Oak Shoot Sawfly
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Hosts
Oak. Found only in two species, Nuttall oak and water oak, which are in the red oak group (Smith and Solomon 1989). However, because this pest also occurs outside the range of Nuttall and water oaks, it likely infests other species in the red oak group.
Range
Newly discovered species known only from Mississippi, Maryland, and Virginia (Smith and Solomon 1989). It undoubtedly occurs elsewhere in the East, particularly between Mississippi and Maryland.
Adult
Female delicate, black and red, wasplike sawfly, 6.5 to 8.0 mm long, with wingspan of 11 to 13 mm; male unknown (Smith and Solomon 1989). Head, antennae, and thorax black with yellow mandibles, palpi, and indistinct markings on pronotum. Abdomen compressed, noticeably deeper than wide; black with at least segments 3 to 6 red; ends with a sharp, sawlike ovipositor. Forelegs and midlegs yellow; hindlegs black, yellow, and orange. Wings hyaline with indistinct, ochre markings and brownish veins; radial vein complete in forewing.
Larva
Nearly cylindrical, thorax somewhat enlarged, slightly S-shaped with head and thorax most noticeably curved downward; white with yellowish head and light brown mandibles.
Biology
Adults emerge from mid-April in Mississippi to mid-May in Virginia (Smith and Solomon 1989). Females lay eggs in the tender terminals and branch ends during spring when shoots elongate. The shoots are girdled by a series of ovipositor punctures apically to the oviposition sites and 3 to 8 cm from the shoot tips. Larvae tunnel basally in the stem pith for 4 to 10 cm, eventually hollowing the shoots and packing frass behind them within the galleries. The larvae girdle the shoots from within one or more times in their travel down the shoots. Larvae are fully grown by mid- to late June and construct thin, white to light brown, partially transparent, cellophane-like cocoons around themselves near the basal ends of the galleries. They spend the rest of the summer, fall, and winter in the cocoons, and pupate in early spring. Adults emerge through round holes in the sides of the shoots. The oak shoot sawfly has one generation a year.
Injury and Damage
Flagging shoots from mid-April and mid-July provide the earliest evidence of attack. New expanding leaves and succulent new growth of terminals and branch ends begin wilting and drooping soon after attack. Affected shoots turn yellowish brown then black, flagging the injured shoots. Most shoots break at girdled sites 3 to 8 cm from the apices and drop to the ground, leaving only blunt stubs. As the larvae tunnel down the stems, the shoot stubs gradually turn dark brown or black, and any succulent side shoots also wither, droop, and darken. By midsummer, new lateral shoots often issue from bud sites just below the blackened stubs. The tunneled portion of the shoot is easily broken at one or more sites that are girdled from the inside by the larva. The girdled site is always nearly perfectly round, with the tightly packed frass protruding from the branch stub in a dome shape; the site is concave in the end of the detached portion. The emerging adults leave round exit holes 1.2 to 1.6 mm in diameter in infested shoots.
Control
Parasitic larvae in sawfly cocoons and very small exit holes in the bark directly over sawfly cocoons have been observed, but no adult parasites have been obtained, and none have been identified. Direct controls will probably be needed for ornamental trees, but none have been investigated.
Gallery
 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.
