L. L. Hyche

Leafy, green crowns of urban forest hardwoods provide shade and many other benefits that contribute to the comfort and well being of human residents. But, as is sometimes the case, comfort and benefits enjoyed are not without problems. Those crowns that provide shade in summer also provide ideal habitat for numerous species of insects. Many of these attack crowns and destroy or reduce the aesthetic and environmental values of the trees, affect tree growth and form, and/or contribute to the annoying problem of tree litter on lawns and grounds.

In late summer and fall, for example, twigs and branches bearing green or wilting leaves sometimes appear in excess beneath some species of trees. On close examination, these structures appear to have been carefully, deliberately, and somewhat mysteriously, “cut off.” And this, indeed, is essentially what has happened, the work of a common but unique group of beetles with peculiar habits that have earned the names pruners and girdlers. While several species qualify, those most commonly encountered during AAES studies on insects of Alabama trees are the twig girdler, twig and branch pruners, and the red-shouldered shothole borer. The beetles themselves are not often seen, but the appearance of pruned twigs and branches beneath trees is an indicator of beetle activity. The manner by which pruning is accomplished is characteristic of each species and aids in determining the identity of beetles responsible. Here is how they do it.

The twig girdler (Oncideres cingulata) prunes by girdling twigs from the outside (Photo 1), thus the common name. Hickory, pecan, and persimmon are favorite host trees, but several other species, including elm, basswood, hackberry, oak, and some fruit trees, are also sometimes attacked. Twig girdler adults are active mainly in September and October. During this period, female beetles girdle live twigs (Photo 2) for the purpose of providing suitable food and habitat for developing larvae. Girdled twigs are usually 1/4 to 1/2 inch in diameter at the girdle.

Photo 1. Twig girdler female at work.

Photo 2. Completed girdle.

Subsequently, the female cuts small niches (Photo 3) in the bark of the girdled twig and inserts a single egg beneath the bark (Photo 4) at each niche. Shortly thereafter, girdled twigs break and fall. Eggs hatch in the fall. Larvae feed, develop, and pupate in fallen twigs. Pupation occurs in late August and new adults emerge to begin another cycle in September. Only one generation of the twig girdler occurs each year.

Photo 3. Egg niche in the bark of girdled twig.

Photo 4. Typical girdler egg.

Twig and branch pruners (Anelaphus villosus, A. parallelus) typically prune structures from the inside, cutting entirely through the wood but leaving the bark intact (Photo 5). Common host trees include oak, hickory, pecan, walnut, basswood, redbud, and hackberry. Females generally lay eggs in small twigs near the ends of live branches in late spring and early summer. The larva consumes the wood of the twig then bores into the center of the branch and tunnels downward. The larva overwinters in the branch and resumes tunneling in spring. When fully grown, in late summer, the larva (Photo 6) severs the twig or branch by tunneling in circles from center outward to the bark (Photo 5), then pupates in the severed portion. Pruned twigs or branches soon break and fall. Stem diameters at the point of cut usually range from about 3/8 to 3/4 inch.

Photo 5. Branch end showing typical pruning by twig pruners; note that wood was cut through and bark left intact.

Photo 6. Pruner larva, the stage responsible.

The red-shouldered shothole borer (Xylobiops basilaris) primarily attacks freshly cut or killed unseasoned wood, but also frequently attacks weak, declining branches and twigs of living trees. Pecan, persimmon, and hickory are the most common host trees, but wood of a variety of hardwoods is also commonly attacked. The adult beetle becomes active in summer and prunes structures by girdling. Girdling, however, occurs in the sapwood under the bark and thus, unlike that of the twig girdler, is not visible. The attacking adult bores through the bark into the sapwood and constructs a deep groove across the grain (Photos 7 and 8). The groove is the egg gallery of the female. Eggs are deposited at intervals in the side of the gallery and larvae feed and develop in the girdled branch. Branches up to 1 1/2 inches in diameter may be completely girdled. Twigs and small branches weakened by girdling break (Photo 9) and fall.

Photo 7. Typical groove, the egg gallery, cut by the red-shouldered shothole borer adult in the sapwood of pecan branch; branch diameter is 1 1/4 inches.

Photo 8. Cutaway exposing groove and adult beetles.

Photo 9. Pecan branch broken at point girdled by red-shouldered shothole borer; note egg groove with adult.

The severity of damage or magnitude of problem caused by the foregoing “pruners” varies by species of beetle and size and health of host trees. Twig girdler and twig and branch pruners attack living trees. On large trees with full healthy crowns, loss of a few twigs or small branches may reduce the ornamental value of trees and create an annoying litter problem, but results in little actual injury to host trees. However, among seedlings, sprouts, and small trees in formative stages of development, damage can be severe. Loss of twigs, branches, and stem leaders results in severely deformed trees, loss in growth (Photos 10 and 11), and reduced chance of survival. Red-shouldered shothole borer attacks are confined to weak, dying, and/or freshly dead branches and twigs. Consequently, activity of this beetle essentially results in early drop of structures already destined to fall.

Photo 10. Hickory sprout severely damaged by twig girdler.

Photo 11. Main stem of oak severed by a twig pruner; diameter of stem at the break is about 5/8 inch.