Oak shoot sawfly
Janus quercusae Smith
From: 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.
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.
Adult(s) female. James Solomon,
USDA Forest Service. |
Damage flagging on branches. James Solomon,
USDA Forest Service. |
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