How to Manage Pests

Pests in Gardens and Landscapes

Poplar and willow borer—Cryptorhynchus lapathi

The poplar and willow borer (a weevil, family Curculionidae) tunnels in and kills limbs of alder, birch, poplar, and willow.


Adults are about 1/5 to 1/3 inch long and a mottled mix of whitish and black or brown or both. The rear one-third of the wing covers is bright white. The oval body is hard and bumpy on the top and sides. The small head is hidden when viewed from above. The head extends into a slender beak, which is characteristic of weevils (also called snout beetles).

The oval eggs are white and about 1/30 inch (0.8 mm) in diameter. The legless larvae have obvious segments. They are whitish with a shiny reddish-brown head. Larvae grow to 1/4 inch long and when exposed are commonly curved or C-shaped. The oblong pupae are white and about 1/4 inch long.

Life cycle

Weevils develop through four life stages: egg, larva, pupa, and adult. Adults emerge from galleries (tunnels) during spring through early fall and chew and feed innocuously on the bark of young shoots. Adults are active mostly in the evening and are inactive during warm parts of the day. When disturbed, they commonly feign death by folding their legs and dropping to the ground. Adults seldom fly and the females commonly lay eggs on the same plant from which they emerged after feeding inside as larvae.

Adult females lay one to several eggs at the bottom of holes chewed in bark which are the depth of their beak. Females then pack the hole with frass (excrement). There are two periods of peak egg laying, about March through April and from July through September.

After hatching and overwintering beneath bark, larvae develop through 4 or 5 increasingly larger instars. Young larvae mine partly around the stem, and then upward in the wood for up to 3 inches. Frass initially is expelled from holes in the stem, but later the frass accumulates packed in the galleries. Mature larvae pupate behind a fibrous frass plug at the opening of their tunnel. Development from egg to reproductive adult takes 1 to 3 years.


This pest prefers willow but also infests alder, birch, and poplar. Adult weevils innocuously chew bark and lay eggs on current-year green shoots. The hatching larvae mine in the host stem, first girdling the phloem then tunneling into the woody xylem. Small-diameter stems can be completely girdled and killed. Lateral shoots commonly grow from where stems were killed, giving shoot terminals a bushy, crooked appearance. Stems not directly killed by larval mining are weakened, increasing their susceptibility to breakage.

Irregular splits commonly develop in bark over the sites of larval feeding. Irregular holes may also be visible in bark. These tunnel entrances may contain a mixture of sap and moist, reddish brown and white frass. Frass may also be found in bark crevices and on the ground underneath tunnel entrances.

Some infested stems become deformed with callused (rough) areas of bark, gall-like swellings, and dead bark. Other boring insects such as bark beetles and clearwing moths and certain plant pathogens such as canker and wood decay fungi are more likely to be present after the attack of poplar and willow borer.


Several parasitic wasps develop in and kill the borer larvae. Ants and birds commonly feed on the adults and larvae. But it apparently is not known whether natural enemies help to suppress the pest's abundance. Because the borer adults are sedentary and seldom fly, cultural and sanitation practices can minimize damage. Prune off and destroy infested stems to reduce the emergence of beetle adults, but do not remove too much of a plant's foliage and wood.

After pruning out infested limbs, where the beetle's damage is intolerable, bark can be sprayed with a persistent, broad-spectrum insecticide such as permethrin or another pyrethroid (insecticides ending in "-thrin") during March and again in July. Potentially effective products are available only to professional pesticide applicators and one must be hired to make the applications. For how to get the services you want, consult Pest Notes: Hiring a Pest Control Company.

Reapplication of insecticide may be warranted in subsequent years to be effective because of the borer's extended life cycle and movement from other infested hosts. Note that some insecticides move after application and contaminate surface water, adversely affect aquatic organisms, or poison bees and natural enemies. See Protecting Natural Enemies and Pollinators from Pesticides for more information.

For more information see Guide to Insect Borers in North American Broadleaf Trees and Shrubs from the U.S. Department of Agriculture.

Branch swollen from feeding by larvae of poplar and willow borer.
Branch swollen from feeding by larvae of poplar and willow borer.

A frass-packed tunnel where a larva of poplar and willow borer pupates.
A frass-packed tunnel where a larva of poplar and willow borer pupates.

Larva of poplar and willow borer exposed in its tunnel.
Larva of poplar and willow borer exposed in its tunnel.

Adult poplar and willow borer.
Adult poplar and willow borer.

Adult poplar and willow borer.
Adult poplar and willow borer.

Statewide IPM Program, Agriculture and Natural Resources, University of California
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