Mysterious leaf spots may be from four-lined plant bug
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
The four-lined plant bug feeds on a wide variety of perennials, shrubs and trees, leaving sets of leaf-spots that look a lot like a disease (Photos 1 – 3). The spots may be circular or angular, light brown to purple or black, or may be ringed. They are caused by saliva that is injected into leaves from the needle-like sucking mouthparts of the plant bug when it feeds. Sometimes the leaves may also be distorted where the plant bugs are feeding. Because the four-lined plant bugs themselves may not be seen, the best clue that these leaf-spots are caused by an insect and not a disease is the pattern of damage. Feeding damage from four-lined plant bug may appear on a single leaf or group of leaves, while the leaves around them are fine. A clumped feeding pattern is much more typical of an insect than a disease. The leaves on some trees may also be affected, but only on low-hanging branches.
The four-lined plant bug is active in May and early June. If you are lucky enough to see them, you are in for a treat. The young larvae or nymphs are red with black markings and the adults are a bright yellow-green with bold black longitudinal stripes. The plant bug feeding damage is rarely bad enough to justify a spray. If spraying is necessary, an insecticide containing carbaryl (Sevin) or a pyrethroid (resmethrin, permethrin or cyfluthrin) should work well.