Honeylocust plant bugs, Diaphnicoris chlorionis, are being found feeding on newly emerging honey locust leaves. Adults should be present in southern Illinois and will appear throughout the rest of the state by early June. In other portions of the state, the nymphal stage is very small right now.
The easiest way to find nymphs is to shake a branch over a white sheet of paper and look for tiny green insects crawling on the paper. Using a 10x hand-lens makes it easier to see the nymphs. The young nymphs are green, with noticeable antennae. Older nymphs are spindle-shaped, with a yellow spot in the middle of the back. The older nymphs possess long legs that allow them to move very fast.
Feeding by honeylocust plant bugs causes severe leaf distortion, chlorosis, and yellow-brown spots on leaves. A heavy plant bug infestation may cause premature leaf drop or failure of leaves to emerge. Defoliated and partially defoliated trees should produce new leaves in June. Trees with less damage retain their damaged leaves throughout the summer, reducing their aesthetic appearance.
Yellow-leaved strains of honey locust are much more susceptible than green-leaved varieties such as ‘Sunset’ and ‘Shademaster.’ In addition, plant location may influence susceptibility to honeylocust plant bug. For example, trees planted in protected areas near buildings tend to be more heavily damaged than trees in parks and other more exposed locations. Wild honey locust trees that are covered with long spines and produce an abundance of seed pods are also attacked by honeylocust plant bug but rarely show any damage that is noticeable from more than a few feet away.
Honeylocust plant bugs are 1/8 inch long, green, and have a flat back. They tend to run and fly readily. Scouting for older nymphs and adults can be done by lightly disturbing the foliage and looking for the insects running up and down the main stem (rachis) of the compound leaf. The mated females lay eggs into the young, green stems of honey locust. The female dies by the end of June. There is only one generation per year, as eggs laid by females do not hatch until next spring.
Honeylocust plant bug nymphs can be controlled with a spray application of acephate (Orthene), bifenthrin (Talstar), cyfluthrin (Tempo), or summer oil. Trunk injections of imidacloprid (Merit, Imicide, Pointer) may also be effective and useful, especially for trees that do not require annual treatment.