Photo: 'Freesia' black locust tree
Some trees take on a dead brownish look along highways in the summer months across the southern Appalachian region (USDA zones 6 and 7). Over the past month, locust leaf miner have been feeding on black locust (Robinia pseudoacacia) trees, stripping chlorophyll from the leaves.
Something to worry about? Generally, no. Spraying insecticides is usually not necessary. By late June black locust trees have completed their "photosynthetic life cycle" and foliar injury does not prevent old established trees from starting fresh next spring.
Generally, American gardeners have always had a low opinion about our native black locust. However, new black locust cultivars are awakening our interests. To list three exciting cultivar introductions- try Twisty Baby™ with lacy foliage and twisted shoots, 'Purple Robe’ with dark red-rose flowers, and golden leaved 'Frisia'. These young trees need insect protection.
A time-saving, environmentally safe treatment is a soil application of imidaclopyrid (Bayer Advanced Garden Insecticide™) in early spring. Simply, the pesticide is applied to the ground under the tree and watered in (according to label directions). This prevents beetle larvae development in the spring, and eliminates the threat of the summer leaf skeletonizing adults.
An alternative summer treatment is to spray any pyrethoid insecticide labeled for landscape trees and shrubs. Spraying large trees are a chore, requiring two or more applications over a 4-6 week period.
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