Now is the time to be aware of two notorious insect pests of landscapes and nurseries: the twig girdler, Oncideres cingulata, and the twig pruner, Elaphidionoides villosus. Although there are similarities, the main difference between these two insects is that girdlers cause damage when adults sever a twig from the outside, whereas the damage from pruners occurs when the larvae sever the twig from the inside. Both the girdler and the pruner attack a wide range of trees, including elm, hackberry, honeylocust, linden, oak, poplar, redbud, sassafras, and sweet gum. Branches up to 3 feet long may drop from a tree because of attack by these insects. They are more problematic in landscapes and nurseries when excessive damage alters the shape of a tree or reduces plant salability.
Twig girdler inflicts damage to twigs or branches as an adult. In fall, adult females chew circular notches around a twig--girdling it. Adult females lay eggs individually underneath the bark in a hole, which they create, in terminal and lateral twigs or branches. A female is capable of laying three to eight eggs into a single twig. Leaves turn brown after the adult female lays eggs and feeds. The eggs are placed beyond the girdled area. Girdled twigs soon die, break off the tree, and fall to the ground. The eggs hatch into larvae that overwinter inside the twigs lying on the ground. Larvae tunnel toward the severed end and feed on woody tissue. Full-grown larvae are cylindrical and 18 to 25 mm in length when mature. In spring and summer, larvae undergo a pupal stage. In late summer and fall, when development is complete, adult beetles emerge, and the cycle is repeated. Females live about 6 to 10 weeks and can lay between 50 and 200 eggs during this period. There is typically one generation per year in Illinois.
Twig pruner adults attack twigs and small branches 6 to 50 mm in diameter. The adult female chews a small hole in the bark of a twig and deposits eggs near the leaf axil. Eggs hatch into larvae covered with long, lemon yellow hairs. The larvae enter trees and feed within the center of a small branch or twig--creating a tunnel toward the base. Larvae then migrate from the center to the sapwood, making concentric circular cuts. Eventually, the larvae migrate back into the center tunnel and plug it with frass. Wind causes the branches or twigs to break and drop to the ground. The larvae continue to feed inside the severed branch and then overwinter as pupae. In spring, the adult emerges from the hollowed-out branch. Similar to the twig girdler, the twig pruner has only one generation per year in Illinois.
Dealing with both the twig girdler and the twig pruner primarily involves removing and destroying fallen twigs and branches. The use of insecticides is not recommended for these two pests because it is very difficult to control the larvae residing inside twigs and branches.