks of young hazelnut trees. However, these can help prevent sunburn and mechanical damage on trunks, which create weak imperfections that the adult beetles can exploit for egg laying. Painting trunks with white latex paint will prevent sunburn. Remove infested trees from the orchard.

Management-chemical control: HOME USE

  • azadirachtin (as a mix with other ingredients)-Some formulations are OMRI-listed for organic use.
  • bifenthrin (as a mix with other ingredients).
  • pyrethrins (often as a mix with other ingredients)-Some formulations are OMRI-listed for organic use.
  • zeta-cypermethrin-Applied as trunk spray may have some efficacy.

Management-chemical control: COMMERCIAL USE

Approaches to management include treatment with systemic insecticides or applying residue barrier sprays to trunks.

  • clothianidin (Belay) at 3 to 6 oz/100 gal water. Use the low rate for smaller infestations or smaller trees. Apply no more than 0.2 lb ai/A per year. PHI 21 day. REI 12 hr. 2(ee) recommendation for OR only.
  • fenpropathrin (Danitol 2.4 EC) at 10.3 to 21.3 oz/A. PHI 3 days. REI 24 hr. At least 50 gal water for ground applications. Note buffer requirements for use near aquatic zones.
  • imidacloprid (Admire Pro) at 1.4 to 2.4 oz/A. Generic labels available. Can be applied as soil application through chemigation system, rates and restrictions differ for this application, see label. PHI 7 days. REI 12 hr. Not specifically labeled for Pacific flatheaded borer but applications for aphid control will help control this pest.