Black locust is commonly seen in the eastern half of the United States and planted throughout the world for its ornamental value. Honey bees transform its nectar into delicious black locust honey. Its durable wood is super resistant to rot and Abraham Lincoln is said to have spent significant amounts of time pondering liberty and equality while splitting black locust for fence posts and rails. As a member of the bean family, black locust has the ability to improve the quality of soil by fixing atmospheric nitrogen in its roots. You may recognize this tree by its fragrant white blossoms which arrive each year in the first week or two in May along major thoroughfares in the DMV, such as interstate 70, and along forest edges in our parks and farmlands.
On a recent trip across South Mountain the black trees looked like they had been assaulted with a blow torch. What manner of devilment is this? Well, the culprit behind this beating is a small beetle called the locust leafminer. The locust leafminer belongs to a clan of beetles known as leaf beetles. Well-known leaf beetles include garden pests such as cucumber beetles and Colorado potato beetles. Ravages caused by locust leafminer began back in spring when adult beetles emerged from their overwintering sites on the ground beneath the locust trees. During May and June beetles fly to the treetops to eat the fresh young leaves. Their feeding produces small holes in tender leaves or rough scrapes on the surface of mature leaves called skeletonization. After dining for many days, females convert the nutritious leaves into small batches of eggs laid on the lower surface of the leaves of locust. After the female deposits her brood on the leaf she defecates on them. Now what would Sigmund Freud have to say about that? No matter, this is probably a way to protect the eggs and larvae developing within from becoming a tasty meal for some roving predator.
Feeding injury by adult and larval beetles known as the locust leafminer are scorching black locust trees along our roadways and landscapes. Feeding by adult beetles externally and as larvae within leaves causes leaves to turn white and then a crispy brown. Watch as an adult beetle grooms its legs before moving off to find the next meal.
The eggs hatch into tiny larvae that tunnel into the leaf and consume the nutritious tissue between the upper and lower surfaces. In general, insects that feed between the surface layers in this fashion are known as leafminers. Leafminers are found in many groups of insects including caterpillars, primitive wasps called sawflies, true flies, and beetles. The leafmining way of life is a clever way to avoid the dangers of occupying the leaf surface where fearsome predators such as lacewing larvae roam. When larval growth is complete, a pupa forms and within a week or so a new locust leafminer adult emerges and the life story repeats. Two generations of the locust leafminer punish locusts in our region each year. Mining produces large whitish blotches on the leaf that later turn brown as the leaf tissue dies. Leaves skeletonized by adults also turn brown. Feeding by larvae and adults in concert give the black locust its scorched appearance. Fortunately, black locust is a very hardy tree and appears able to withstand the beetle’s periodic onslaughts.
“Managing insects and mites on woody plants” by J. A. Davidson and M. J. Raupp was consulted for this episode.
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