The Twig Girdler
The Twig Girdler:
A Guide to Recognition and Habits in AlabamA
L.L. Hyche, Associate Professor
Department of Entomology
The twig girdler1 is appropriately named, because it is indeed a girdler of twigs of several species of hardwoodtrees. The beetle, a common member of the longhorned wood borer family, occurs over much of the eastern and southern United States from New England to Texas and Arizona. It is most common in the South, and is found throughout Alabama wherever its host trees occur. Pecan, hickories, and persimmon are the favored hosts in the state, but elm, hackberry, basswood, sourwood, oak, honeylocust, dogwood, and some fruit trees may be attacked. Girdling occurs in late summer and fall, and is done by the female beetle in preparation for laying eggs. Girdled twigs usually break and fall. Presence on the ground of twigs that appear to have been chewed off (Photo 1) from the outside is a good sign that the girdler is or has been active.
Activity and habits of the twig girdler in Alabama have been noted and recorded periodically over several years. The following is a description of the beetle, its habits, damage caused, and general seasonal cycle.
Description of Life Stages
There are four life stages: egg, larva, pupa, and adult. Adults (Photo 2, left and right) are grayish brown with a lighter band across the back, and 12-17 mm long (average length of 20 adults collected at Auburn was 15 mm).2 Color is near that of bark of host twigs, thus adults on trees are inconspicuous and easily overlooked (Photo 2, right). Antennae (feelers) are slightly longer than the body of the beetle, and each antenna has 11 segments.
Eggs (Photo 3) are elongate-oval, about 2.5 mm long (mean of 20 measured), and white to cream in color. They are found in the bark of twigs girdled by the female.
Larvae (Photo 4) are white, legless grubs, 16-25 mm long when fully grown. They are distinctly segmented, and taper toward the rear.
Pupae (Photo 5) are about as long as full-grown larvae. They are bare (no covering cocoon), and the antennae, legs, and elytra (wing covers) are visible. Color is white to cream but becomes darker as pupae near the change to adulthood.
Life Cycle and Habits
The twig girdler requires a full year to complete its life cycle (Figure 1). Most of this period is spent as larvae tunneling and feeding unheard and unseen in dead girdled twigs on the ground. Adults begin to appear during the first half of September, and are often present throughout much of the fall. They feed on live twigs of host trees. removing small patches of bark (Photo6) near ends of twigs. Damage from adult feeding is usually minor and often goes unnoticed.
Girdling and oviposition begin in mid-September and may continue into November (Figure 1). The female beetle cuts a V-notch completely around the twig, usually leaving it attached to the branch by a core of wood (Photo 7). Bark adjacent to the girdle often has several transverse markings (Photo 7). These apparently are made by the girdling female; the reason or purpose has not yet been explained. Several hours may be spent girdling a single twig and one female may girdle several. Diameter of twigs at the girdle is is usually around 9-10 mm, Figure 2A (range 6-13 mm for over 300 twigs measured at Auburn). Length of severed twigs varies from around 30 cm to one m.3
Eggs are laid in the bark of girdled twigs. The female cuts a small pit or niche (Photo 8) and inserts a single egg under the bark in the bottom of the pit (Photo 3). The opening to the egg is then sealed with a substance secreted by the female; sealed sites tend to glisten. Oviposition niches are usually located at the vase of leaf buds or secondary twigs (Photo 8). Bark below each niche often shows crosswise markings similar to those seen at the girdle (Photo 7). Oviposition may occur all along the length of twigs, but its outermost extent appears to be limited by twig diameter. Of 200 twigs examined, few eggs were found beyond where twig diameter had decreased to less than 6 mm (Figure 2B). The number of eggs per twig varies greatly. Among 480 twigs collected in Auburn and Lee County, the number of egg niches per twig ranged from 0 to 17, but about 60% contained 3 to 6 niches. Girdled twigs usually break and fall, but a few may remain attached throughout the year (Photo 9).
Eggs begin to hatch in 20-25 days (around October 1, at Auburn). In Alabama, most eggs hatch during the fall; however, unhatched eggs have been found during winter into early March (Figure 1). Larvae tunnel and feed in girdled twigs throughout most of the year, becoming fully grown by mid-August to mid-September. During development, larvae cut small holes in walls of tunneled twigs to expel frass and wood particles (Photo 10). Full-grown larvae then plug tunnels with wood chips (Photo 4), and pupate. Pupae transform to adults in 10 to 14 days, but newly formed adults may remain in larval tunnels for several days before emerging. Pupae and new adults may be found in twigs on the ground from about mid-August through September (Figure 1). During the first half of September, new adults begin to chew out of the old girdled twigs and move to live host trees to feed, girdle, and start a new cycle. Only one generation occurs each year.
Typically in Alabama, girdler damage is most common among young hickories in natural stands, and pecans and other hosts maintained as shade and ornamental trees. Damage is the result of girdling of twigs, small branches, and stems by the adult female. Structures beyond the girdle die and usually break and fall. Among large established trees, girdling is confined to small twigs in the periphery of the crown. In trees with well-developed crowns, loss of a few twigs among many results in little or no real damage. However, among seedlings, sprouts, and small, young trees, damage can be severe. In host trees of this size, stems and main branches of many are of the diameter (6-13 mm) preferred by females for girdling and oviposition (Figure 2A). Girdling of these structures deprives the tree of all or much of its crown. As a result, trees may die, or, become stunted and severely deformed (Photo 11, left and right).
1Oncideres cingulata. Order Coleoptera; Family Cerambycidae.
2One inch equals about 25 mm.
330 cm equals one foot; one meter (m) is about 40 inches.
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