Cottonwood Leaf Beetle on Oak

Identification

• Cottonwood leaf beetles have yellow wing covers with broken black stripes. These insects are about 1/2 inch long and have black heads. Larvae are black with a white spot on each side, and are less than 1/2 inch long. Older larvae are yellowish with black and white spots. The yellow eggs are laid in clusters of 75 to 100 on the undersides of leaves. Watch for leaves with brown patches, or leaves with ragged gaps between the veins (skeletonizing). You may also find black droppings on leaves.

Damage

• Adult cottonwood leaf beetles begin feeding on tender new foliage and buds in early spring, after spending winter hibernating under plants and other debris. Within a few days of their emergence, the females lay their eggs. As soon as the larvae hatch, they begin feeding in groups, quickly skeletonizing the tree's leaves. Older larvae leave the group to feed separately, and each one can consume an entire leaf, minus the largest veins. The adults and larvae can defoliate large portions of the tree's canopy in just a few generations, and cottonwood leaf beetles produce up to 7 generations per year. Seedlings suffer the worst damage from cottonwood leaf beetles, stunting the trees' growth and causing the trees to produce odd lateral branches.

Chemical Controls

• Watch for adult beetles in spring. As soon as larvae appear, spray the tree with an insecticide. The North Dakota State University College of Agriculture recommends using insecticides containing acephate, diazinon, methoxychlor, chlorpyrifos, carbaryl or malathion. Follow manufacturer's directions for effective application. Chemicals, though, will kill not only the cottonwood leaf beetle and its larvae, but also beneficial insects that may be preying on the beetles.

Organic Controls

• If the beetle population is low, you can handpick the adults from the tree or knock them down with a hard spray from the garden hose and collect them. Crush the beetles or drop them into a bucket of soapy water to kill them. Pyrethrins and rotenone are considered organic and may also be effective in controlling cottonwood leaf beetles. Clean up all plant and other debris around the oak tree in winter and dispose of it to get rid of beetles that may be hibernating. Ladybugs eat cottonwood leaf beetle eggs. Other predators include stink bugs, assassin bugs, ants, spiders, tachinid flies and parasitic wasps. A milky spore disease called BT -- Bacillus thuringiensis var. tenebrionis (=san diego) -- can also control beetle larvae. (Ref. 5, 6)