Cottonwood Leaf Beetle

United States
Department of
Agriculture

Prepared by
Forest
Service

Alaska Region
Leaflet
R10-TP-8

COTTONWOOD LEAF BEETLE

Leaf beetles periodically attack cottonwoods and willows in southeast Alaska during the summer. Leaf beetles also attack balsam poplar, quaking aspen, and birch throughout south-central and interior Alaska. Although damage is usually not severe, leaf beetle feeding effects are unsightly and sometimes alarming to homeowners. This brochure will familiarize homeowners with leaf beetles, one of the more common insect pests on cottonwood, balsam poplar, and willow; and provide some ideas for reducing damage to host plants.

Figure 1. Adult cottonwood leaf beetle

LIFE HISTORY: Adult leaf beetles are small black insects with orange markings on their wing covers (Figure 1). These orange patterns vary considerably and may be hard to see on some beetles. The beetles average about 1/4-inch long. Males are usually slightly smaller than females. Adult beetles are first visible in May when they emerge from their overwintering sites. They fly to cottonwoods and other hosts just after the leaves have come out. Throughout May and early June, the beetles are found on leaves and along twigs and branches. Shortly after mating, the females lay clusters of pale yellow eggs (Figure 2) on the undersides of the leaves. Each female deposits a cluster of 15 to 75 eggs. Eggs begin to hatch by mid-June and small black larvae emerge and begin feeding on the undersides of the leaves. As they grow older, the larvae migrate as a group to the upper leaf surface (Figure 3) and continue to feed throughout the month of July. When the larvae are disturbed they give off a pungent odor. The odor comes from small white glands inflated in response to disturbance (Figure 4). By the end of July, the larvae firmly attach their skin to the upper leaf surface and form small black cocoons. Their pupal stage lasts for about one week and new adults emerge by mid-August. Adults leave the host plants by late August, seek cover, and hibernate for the winter.

Figure 2. Leaf beetle eggs are laid in early June

Figure 3. Larvae feed in groups and skeletonize host foliage

Figure 4. Beetle larvae are protected from predators by specialized glands

DAMAGE: Leaf beetle damage is usually not serious. Trees are rarely killed or weakened by this insect. Adults feed along the outer margins of leaves in the spring (Figure 5). Larvae consume everything but the larger veins of the leaf during the summer. Heavily infested trees will appear scorched (Figure 6) by the end of the summer after the larvae have finished feeding. Although skeletonized leaves can be found throughout the crowns of affected trees, leaf beetle damage is usually much heavier in the tops of trees. The same trees are likely to be attacked repeatedly. Beetle populations will fluctuate considerably from year to year.

Figure 5. Damage caused by adult feeding

GUIDELINES FOR REDUCING DAMAGE: Although the cottonwood leaf beetle does not seriously affect trees growing in the forest, trees in urban areas may be more susceptible to the pest. Urban owners of ornamental cottonwoods should insure the best possible growing conditions so that the trees can withstand periodic heavy defoliation. Care should be taken to avoid injuring the roots with mechanical devices or soil compaction. Soil in the area beneath the crown of the tree should not be removed or covered with more soil. To avoid moisture stress, trees should receive adequate water throughout the growing season. Spring fertilization also helps promote tree vigor and minimizes the weakening effects of such pests as the cottonwood leaf beetle. The USDA Cooperative Extension Service should be consulted for specific information on the type and amount of fertilizer to be applied. The factors that regulate leaf beetle populations are not well known, but heavy mortality in overwintering sites appears to be one of them. Beetles probably overwinter near the trees they fed on during the previous summer-either beneath loose bark flakes or on the ground near the tree. If damage has been heavy during the summer, the homeowner can help reduce the overwintering population by disturbing the litter layer with a rake or overturning material on the ground which may harbor the hibernating beetles. Summers characterized by heavy rainfall appear to have few leaf beetle problems. Heavy rainfall may dislodge many of the larvae. Consequently, a high-pressure water spray would be effective against small larvae as they begin to feed in the spring. If they are dislodged from the leaves, they will probably be unable to get back to their food source. Washing off adults would have little effect, since the beetles are very mobile and would return to their host plants. Currently, no insecticides are registered specifically for use against the leaf beetle.

Figure 6. Heavy leaf beetle infestation produces scorched appearance in tops of cottonwoods.

Cottonwood Leaf Beetle, by Andris Eglitis and Edward H. Holsten, Entomologists, Forest Pest Management, State & Private Forestry, USDA Forest Service, Alaska Region.

Additional information on this insect and control alternatives can be obtained from your local USDA Cooperative Extension Service office, Alaska State Forestry office, or from:

Forest Health Protection
State and Private Forestry
USDA Forest Service
3301 C. Street, Suite 202
Anchorage, AK 99503-3998.
Phone: (907) 743-9455

Forest Health Protection
State and Private Forestry
USDA Forest Service
2270 Sherwood Lane, Suite 2A
Juneau, AK 99801
Phone: (907) 586-8811

CAUTION: Pesticides can be injurious to humans, domestic animals, desirable plants, and fish or other wildlife-if they are not handled or applied properly. Use all pesticides selectively and carefully. Follow recommended practices for disposal of surplus pesticides and pesticide containers. Mention of a pesticide in this publication does not constitute a recommendation for use by the USDA, nor does it imply registration of a product under the Federal Insecticide, Fungicide, and Rodenticide Act, as amended. Mention of a proprietary product does not constitute an endorsement by the USDA.