Ash and honeylocust plant bugs
Published in Yard & Garden Brief October 2002
Plant bugs (family Miridae) are a group of insects that pierce plant tissue and feed on sap. Two plant bugs which commonly occur on shade trees in Minnesota are the ash plant bug, Tropidosteptes amoenus Reuter, and the honeylocust plant bug, Diaphnocoris chlorionis (Say). Ash plant bug occurs only on ash trees (Fraxinus spp.), while honeylocust plant bug occurs only on honeylocust (Gleditsia tricanthos).
Both plant bug species are oval and 3/16 to 1/4 inch long. Ash plant bug adults are pale yellow to brown to almost black with indistinct yellow or pink markings on the back. Honeylocust plant bug adults are pale green and are difficult to see on foliage. Immature plant bugs (nymphs) are similar in habit and appearance to the adults, but are smaller, wingless, and sometimes lighter in color. Nymphs are sometimes described as looking like large, mobile aphids. It takes about one month for plant bugs to mature from egg to adult.
Both ash plant bugs and honeylocust plant bugs feed with a needle-like mouthpart (stylet) on the underside of leaves. The stylet is inserted into the leaf tissue and plant fluid is removed. Tiny, distinct circular discolorations are left where feeding occurs. Also, both plant bugs produce small, shiny, dark, varnish-like specks of excrement on the undersides of infested leaves.
Plant bugs generally do not seriously injure vigorously growing trees, although they can detract from their appearance. Young, recently transplanted, and stressed trees are most likely to be damaged.
Ash plant bug
In spring, ash plant bug eggs hatch shortly after the leaves begin to expand. Ash plant bugs produce two generations each year with some overlap between them. First generation adults appear in early May, depositing eggs on leaf midribs. Second generation adults develop by late summer and insert eggs into twigs, bud scales, or other protected places on trees where they overwinter. Adults remain active until the first hard frost. The first generation of ash plant bug is more damaging than the second generation because nymphs and adults of the first generation feed on easily damaged young leaf tissue.
Nymphs feed on new shoots, leaf stems, and the undersides of leaves. Light to moderate feeding causes yellow stippling or mottling on leaves. Young leaves may wilt and turn brown, developing a scorched appearance even before they become fully expanded. As feeding continues, stippled areas become connected and large areas of the leaf can be damaged. Damaged leaves often linger until autumn. Ash plant bug does not normally cause leaf drop in the spring. Although ash plant bugs may be present, ash leaves dropping in the spring is normally caused by anthracnose or stressful environmental conditions. For more information on ash anthracnose, refer to Yard and Garden Clinic Brief P413A.
Honeylocust plant bug
Honeylocust plant bugs have only one generation per year. In spring, overwintering eggs hatch soon after the leaf buds open, and the nymphs crawl into unfolding leaves and feed. Feeding on the young leaves causes them to be distorted and dwarfed, and can mimic injury caused by herbicides. Feeding nymphs and adults also cause mottling and discoloration to the leaves. The most significant damage occurs at the time of leaf expansion, when the bugs are hidden. In June, adult females lay clusters of eggs under the bark of young twigs, where they remain until the following spring. Adults are rarely seen after late July.
Trees grown in exposed, sunny locations are more prone to plant bug attack, and yellow-leaved varieties of honeylocust, e.g., "Sunburst" are more attractive to honeylocust plant bugs than are green-leaved ones like "Shademaster" and "Skyline."
Ash and honeylocust plant bugs can damage individual leaves but generally do not threaten the health of mature trees. Therefore, the best course of action is to tolerate damage from these insects.
Insecticides can be used to protect the appearance of infested trees. Use insecticides judiciously as they kill natural enemies as well as the plant bugs. When using insecticides, good coverage and timing is critical. Carefully check trees for insects in early spring before or as leaf buds open. If insect numbers are high enough to justify spraying them, insecticidal treatments should be timed for 7-10 days after the leaf buds open for ash plant bugs and just after bud break for honeylocust plant bugs. Later applications of insecticide are not as effective. Be sure to spray the undersides of the leaves where plant bugs are normally found.
Use less toxic, effective products first. Pyrethrins have relatively low toxicity and have little impact on many beneficial insects including natural enemies. Broad spectrum, higher toxicity insecticides such as carbaryl (e.g., Sevin), cyfluthrin, permethrin, or acephate (e.g., Orthene) are also effective. Use these insecticides carefully and judiciously to preserve natural enemies for good long-term management. In the case of honeylocust plant bugs, applications of some broad-spectrum insecticides can kill the natural enemies of spider mites. This can result in an increase in numbers of spider mites, which can increase damage to a tree.
CAUTION: Read all label directions carefully before purchasing and again before using any insecticide. Information on the label should be used as the final authority. Treat only plants listed on the label of the insecticide.
If you do not want to or cannot treat a plant bug problem yourself, contact a reputable landscape care service. They have considerable experience treating tree and shrub pests.