Common Insect Species of Northwestern Ontario





Forest Pests

Birch Leafminer
Fenusa pusilla - Order Hymenoptera

HOSTS: In NW Ontario, paper birch is preferred.

DamageEVIDENCE: The most obvious sign of infestation is severe browning and distortion of foliage beginning in mid-May. Larval feeding causes irregular blotch mines that are translucent green at first, turning to gray and eventually brown. Mines contain black waste material. Larvae are flat and light greenish-yellow with four black spots on the underside near the front. Eggs are laid singly in slits made in developing leaves. Both eggs and larvae show up as translucent spots when the leaf is held up to the light. Browning may increase as damaged leaves dry out over the growing season, or defoliation may occur.

Larva LIFE CYCLE: There are 3 to 4 generations per year. The insects overwinter as prepupae in soil under infested trees. The small black sawfly adults emerge in May and early June, and lay eggs in individual slits cut in the upper surface of developing leaves. Adult females require newly developing foliage for successful oviposition. Larvae eat the tissue between the upper and lower leaf surfaces, and eventually chew through the leaf surface and drop to the ground to pupate. New adults emerge and lay eggs to continue the cycle.

MANAGEMENT: Damage tends to be more serious on open-growing ornamental birches than on trees in forest stands. Trees weakened by birch leafminer may be more susceptible to attack by other insects and pathogens. Control of the first generation greatly reduces damage by subsequent generations. Prevention of egg-laying at budbreak may be accomplished using systemic pesticides. This insect may also be controlled by applying appropriate pesticides as the new leaves emerge.

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