Cottonwood Clearwing Borer
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Hosts
Poplar, willow. Eastern cottonwood is the major host, but balsam poplar and the hybrid poplars are also attacked. Many other poplars probably susceptible. Observed occasionally in black willow; probably occurs in other willow species.
Range
Throughout the eastern half of the United States westward to the edge of the Great Plains (Engelhardt 1946); most destructive in the South (Solomon 1988a).
Description
Adult
Reddish clearwing moth with opaque, dark brown forewings with violet or coppery reflections. Hindwings semitransparent and reddish brown. Wingspans from 30 to 40 mm. Male antennae robust, bipectinate, and strongly dilated at tips; female antennae simple. Head black and shiny with rust red fringe just behind top. Thorax black with lateral buff and reddish tufts. Abdominal segments 1, 2, and 3 black; 4, 5, 6, and 7 reddish; and segments 2 and 4 narrowly ringed with reddish yellow. Legs pale red with black femora (Engelhardt 1946).
Egg
Oval and dark brown (Morris and others 1975), measuring 0.7 to 0.9 mm wide and 1.0 to 1.2 mm long (Eroles-Harkins 1983).
Larva
White to light pink with brown head and thoracic shield; 25 to 32 mm long and 4 to 5 mm wide at maturity (MacKay 1968, Morris and others 1975). Thoracic and abdominal spiracles elliptical, former being larger than latter. Ventral prolegs on abdominal segments 3 to 6 bear parallel rows of well-developed uniordinal crochets; anal prolegs posses only one row of crochets (Peterson 1962).
Pupa
Brown, smooth, shiny, and 20 to 25 mm long. Upper surface of each abdominal segment with two parallel transverse rows of spines.
Biology
Adult life span is 10 to 18 days (Eroles-Harkins 1983). Adult males were caught in pheromone-baited sticky traps in north central Florida from early April to late June and from mid-September to mid-November (Sharp and others 1978). Similar trapping by Eroles-Harkins (1983) in west central Mississippi showed three peaks: mid-April to mid-May, late June to mid-July, and mid-August to early September. Moths are diurnal; males confine their flight mostly to 12 noon to 4 p.m. (Solomon and others 1982). Females oviposit for 2 to 6 days in deep cracks, crevices, and other cavities, usually in the basal meter of the tree. Larvae have 12 instars in the field and 15 to 17 instars in the laboratory (Eroles-Harkins 1983). Upon hatching, larvae feed initially in bark and later enter the wood. Galleries in wood at the tree base meander, whereas those made higher on the trunk tend to follow the pith. Several larvae often infest a stump, but galleries generally do not intersect. Galleries are about 10 cm long for mature larvae. Mature larvae, before constructing silk-lined pupal chambers near the distal end of the galleries, make short tunnels almost to the bark surface, keeping exit holes closed with flimsy bark flaps. When the pupal stage nears completion, the pupae work their way up the galleries with the aid of abdominal spines and protrude through exit holes for emergence. In its northern range, this species requires 2 years for its life cycle (Engelhardt 1946), whereas, in Mississippi, one or two generations may be completed in 1 year (Cook and Solomon 1976). Biological observations are confounded by the occurrence of various-sized larvae in trees throughout the growing season and the occurrence of at least three peak male moth catches in pheromone-baited traps.
Injury and Damage
Early signs of attack are sap flow and frass pushed from the entrance holes; attacks may occur at almost any point on the stem but are most common at the base (Morris and others 1975). Stems may have multiple tunnels from repeated attacks. As larvae grow, clumps of granular frass accumulate at the base. Galleries are partially filled with small, round, reddish pellets of excrement and woody fragments (Engelhardt 1946). Stems less than 4 cm in diameter frequently develop galled or cankerlike swellings. Large stems exhibit only slight (or no) swellings (Cook and Solomon 1976). A survey revealed that 12% of 1- and 2-year old trees in cottonwood plantations in Mississippi River bottoms of Arkansas and Mississippi were infested (Abrahamson and Newsome 1972). In nurseries, populations in 1-year-old plants are seldom heavy; sizeable infestations build up in 2-year-old and older rootstocks. After several harvests, every rootstock may be attacked, requiring clearing and destruction of stumps. Infested trees are weakened and sometimes break off or may be killed by fungi that invade entrance holes. Plantation trees occasionally are deformed or killed. In nurseries, losses average about 12% from culling of infested cuttings (Cook and Solomon 1976).
Control
Woodpeckers are one of the most important natural enemies of the cottonwood clearwing borer, particularly of overwintering larvae, but the holes they excavate may aggravate infestations by providing more oviposition sites and entrance courts for fungi (Cook and Solomon 1976). The parasite Apanteles paranthrenidis Muesebeck has been recorded, but nothing is known of its effectiveness (Marsh 1979). In heavily infested nurseries, the rootstocks should be rouged out and burned about every 3 years to destroy the borers (Solomon 1988a). Systemic insecticides incorporated into soil provide some control (Cook and Solomon 1976).
Gallery
 Photo by James Solomon, USDA Forest Service, Bugwood.org |  Photo by James Solomon, USDA Forest Service, Bugwood.org |
References
Solomon, J.D. 1995. Guide to insect borers of North American broadleaf trees and shrubs. Argic. Handbk. 706. Washington, DC: U.S. Department of Agriculture, Forest Service. 735 p.
