What is "killing" all of those trees?

Mark NaleBeing a retired biology teacher with a natural bent, I end up answering this question every August and September — what is killing all of the locust trees?

Take a drive along almost any rural highway in early September, and you will notice trees that are prematurely brown and look like they are dying.

Are our forests in trouble?

Pennsylvania's forests have numerous serious threats, including the emerald ash borer, gypsy moth, Dutch elm disease, the chestnut blight and last, but certainly not least, the hemlock woolly adelgid. Fortunately, the insect that is causing this late summer damage — the locust leafminer — is a relatively minor problem compared to the others just mentioned.

The locust leafminer is a quarter-inch-long black and orange beetle that feeds primarily on the leaves of the black locust tree. The adult beetles overwinter and become active about the time that locust leaves emerge in the spring. The beetles mate and their eggs are deposited on the underside of the locust leaves.

The eggs soon hatch and the tiny larvae, as the name "leafminer" suggests, excavate or "mine" their way through the inner tissues of the leaf — living a protected life in the narrow space between the upper and lower surfaces of the black locust leaf. Larvae pupate inside the leaves in July.

Up until this time, the damage is usually not noticeable. It is the second generation of larvae and adults that cause the destruction that is so visible at this time of year. As the billions of larvae feed, they leave behind translucent trails, which later turn brown.

The second generation of adult beetles also feeds on the leaves — sucking the life out of them. If enough larvae and then adults are present, the entire locust tree turns brown.

Black locust trees are not truly a species of the forest, but they grow in reverting fields and in edge habitat, such as along roadsides. This habitat preference makes the leafminer damage very visible to even the most casual observer.

The wood of black locust is an excellent firewood, and because of its resistance to decay, it is prized for fence posts and pole barns.

Various species of leafminers (the larvae of moths, flies, sawflies, wasps, and even a few beetles) attack the leaves of almost every tree and plant, but none cause the visible damage that the locust leafminer causes. Common and easily visible garden examples are the leafminers that eat columbine and tomato leaves. As evidenced by the sheer number of insect species that "mine" leaves, eating the inner tissues of leaves is a successful adaptation.

Although ugly now, the black locust trees are not in danger and will be back next spring — almost as good as new.

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