Aphids are small insects found in a variety of colors that use mouth parts to pierce tender plant parts and suck out fluids. Small numbers of aphids are generally not cause for alarm, but large populations can cause the wisteria leaves to curl, yellow and become distorted. Additionally, aphids secrete a sticky, sugary substance known as honeydew that acts as a food source for sooty mold. Aphids can be controlled naturally by predatory insects like ladybugs, lacewings and the syrphid fly. Prune out heavily infested leaves if the aphid population is extremely localized or knock off aphids with a strong spray of water. Horticultural oils, insecticidal soaps and a number of insecticides can be used to treat aphid infestations.
Mealybugs, members of the genus Pseudococcidae, are covered with waxy, cottony filaments and, like aphids, produce honeydew. Mealybugs can cause significant cosmetic damage by turning leaves yellow and forcing leaf dieback. Under favorable conditions natural predators like the ladybug beetle and parasitic wasps can control mealybugs. Mealybugs can be addressed with thorough applications of horticultural oil, insecticidal soap or one of several insecticides.
Longhorned borers, also known as roundheaded borers, are large, cylindrical, elongated and frequently bright-colored insects. These beetles have long, noticeable antennae. Borer presence is indicated by holes, stains or oozing liquid on the wisteria stem. Leaves may discolor and wilt and parts of or the entire wisteria can die. The best way to prevent a borer infestation is to maintain the wisteria using proper cultural practices. Remove any dead portions of the vine promptly and do not store any cut wood near the plant. Pesticide applications will likely not affect the borer.
Wisteria can be attacked by certain species of both soft and hard scales. Armored scales, generally smaller than soft scales, secrete a removable protective covering while soft scales produce a hard waxy layer that is not removable. Although scales pass through a crawler stage, they eventually become immobile to feed and cluster in crevices or along veins. These insects use piercing mouth parts to feed. As a result, leaves turn yellow and suffer from dieback. Soft scales also produce honeydew. Naturally occurring predatory insects can help control scale populations. Scales are most susceptible in their early crawler stage, so a well-timed insecticide is crucial for thorough scale treatment.