Birch cambium miner
Phytobia betulivora Spencer
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. Birch. River birch is possibly the only host
(Greene 1914, Spencer and Steyskal 1986). Sweet birch, red maple, and wild
cherry have been mentioned, but references are not clear on Phytobia
species (Frick 1959, MacAloney and Ewan 1964, USDA FS 1985).
Range. Canada and New York south to the District of Columbia and
west to Illionois and Kansas (Spencer and Steyskal 1986). Probably
throughout the range of river birch. Identical injury in river birch has
been found in North Carolina, Mississippi, Arkansas, and the Great Lakes region,
but the miner species has not been confirmed as P. betulivora (Beal and
others 1952, MacAloney and Ewan 1964).
Description
Adult. Small, blackish fly with large compound eyes that occupy
most of head; measures 3 to 4 mm long (Greene 1914, Spencer and Steyskal
1986). Females slightly larger and more robust with a shiny, black,
slightly flattened ovipositor that extends about 0.5 mm beyond end of abdomen
and slightly wider at apex than at base. Head blackish gray with reddish
orange frons and five or six long bristles. All antennal segments reddish
orange; arista slightly swollen at base. Legs blackish brown with pale
orange "knees." Wings hyaline with dark veins.
Egg.
Larva. Opaque white, elongate, filiform to cylindrical, 20 to 30
mm long, and 1 mm wide when mature. Anterior and posterior ends of body
taper slightly. Head small; mouthparts consist of a large, shiny, black,
chitinized, hooklet with two smaller toothlike processes, one on each side and
slightly back of the large hooklet (Greene 1914). Two slightly raised
padlike surfaces covered with brown hooklike setae on the last two abdominal
segments.
Puparium. Barrel shaped, 4 to 5 mm long, and 2 mm in
diameter. Posterior spiracles have three bulbs each but are slightly less
prominent than the anterior pair.
Biology. Adults emerge from mid-April to mid-May (Beal and
others 1952, Brown and others 1949, Greene 1914, Snyder 1954). Oviposition
apparently occurs most commonly in branch forks in the upper portion of tree
crowns. To oviposit, females perforate the periderm of young branches with
the ovipositor and deposit an egg in the living tissue beneath. Newly
hatched larvae burrow directly into the cambial area between the phloem and
xylem where they feed throughout their development. As they grow, larvae
mine from the branches down the bole to the basal part of the trunk and finally
into the roots. When larvae reach the root collar, many turn and mine
upward for 1 to 2 m or more before turning again and mining into the
roots. Larvae mine along the roots, sometimes encircling them, and move as
far as 60 cm from the root collar. When maturing, larvae burrow through
the bark to exit, sometimes on the top or side, but usually on the underside of
the root. Puparia are formed in the soil from 12 to 25 mm from the exit
sites on the roots. Pupation occurs during August, and puparia overwinter
in the soil. Although 3 years are reportedly required for development, a
1- or 2-year life cycle seems likely.
Injury and Damage. There are no external symptoms of miner
infestations on standing trees; larval mines can only be detected by peeling
bark (Beal and others 1952, Brown and others 1949, Greene 1914, MacAloney and
Ewan 1964). Mines begin in branches as tiny, hairline burrows, pale and
difficult to detect. Mines become larger and darker as larvae progress
down the bole and may be more than 12 m long and 2.2 to 3.2 mm in diameter at
the base of the tree. Mining larvae reverse directions in the basal part
of the trunk, damaging the butt log, which is typically the most valuable part
of the tree. In cross section, mines are small semicircular or lunate pith
flecks orientated so that the long diameter is directed tangentially.
Damage is visible in sawn wood products as brown to yellowish brown flecks,
marks, and streaks (known in the lumber industry as "pith ray flecks")
that degrade the product and reduce its value. Logs with numerous pith
flecks defects are unsuitable for veneer. The defects do not affect the
strength of the wood but detract from its beauty.
Control. One hymenopterous parasite--Sympha agromyzae
Rohwer--is the only recorded natural enemy of this miner (Greene
1914). No direct controls have been developed.
Damage pith ray flecks in birch lumber. James Solomon,
USDA Forest Service. |
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