Reposted from http://ecoipm.org/blog/
Imported willow leaf beetle (Plagiodera versicolor) are common on willows in landscapes and natural areas. Most of year adults and larvae are feeding on willows. The adults are iridescent black to blue and about ¼ inch long. The larvae are dull gray. The eggs are yellow and resemble lady beetle eggs.
The adult beetles overwinter outdoors under bark or in leaf litter. They and emerge from hibernation sites in spring around the time willow leaves start developing since adults prefer new leaves. Adults and larvae skeletonize leaves, leaving larger veins intact. This gives trees a brown cast as damaged leaves crisp in the sun. In some cases though they can defoliate trees like the one I pass walking to work each day. This tree has been defoliated for the past 5 years and this year is dead.
Pubescent varieties of willow may be less affected than glabrous varieties. Also when you inspect infested willow trees you often see a lot of lady beetle larvae, pupae, and adults that eat the eggs. Thus, if the habitat is suitable to sustain these and other predators insecticides are often not necessary.
Insecticides labeled for leaf feeding beetles such as spinosad, imidacloprid, and chlorantraniliprole, and others in the NC Agricultural Chemical Manual can be used if needed. Unfortunately, these beetles are here to stay so efforts to prevent any damage to willows is in vain. If you plant a willow in a landscape these beetles and some damage are practically guaranteed. The goal should be to keep populations below a level that cause substantial defoliation.