Honeylocust plant bug is present throughout the state. It is even present in the Morton Arboretum, an area that is far enough north normally not to have this insect away from Lake Michigan. This insect is reliably found throughout southern and central Illinois at least to Peoria. North of there, it is primarily found along Lake Michigan, where lake effect moderates winter temperatures. It does become numerous in other areas of northern Illinois after a series of less-severe winters.
Honeylocust plant bug overwinters as eggs in the first-year twigs of honey locust. The eggs hatch about a week after honey locust bud break. The first two nymphal instars are small and not very mobile, making them less likely to be noticed. Third through fifth instars are larger, about 1/16 to 1/8 inch long, and actively run up and down the twigs and leaf rachises when disturbed. They are oval and light green, with a small yellow spot in the middle of the back.
By late May, they have matured into adults, which are light green and about 1/8 inch long. They also run actively on the twigs and foliage, easily flying when disturbed. A person brushing the foliage or even walking close to the tree will likely have several of these insects on their clothes and in their hair. By late June, the insects have inserted their eggs into the new, green, first-year twigs, where they spend the rest of the summer, fall, and winter.
Honeylocust plant bug feeds on the developing foliage, causing stippling on the leaflets, distorted leaflets, and leaflet drop in severe infestations. Branches or trees that drop all their leaves will releaf within a few weeks with no obvious effects. Stippled and distorted leaflets stay on the tree until fall leaf drop. There is typically a leafhopper present at the same time, but this insect feeds primarily on the leaf rachis, causing no noticeable damage.
Control these insects with sprays of acephate (Orthene), bifenthrin (Talstar), cyfluthrin (Tempo), or summer oil.