Spotworm borer
Agrilus acutipennis Mannerheim
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. White and overcup oaks are specific hosts
(Donley and others 1974, Morris 1964). Other white oaks probably also
serve as hosts. Adults have been collected from the foliage of birch,
poplar, and hazelnut, but it is doubtful that they are larval hosts (Fisher
1928).
Range. Widley distributed from Maine south to Florida and and
west to Texas and Colorado (Fisher 1928, Mutchler and Weiss 1922). Also
reported from Ontario, Quebec, and Newfoundland.
Description
Adult. Slender to moderately robust beetle, slightly flattened
(Fisher 1928). Feebly shining, dark blue to black with less distinct
greenish tinge. Antennae extend to about middle of pronotum on male;
slightly shorter on female. Adults 7.5 to 12.7 mm long.
Larva. Slender, white, and extremely elongate, ranging from 25.4
to 33.0 mm (Morris 1964). Body noticeably flattened, with prothorax
slightly wider than body.
Biology. Adults emerge in May and June and sometimes through
July (Chittenden 1900b, Morris 1964). Females deposit eggs in bark
crevices on host trees (Morris 1964). Larvae bore through the bark and
excavate patches of the inner bark about 13 mm in diameter. They soon
enter the cambium and tunnel spirally, primarily in the outermost growth
ring. Gallery lengths have not been measured adequately, but individual
galleries are at least 1 m or more in saplings and small trees. Larvae
pupate in enlarged chambers in the tunnels, and new adults emerge in late spring
and early summer. A generation requires 2 years.
Injury and Damage. Trees growing on river bottomlands subjected
to backwater flooding from December through June are most apt to be infested.
Larvae cause injuries by tunneling the outermost growth ring of the sapwood
(Morris 1964). By cutting away the bark of infested saplings, one can
observe larval feeding sites. Galleries are long, crooked (occasionally
spiralling around the stem), flattened, and packed tightly with frass.
Adults leave small D-shaped emergence holes in the bark, but there is little
other evidence of infestation on the bark surface. Ends of fresh-cut logs
usually show irregular lines of stained wood where the spots are exposed.
On large sawlogs, the stains resemble scribbled handwriting on log ends, and
woodsmen read them as a sign of infestation and poor-quality wood. Cross
sections of small stems may reveal tiny frass-packed galleries surrounded by
dark stain. In sawn lumber, spotworm damage is characterized by dark
stains, oval to diamond or spindle shaped in cross section and about 25.4 to
43.2 mm long with a 1.5-mm frass-packed hole in the center of the stain.
Larvae tunnel the outer sapwood of host trees, leaving defects called
"grease spots" or "worm spots." Lumber with heavy
spotworm defect is graded as Sound Wormy or No. 3A Common, which is worth about
$60 less per thousand board feet than lumber without the defect (Morris
1964). Defect has been reported as particularly prevalent in overcup oak
from river bottoms of the lower Ouachita, White, Arkansas, Alabama, and Pearl
Rivers and in smaller river bottomlands in Alabama, Arkansas, Louisiana, and
Mississippi. Lumber degraded by spotworm defects has been suggested for
use as character-marked paneling that could help lumbermen recover some of the
losses from degrade (Solomon 1986).
Control. Evidence of woodpecker predation has been observed, but
no other natural controls have been recorded. Direct controls have not
been investigated.
Galleries on surface of sapwood. James Solomon,
USDA Forest Service. |
Adult(s). James Solomon,
USDA Forest Service. |
Damage in white oak lumber. James Solomon,
USDA Forest Service. |
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