Willows are deciduous woody trees and shrubs with alternate lance-shaped leaves. They vary in size and form depending upon the species. Willow trees are susceptible to attacks from a variety of different insects.
The giant willow aphid Tuberolachnus salignis is approximately 0.2 inch long, with light brown legs and a greyish or black body. The willow sawfly, or Nematus ventralis, is a 0.75-inch larval insect with a greenish-black body and yellow spots. Imported willow leaf beetles (Plagiodera versicolora) are 0.16-inch metallic turquoise insects that produce larval black grubs, while willow flea weevils (Rhynchaenus rufipes) are 0.1-inch long orange-legged weevils with black bodies.
Giant willow aphids feed on plant juices from various willow species. Willow sawfly larvae feed on foliage, occasionally defoliating trees. Imported willow leaf beetle larvae eat the leaf tissue between the veins and skeletonise the foliage; willow flea weevils chew holes in leaves.
Gardeners control giant willow aphid and sawfly larvae infestations by spraying trees with a steady stream of water and by handpicking sawfly larvae from trees. Many types of predatory insects such as green lacewings and lady beetles feed on giant willow aphids. The Pacific Northwest Insect Management Handbook does not recommend using insecticides to kill giant willow aphids, because the sprays also kill beneficial predatory insects; horticultural oils and insecticidal soaps control severe infestations. Traditional insecticides kill sawfly larvae, imported willow leaf beetles and willow flea weevils.