Chrysobothris mali
Chrysobothris femorata
Pest description and crop damage Flatheaded borers represent the metallic wood borer beetle family (Buprestidae), and there are two native species that affect hazelnuts. The Pacific flatheaded borer (Chrysobothris mali) is the more common species in the PNW, and the flatheaded apple tree borer (C. femorata) is also found attacking hazelnut. Both species are pests of many different trees and shrubs, including fruit trees and hazelnuts. In hazelnuts, flatheaded borers have been most destructive in new orchard plantings, where small trees are attacked and often killed at a high rate. Larger trees are also attacked on trunks and scaffold branchess, which is damaging but typically does not kill the tree. Adults are flat, 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 beetles are highly active and rarely observed. Adults may feed on leaf margins. The females lay their eggs on the trunk of young hazelnut trees, or imperfections on the wood of older trees. The larvae enter the wood, boring out the cambium as they feed and molt, creating a gallery that increases in size. Larvae are whitish to pale yellow and about 0.5 inch long when fully developed. The thorax of the larva is enlarged and flattened giving the "flat-headed" appearance. Larval feeding beneath the bark can cause partial or complete girdling, especially on young trees. The percentage of the trunk circumference that is girdled determines the outcome of the damage. A complete trunk girdle in newly planted trees causes certain tree death, and partial girdles can lead to a prolonged decline. In older trees, the feeding site obstructs the flow of water and nutrients from the roots to the leaves and branches. Damage symptoms 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. Young 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. The trunk may break easily at the damage site. In older trees, stress symptoms in the canopy can indicate flatheaded borer attack on scaffold branches or trunks. However, there are many different potential causes of such symptoms.
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 eggs in imperfections on the bark, and the larvae hatch from the eggs and bore 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 heartwood to overwinter in the larval stage. In the spring, the larvae pupate, and the adult chews its way out of the host leaving a distinct "D" shaped emergence hole.
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. Sometimes trunk guards are too short to adequately protect trunks. Painting trunks and exposed wood with white latex paint will prevent sunburn. Manipulation of the tree canopy through pruning can expose wood that is normally shaded, resulting in sunburn. Straighten leaning trees to prevent burn. Remove infested trees from the orchard and destroy.
Management-chemical control: HOME USE
- azadirachtin (as a mix with other ingredients)-Some formulations are OMRI-listed for organic use.
- bifenthrin (often as a mix with other ingredients).
- emamectin benzoate
- imidacloprid (often as 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 (0.2 to 0.4 lb ai/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.