The Scourge of the Black Locust

It happens every year at about this time. Every single black locust tree on our road turns brown. This occurs gradually over about a 2-3 week period.

The feeding of locust leafminers causes whole trees to turn brown.

I first notice it when the leaves begin to turn a dull green. Slowly a light brown cast spreads over the trees and by late July, the foliage is completely brown. The damaged trees really stand out against the dark green oaks, cedars, and pines that make up the majority of the tree species along the road. It is especially obvious because the black locusts are among the tallest of the trees.

You can also see quite a few of these crispy black locust trees as you drive along many of the highways in Virginia. It looks like the trees have all died, but not so. The damage is caused by a small beetle called the locust leafminer (Odontota dorsalis).

As the larvae feed, the mines eventually run together turning the whole leaf brown.

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Most of the noticeable browning is caused by the larvae of these beetles which tunnel through the leaves feeding on the tissue between the upper and lower leaf surfaces. It is the “mines” left by their feeding that causes the leaves to dry out and turn the rusty brown color. The larvae pupate in the mines and when the adult beetles emerge in midsummer, they feed on the underside of the leaves leaving them skeletonized. It’s a double whammy! Quite often the trees eventually lose all their leaves.

The adult beetles skeletonize the leaf
when they feed on the leaf tissue.

Despite this seemingly major damage and defoliation, these trees are normally not adversely affected and are able to recover often putting out a second set of leaves (which are sometimes attacked by a second generation of the leafminers). However, if a tree is stressed for any reason, including stress from drought or lack of adequate nutrition, repeated defoliation due to these pests may further weaken the tree and result in its death.

In our area, locust leafminer infestations have been occurring year after year with little or no long-term effects on the trees. For the most part, they seem to be able to withstand the damage from these pests.

I sort of wish the leafminers would kill off a few of our black locusts, though! I’m not at all a big fan of these trees. We consider them a weed tree that pops up all over the place – mostly where we don’t want them. This is because new shoots (which eventually grow into new trees) develop along the extensive root system. Through these root sprouts, a single locust tree can give rise to an entire grove of trees. We find them popping up everywhere – in our vegetable garden, in our flower beds, in our orchard … They’re very annoying. If you cut them down, it just seems to encourage the development of new sprouts. Hmmm, pretty good survival tactic!

Despite its negative qualities, the tree has beautiful flowers in the spring.

Normally, black locust is not grown as an ornamental tree so control of the locust leafminer is rarely necessary. However, if these trees are in your landscape, the browning of the foliage can be quite unattractive! If desired, you can protect your trees from leafminer damage by applying Bayer Advanced 12 Month Tree & Shrub Protect & Feed II at the base of the tree according to the label directions. This will give systemic protection against these destructive pests. Always read and follow the label directions. This product may not be available in all states.

Until next time – Happy Gardening