Alder (Alnus)-Leafminer

Fenusa dohrnii


Pest description and crop damage The adult insect is a black sawfly about 0.25 inch long. The larvae are small, and light colored. The larvae feed between epidermal layers, creating large dark blotches in the leaves. After the insects leave, the affected areas turn light brown and crinkle. Aside from the effects on the appearance of the foliage, some defoliation and weakening of the tree can occur.

Biology and life history The insect overwinters in the soil as a pupa. Adults first appear in spring, when leaves are partly formed. The females lay eggs on upper leaves, after the eggs hatch they mine out the middle layer of the leaf. Several mines may combine to form a blotch. After 2 to 3 weeks, the larvae drop to the ground and pupate. There are normally two generations per year.

Management-chemical control