Issue 15, September 2, 2011
Twig Pruner and Twig Girdler
Large numbers of branches from one to three or more feet long dropping from oaks at this time of year are likely to be caused by twig pruner or twig girdler. We have had a few calls about these insects in the last couple of weeks. In Illinois, these insects seem to prefer oaks, but will also attack elm, linden, hackberry, redbud, hickory, pecan, persimmon, honey locust, and flowering fruit trees. Twig girdler also attacks poplar and dogwood. Twig pruner also attacks chestnut, maple, sweet gum, sassafras, and wisteria.
Both of these insects are roundheaded borers; the adults are called longhorned beetles. Generally, roundheaded borers require a high moisture level in the wood, but cannot handle the high sap flows and pressures found in healthy trees. For that reason, they tend to attack dying or recently dead trees or parts of trees. An exception to this is the Asian longhorned beetle which attacks and healthy trees. We appear to have eradicated Asian longhorned beetle in Illinois, but it is currently a problem in the Cincinnati, Ohio and New England areas of the U.S. Twig pruner and twig girdler are interesting in that they create dying branches for the larvae to live in.
The twig girdler, Oncideres cingulata, adult female lays an egg near the end of a branch in the late summer to early fall. After laying the egg, the adult crawls up the branch to a location where it chews a groove all of the way around the branch, cutting through the bark and much of the sapwood. This effectively girdles the branch, causing it to die. This eliminates most of the sap flow, allowing the hatching larva to feed in the branches without drowning in the sap or being crushed by the internal wood pressures caused by healthy sap flow. Because the girdling by the adult female has weakened the branch, it commonly snaps off at that location and falls to the ground. The end of the branch will have the external smooth cut made by the female beetle, but the pith and inner wood portion will be jagged. The twig pruner larva continues its development in the fallen twig through much of the following summer.
The twig pruner; Elaphidionoides villosus, adult female beetle lays an egg at a leaf axil towards the end of a branch in the spring and then flies off. The hatching larva tunnels through the center of the branch, where the sap flow is less, towards the base of the branch. Late in the summer, the larva tunnels outward through the sapwood, stopping at the bark. This internally weakened branch snaps off in the wind and falls to the ground. The end of the branch will show a smooth cut, but the bark breakage will be jagged. The twig pruner larva continues its development in the fallen twig, pupating in late fall, and spending the winter as a pupa in the twig. The adult beetle emerges in the spring.
Squirrels will also clip off large numbers of twigs. The end of these twigs will be cut smooth at a slight angle, looking like they were cut with hand pruners. Neither the center nor the bark will have jagged edges.
Control of twig pruner and twig girdler is accomplished primarily by raking up and destroying the fallen twigs. This kills the developing larvae of both species. Insecticide applications are not practical as damage is sporadic in both location and time. The year in which these insects become numerous is not predictable and neither are the individual oaks or other trees that are attacked. (Phil Nixon)