Clearwing Borers

Host Plants

Alder, ash, birch, dogwood, hawthorn, lilac, mountain ash, maple, oak, pine, poplar, sycamore, viburnum, willow, and fruit trees such as apricots, cherries, peaches, and plums. Fruit and cane borer larvae can also tunnel into the stems of grapes, raspberries, strawberries, currants, and gooseberries.

Regions impacted

Found throughout North America.

Description

Adult clearwing borers are moths resembling small wasps. They are active during the day, unlike most moths. The adult moths rarely cause damage to flora, but their larvae can be very destructive to trees and shrubs. Larvae are whitish, hairless caterpillars sporting brown heads and are 1/8th to an inch long.

Life Cycle

Larvae overwinter in host trees and pupate in spring or summer; adults emerge two weeks later. In late summer, females begin laying eggs on crevices or pre-existing wounds of tree trunks. Eggs hatch 10 to 15 days later, and new larvae burrow into tree trunks to feed and overwinter.

Damage and Detection

Wood-boring larvae are very harmful to ornamental trees and shrubs, tunneling and feeding under the bark and destroying water and sap-conducting tissues. Damage, which can involve branch dieback, girdling, and structural weakness in the host, can result in the decline and eventual death of susceptible plants.

Infestation sites can provide entry points for disease and other insects. Immature or weak trees are the most susceptible to damage by the borers; more established trees can often resist and remain unaffected.

Recent transplants or trees with pre-existing injuries can be more susceptible to attack. Borers tunnel beneath the bark of trees and into the main roots near the surface. Once inside, borer larvae are much less vulnerable to insecticides and can remain undetected until serious damage has been done.

Signs of borer infestation include wilting or broken branches, cracked bark, and crown dieback. Entrance sites can ooze sap mixed with saw-dust-like filings (the borer’s excrement). Feeding holes trafficked by woodpeckers or other birds may indicate that a tree is infested.

Prevention and Control

The best prevention is to keep plants healthy and, if necessary, to treat them with insecticides.

• Use bark sprays with contact insecticide in late spring to kill eggs and newly hatched larvae. Application timing must intercept the newly hatched borers before they make their way into the bark. Pyrethroids, including bifenthrin and permethrin, work well. Chlorantraniliprole is an effective, bee-friendly option.
• Prune and burn all affected areas of impacted shrubs or trees.
• Remove infested plants.
• Avoid damage to the bark of trees from lawnmowers.
• Inspect your shrubs and trees in late summer and into fall.
• Cultivate soil around the base of the trunk in fall and spring to expose and destroy larvae and pupae
• Attract native parasitic wasps and predators.