Imported Willow Leaf Beetles
You can find a lot of damage on willows this time of year because imported willow leaf beetles have been feeding all summer. Imported willow leaf beetles (Plagiodera versicolor) are common on willows in landscapes and natural areas. Adults and larvae feed both on willow leaves but they look very different. The adults are iridescent black to blue and about ¼ inch long. Quite attractive. The larvae are dull gray to brown to army green. Not attractive. 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. Repeated defoliation, particularly in combination with other stress can kill trees or reduce growth.
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 can be found in the 2017 Southeastern US Pest Control Guide for Nursery Crops and Landscape Plantings. If you plant a willow in a landscape these beetles and some damage are practically guaranteed. But, willows are great trees for certain locations, they grow fast, and often damage is hard to notice.