Poplar and willow borer
All species of willow, most poplar (but rarely quaking aspen), and occasionally birch and alder are susceptible to attack by the poplar and willow borer, Cryptorhynchus lapathi. This insect, a type of weevil, is 5/16–3/8 inch long with a slender snout as long as its head. It has a roughly textured black body with mottled cream to tan colored patches on its body and its legs, including the back 1/4 of its wing covers.
Poplar and willow borers overwinter as larvae in small cavities they excavate under the bark. The larvae continue feeding in the spring, expelling frass (a mixture of sawdust and excrement) out of openings as they tunnel around stems or branches. They eventually pupate in June and then emerge as adults in late July or August. Adults feed on young stems, laying eggs in slits in the bark. The larvae tunnel under the bark, creating galleries in all directions. They chew exit holes in order to push the frass out.
These borers commonly attack trees between 1–4 inches in diameter and are particularly common on the lower trunk, especially near the root collar. Their tunneling can result in irregular splits, cracks, and dead patches on the bark and around exit holes. Stems can become deformed and some trees may become bushy due to epicormic sprouting. Extensive tunneling can cause small stems or branches to die or break. Damage is most severe on newly planted trees and nursery stock.
Management can be challenging. Remove and destroy infested stems. This is most effective if there is not a lot of susceptible trees in the area and poplar and willow borer populations are relatively low. You may be able to kill or remove larvae in their tunnels by poking a wire into holes (the larvae are active in the tunnels where the frass is being expelled). Another option is to treat trees with a residual insecticide, such as permethrin, when adults are first active during late summer.
Originally published in Yard and Garden Line News, July 2009