Every year in late summer, an unusual type of insect makes an appearance in many landscapes. Actually, similar damage is caused by a small group of insects, called twig girdlers and twig pruners. Fortunately, these insects are not a serious problem, but their presence can be seen as small twigs at branch tips turn brown and die. In eastern Nebraska, oaks are the main host, but these insects also can be found potentially in persimmon, pecan, elm, hickory, honeylocust, hackberry, poplar, linden, redbud, basswood, dogwood and various fruit trees.
Two different insects have been given the common name of twig girdler. The first is a small, metallic wood boring beetle, while the other is a larger longhorn beetle.
The metallic wood boring beetle, Agrilus angelicus, has a two-year life cycle. In our area, it prefers red oak, but also attacks live oaks in the western and southern United States, and several other introduced oaks. The slender, bronze-to-black beetles are about ¼-inch long at maturity, and emerge from May to September. They deposit eggs on twigs at the junction between the current and previous year's growth. After hatching, larvae bore into twigs and as the insects grow they mine spirally around the twig so that terminal clusters of dead leaves ("flags") appear during August and September. During the next year, the larvae continue to mine into the twigs, but turn and tunnel back into the dead tip before completing development, pupating in the autumn. No girdling on the outside of twigs is evident, as with our next twig girdler, the longhorn beetle.
Longhorn beetle adults, Oncideres cingulata, have only one generation per year, and at maturity are grayish-brown, stout-bodied beetles, about ¾-inch long. Adults appear in late summer from mid-August through early October. The female beetle prepares to lay eggs by chewing through the bark of a small twig, in a grooved channel that goes all the way around the twig, girdling it. She lays an egg in the girdled twig section, which quickly wilts, turns brown and dies. Larvae, which are creamy white, grublike borers, cannot survive in healthy wood, but do fine in the dead twig even after it falls from the tree. When twigs fall from the tree, a close inspection of the twig’s cut end looks a lot like beaver damage, in miniature.
Another similar insect is the twig pruner, Elaphidionoides villosus, which also has only one generation per year. Adults are grayish-yellow and about ½-inch long. In spring when new growth is emerging on trees, female beetles emerge and lay eggs in leaf axils, the angle where a leaf joins the stem. Larvae hatch within a few days and tunnel into the twig. Young insects feed on the twig’s wood and tunnel toward the base of the twig.
By late summer the larvae are fully grown, and begin to make concentric cuts in the wood of the twig, spiraling outward until they almost reach the outer bark. The larvae then retreats into the outer portion of the twig. The larvae overwinters inside the twig, pupating and emerging as an adult in spring.
On close inspection, twigs severed by the twig pruner will have ragged outer edges but a smooth concave inner surface. Tunneling also will be evident. Affected twigs may range in diameter from one-half inch to two inches.
Twigs girdled by any of these insects may stay attached to the main branch for several weeks, or be broken out of the tree by wind. Tunneling may not be evident in fall, if twigs fall out of the trees before the insect eggs have hatched. Homeowners are usually first aware of these insects, due to the many dead twigs that appear at branch tips in late summer.
Heavily infested mature trees can look a little ragged, but the damage is not a serious health problem so chemical control is not recommended or practical. The best way to minimize insect activity is to remove, and burn or discard twigs in fall and spring that contain the developing larvae.