Why the Brown Trees in the Green of Summer?

Right now, if you scan the hills and roadsides before too long you are likely to see clusters of small to medium-size browned trees that look sickly. The black locust (Robinia pseudoacacia L.), a common tree of western PA, is again being eaten this year by large numbers of the locust leaf-miner beetle (Odontota dorsalis (Thunberg)). This tree of the pea family (Fabaceae) was originally confined to very southern Pennsylvania and in more southern states, but has expanded north to many other PA counties.

The locust leaf-miner beetle is a member of the leaf beetle family (Chrysomelidae, with about 1500 N. American species) and is in the leaf-mining leaf beetle subfamily (Hispinae). The adult beetle is 6-7 mm long.

Both the adults and the larvae (grubs) feed on the leaves. The eggs are attached to the locust tree's leaves and, upon hatching, the young bore into the middle layer of the leaf tissue creating tunnels as they feed; thus the term "leaf-miner." Upon transforming to adults, more feeding occurs as the orange and black colored beetles chew holes into the leaves. The adult beetles can be found by examining the locust leaves and trunk. The locust leaf-miner focusses almost entirely on the black locust.

Unlike non-native insects, which are frequently lethal to our native plants, the native locust leaf-miner is a less serious pest. During years of abundance, the beetles turn locust trees brown or defoliated them, but rarely are the trees killed. This is probably due to the fact that the history of beetle and tree go back countless eons, and perhaps to some degree they have "worked things out" in a way that avoids death to most black locust trees. However, somewhat mysteriously, early research suggests that this beetle has much less impact on black locusts in the heart of the tree's natural range to the south of Pennsylvania.

Today's photo's of black locust were taken by Ephraim Zimmerman, WPC Ecologist. The close-up of the adult beetle is by Arnold T. Drooz, USDA Forest Service.

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