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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.
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Photo
1. Twig girdler female at work.
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Photo
2. Completed girdle.
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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.
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Photo
3. Egg niche in the bark of girdled twig.
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Photo
4. Typical girdler egg.
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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.
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Photo
5. Branch end showing typical pruning
by twig pruners; note that wood was
cut through and bark left intact.
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Photo
6. Pruner larva,
the stage responsible.
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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.
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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.
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Photo 8. Cutaway exposing
groove and adult beetles.
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Photo
9. Pecan branch broken at
point girdled by
red-shouldered shothole borer;
note egg groove with adult.
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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.
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Photo
10. Hickory sprout severely
damaged by twig girdler.
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Photo
11. Main stem of oak
severed by a twig pruner;
diameter of stem at
the break is about 5/8
inch.
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Hyche
is an Associate Professor of
Entomology |
