Honeylocust plant bug nymphs are feeding on newly emerging honeylocust leaves. They are very small right now. The easiest way to find the very young bugs is to shake a branch over a white piece of paper. When you see a tiny green insect crawling on the paper, look at it through your hand lens. These young nymphs are green with obvious antennae. Older nymphs are spindle shaped with a yellow spot in the middle of the back. The long legs on the older nymphs make them very fast moving.
In southern Illinois, the adults should be emerging and will appear in the rest of the state by early June. They are 1/8 inch long, green, and have a flat back. They are very active in that they run and fly readily. With older nymphs and adults, scouting is easily done by lightly disturbing the foliage and looking for the insects running up and down the main stem (rachis) of the compound leaf. The adults mate; lay eggs into young, green stems of the honeylocust; and die by the end of June. There is no second generation as these eggs do not hatch until next spring.
The insect’s feeding causes severe leaf distortion, chlorosis, and yellow–brown leaf spots. A heavy plant bug infestation may cause premature leaf drop or failure to leaf out. Defoliated and partially defoliated trees will releaf in June. Trees with less damage will retain their damaged leaves throughout the summer, reducing the aesthetic appearance of the tree.
Yellow-leaved strains of honeylocust are much more susceptible than green-leaved varieties such as ‘Sunset’ and ‘Shademaster’ although there do not appear to be any varieties that will not show obvious damage in some years in some locations. Trees planted in protected areas near buildings tend to be more heavily damaged than those in parks and other more exposed locations. Wild honeylocusts that are covered with long spines and produce lots of seed pods are also attacked by this insect but rarely show any damage that is obvious from more than a few feet away.
Control the nymphs with an application of acephate (Orthene), bifenthrin (Talstar), cyfluthrin (Tempo), or summer oil. Trunk injection with imidicloprid (Merit, Pointer, Imicide) should also be effective and useful, particularly for trees that do not require annual treatment.