Alder Leaf Beetle Agelastica alni
The Alder Leaf Beetle is a rather small black leaf beetle, measuring 6 or 7 mm. Like many other black beetles it looks blueish in the right light. The larvae are usually found on alder, but may feed on hazel or birch reluctantly. The adult beetle overwinters after pupation. The first appear in April. The females deposit their eggs on the underside of a leaf, the larvae appear some 6 days later. The larvae are green in the first stage, later become black. They feed from the leaves of the host plant, leaving oblong holes. The larvae are full grown after three weeks and drop on the ground, where they pupate in debris. Pupation only takes one or two weeks. The adults only appear in spring, by the end of June they are all gone.
From the human perspective the Alder Leaf Beetle is not very harmful, because of its diet. Sometimes adults may feed on other plants, but they cause minimum damage. This is a common species in Europe and Northern Asia. Has been introduced in the United States and is mainly found in the North West.