The alder leaf beetle (Altica ambiens), also called the alder flea beetle, is an insect pest native to the western United States from New Mexico to Alaska. Alder leaf beetles and their larvae are known for defoliating trees from late spring through early fall.
Adult alder leaf beetles are 1/5 inch long and metallic deep blue to deep greenish-blue in colour. The shiny, caterpillar-like larvae are slightly longer and more narrow, with brownish-black colouration on the dorsal side and yellowish colouration on the ventral side.
Adult beetles emerge from hibernation in late spring or summer, mate and lay their eggs under host leaves. The larvae hatch within a few days and begin feeding on the leaves, maturing and entering the pupae stage in August. Adult beetles emerge in about 10 days, feeding on leaves into early fall and overwintering at the base of the tree.
Alder leaf beetle larvae feed on the underside of leaves after hatching. Mature beetles chew through the leaves, completely skeletonising them. Fortunately, insect damage does not cause permanent, long-term damage, and trees recover quickly from the defoliation.
The U.S. Forest Service recommends watering and fertilising trees on a regular schedule to keep them robust. Since the alder leaf beetle does no permanent damage to the tree, there is no need to treat trees with insecticides. Insecticide treatments can also kill off the beetle's natural predators, including praying mantises and ladybirds.
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