Cottonwood leaf beetle larvae are present on poplar leaves at The Morton Arboretum. These larvae can be serious leaf skeletonizers of poplars, cottonwoods, and willows, although they have a lot of natural enemies, such as lady bugs, ants, spiders, and lacewings. All stages—eggs, various larval stages, pupae, and adults—are present. Cottonwood leaf beetle larvae are about 1/4 inch long. They are pale yellow with tiny black spots when they are fully grown, and they have small, white bumps along their sides. The four white bumps closest to its head are large scent glands, which secrete a foul-smelling fluid that repels predators. Large larvae skeletonize the leaves.
Young larvae are gray to black and window feed by eating the leaf’s lower surface and interior, leaving the upper surface intact. Fully grown larvae form pupae that are whitish with black spots and look like roundish blobs of bird manure on the leaf surface. The adult beetle is about 1/4 inch long. It is light yellow with a black line down the middle of its back, and it has three black lines in the form of dashes and dots on each side. The pronotum, the area just behind the head, is red with a large black spot in the center. The adults lay masses of yellow eggs that are 1/4 inch in diameter. There are two to three generations per year.
You can ignore damage to leaves at this time of year; there will be little effect on tree health because leaves will drop soon anyway. This beetle is an unusual insect for Illinois. It is much more common in Kansas and Colorado. At this time, it is more of a curiosity than anything else, but at least you will know what it is when clients ask about it. It has multiple generations per year, so keep an eye out for its presence and damage next fall. Just like its relatives, the elm leaf beetle and the imported willow leaf beetle, it overwinters as an adult. As the leaves approach full size next spring, damage should appear as holes and eaten leaf margins.
Control is probably not needed at this time of year. Bacillus thuringiensis tenebrionis and insecticidal soap can be used to control young larvae, but they are not as effective against older larvae. Carbaryl, sold as Sevin, is effective against all larval stages and adults. Handpicking works in a light infestation and is environmentally safe.