Apple-Pacific flatheaded borer

Chrysobothris mali

Pest description and crop damage The Pacific flatheaded borer is a pest of many different trees and shrubs, including most fruit trees. Adults are reddish bronze beetles with copper-color spots on wing covers, and about 0.25 to 0.5 inch long. Larvae are whitish to pale yellow and about 0.5 inch long when fully developed. Just behind the head is a broad, flat enlargement giving a "flat-headed" appearance. Larvae feed beneath the bark and may girdle the trunks and branches of trees. Young, recently planted trees are most susceptible. Trees that are stressed because of drought or suffering sunburn or sun scald on the trunks are particularly susceptible to attack. Young trees are sometimes attacked at the graft union.

Biology and life history The insects overwinter in host plants as mature larvae. In spring, they pupate and then bore "D" shaped emergence holes in the tree. Some borers may spend more than one season in tree trunks. They emerge usually from late May through August. Females lay eggs from June through July in crevices in the bark, usually on the sunny side of tree trunks below the lowest branches. Stressed trees are the preferred egg-laying site. The larvae hatch, burrow into the wood, and feed on the cambial and vascular tissues until full grown. Burrows are characteristically broad and irregular.

Scouting and thresholds Watch for swellings under bark on young trees or cracks through which frass may be seen. Fine sawdust frass piles may be evident around the base of infested trees.

Management-biological control

Birds peck the larvae from under the bark with their beaks. Some wasp parasites attack the borer. Carpenter ants eat both larvae and pupae from the wood.

Management-cultural control

Beetles are attracted to weakened, sunburned, or injured parts of the trunk and lay eggs in cracks on bark exposed to the sun. Protect young and newly planted trees from sunburn by whitewashing or painting with white interior latex paint. Paper trunk protectors or shading the trunk with boards also may be helpful. Avoid drought stress and nutrition deficiency on young trees. Remove infested trees where practical to reduce pest pressure.

June through July sprays

Management-chemical control: HOME USE

  • azadirachtin (neem oil)-Some formulations are OMRI-listed for organic use.
  • imidacloprid-Soil drenches may have residual activity in woody plants lasting for 12 or more months. If short-term management is the goal, consider other approaches.
  • pyrethrins-Highly toxic to bees. Some formulations are OMRI-listed for organic use.
  • zeta-cypermethrin -Highly toxic to bees.

Management-chemical control: COMMERCIAL USE

  • imidacloprid (Admire Pro and generics) at 7 to 10.5 fl oz/a chemigation or root drench, depending on label. Do not apply prior to bloom or when bees are actively foraging. Do not apply more than 10.5 oz product/a per growing season. REI 12 hr. PHI 21 days.