Clearwing Borers (Parathrene dollii dollii (Neum.) and P. tricincta (Harris))
Morris, R.C.; Filer, T.H.; Solomon, J.D.; McCracken, Francis I.; Overgaard, N.A.; Weiss, M.J. Insects and Diseases of Cottonwood. New Orleans, LA. U.S. Department of Agriculture, Forest Service; Southern Forest Experiment Station; State and Private Forestry Southeastern Area. Gen. Tech. Rep. SO-8. 1975. 41 p.
Two clearwing borers prevalent in the Southern United States cause serious losses in cottonwood plantations and nurseries. P. dollii damages the tree base, and weakened trees may break off at the ground. P. tricincta attacks terminals, and breakage can occur at the entrance hole 18 to 24 inches (45 to 60 cm) below the terminal tip. The borer holes also provide infection sites for stem canker diseases caused by a complex of fungi.
Nursery plants become infested during their first year. Populations build up during the second and third years in the stools left after cuttings are harvested. Heavily infested stools cannot support vigorous growth from sprouts, and large stems break off and die before cuttings are harvested. Borer attacks also develop in the basal third of the shoots, and cuttings made from this material are rejected. As many as 10 percent of the cuttings produced may be damaged and discarded.
Early signs of clearwing attack are sap flow from entrance holes and borings (frass) pushed out by the caterpillar. These signs will appear at the base of young trees infested by P. dollii or about a foot below the tip of terminals and branches attacked by P. tricincta. As the larvae grow, their galleries enlarge, and piles of frass accumulate at the tree base. A swelling of the stem may indicate the presence of P. tricincta in a terminal. When the terminal breaks off, the borer will be in the stub below the break. Evidences of adult emergence are brown pupal skins protruding from the exit holes. Infested trees will be drilled by woodpeckers feeding on the caterpillars during the winter.
The adult of P. dollii dollii has narrow, dark brown forewings and partly transparent hindwings. It's wingspan is about 1 1/4 inches (32 mm). The body is brown, and orange crossbands borered with yellow and dark brown mark the thorax and abdomen. The eggs are dark brown heads attain a final length of 1 to 1 1/4 inches (25 to 32 mm).
The adult P. tridncta has blue-black forewings, nearly transparent hindwings, a black body with three lemon yellow crossbands in the female and two in the male, and a wingspread of about 1 inch (25 mm).
The life cycles of both species are similar. The female lays eggs in bark crevices, especially near trunk scars. The larvae molt fives times while tunneling in the wood and pith and then prepare pupal cells at the gallery ends. Larvae overwinter in the galleries and pupate in early spring. Adults emerge in April, leaving the empty pupal skins protruding from their exit holes. Adult emergence peaks again in August, indicating two generations per year. There is, however, considerable overlapping: and larvae of several stages can be found in the trees during the late summer and fall.
Control. - There are no effective natural controls. Woodpecker predation reduces populations in nurseries and plantations but may aggrevate the damage by providing oviposition sites and additional entries for canker fungi. Sanitation measures in nurseries, especially burning infested cull stems, tips, and stools, will help hold down borer populations. Control chemicals are listed in the insert.
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