Flatheaded Borers / Metallic Wood Borers
Flatheaded wood borer beetles attack stressed, dying, or dead hardwood trees (evergreen, deciduous, and hardwood forests).
Found throughout North America.
Flatheaded borers are beetles that tunnel just under the bark of tree trunks, branches, and roots. Adult flatheaded borers are small to relatively large beetles (1/4-2 1/2 inches [6-64 mm]) with small antennae and a characteristic oval body shape. They live up to their name as they sport a prominent head with a low profile and eyes on each side. The adults are sometimes referred to as metallic wood-boring beetles, in deference to the metallic or iridescent color of many common species. Most adults are dark in color, although some species have green patches and others are highly speckled. There are many species of flatheaded borer, while the most common is the flatheaded appletree borer. Larvae are white, legless grubs of up to 1 ¼ inches (3ccm) with an elongated body shape and a small retracted head.
- The life cycles of different species vary from 1 to many years. Flatheaded borer adults attack their host from spring through fall, depending on the species. Adults are particularly attracted to weakened, dead, or dying trees and stumps.
- Females lay their eggs in the outer layers of the bark, and most larvae develop and overwinter under the bark. Several species also tunnel into the sapwood and heartwood.
- The larvae remain inside the tree, feeding through the winter, then pupate the following spring before emerging as adults to repeat the cycle.
- The adult beetles emerge from infested trees by chewing out through the bark. They typically begin in May and continue to emerge for several weeks.
Damage and Detection
- Flatheaded wood borer beetles attack weakened, dying, and recently cut or killed trees.
- The beetle's larvae feed on the inside of hardwood tree trunks, leaving tunnels etched in the outer surface, just under the bark.
- Through their tunneling and feeding efforts, the larvae also leave behind a pile of boring dust (feces) on the outside of the trunk that looks a lot like sawdust.
- The larvae also leave entrance holes when they penetrate the wood; the emerging adults follow suit, leaving cleanly cut exit holes as they emerge.
- Most flatheaded borer species are not significant threats to the hosts they inhabit, although larvae that tunnel into sapwood and heartwood can damage logs and wood products for commercial or other purposes.
- Injured or dying trees can have their lives shortened through the activities of the borer beetles and larvae.
Prevention and Control
- Because flatheaded beetles do not generally attack healthy hardwood trees, the best management involves maintaining good tree health and hygiene.
- Actions that promote tree health include site and species selection, proper planting, application of mulch around trees and plants, watering as appropriate, fertilizer to correct nutrient deficiencies, and active defense against injury (from lawnmowers, trimmers, etc.).
- It is prudent to eliminate targets of beetles by pruning and removing dead and dying limbs and trees. Once borers have infested a tree, they are difficult to eradicate.
- External sprays are only effective if applied to the tree when the adult beetles are active and laying eggs.
- To prevent the spread of borers, do not move firewood or lumber from infested areas unless it is removed fully from the property.
Frank Vassen, Flickr
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