Birch Cambium Miner
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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.
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.
