Home gardeners grow wisteria as landscape shrubs or hedging. It's an ornamental plant with green, multi-leaflet foliage that becomes yellow in autumn. Wisteria produce blossom bunches in white, blue and pink in the spring. It's a relatively easy-to-grow-and-maintain shrub, yet it's also susceptible to invasions by certain pests.
Aphids are tiny insects that seek out wisteria and other plants for their juices. They have long, thin bodies and can range in colour from green and yellow to red, brown and black depending upon their species. They often invade wisteria in the spring and fall when insect populations are greatest. They can cause leaf distortion and inhibit shrub growth. They can also contaminate the plant with other viruses that weaken and even kill the plant. Inspect the underside of leaves for aphids every other week in the spring and fall. Manually remove pests when found in low quantities. Treat wisteria with an insecticide to reduce chance of aphid infestation.
Scale insects often inhabit wisteria plants. Once they invade a plant, scales usually multiple quickly. However, they do relatively little damage to the plant itself other than deposit a sticky sap on it, which you might find visually unappealing. However, large populations can cause leaf curl, foliage discolouration and premature leaf drop. Scales do not look like a normal insect. They appear on leaves and branch stems in clusters that look like raised red-pink blemishes. Proper watering and periodic pruning of decayed branches can minimise scale infestation. Apply an insecticide to plants with large populations of scales to eliminate the pest.
Longhorned borers, also called roundheaded borers, are a type of beetle that occasionally infests wisteria. The beetle has a long, thin body with a bright red or brown spot near its back-end. The beetle bores holes in wisteria's bark and emits liquid on the plant's leaves, buds and stems. Longhorned borers can be incredibility destructive to wisteria. They can cause leaves to yellow, limbs to retreat and branches to weaken and die. For the most part, these pests usually attack only diseased trees, according to the University of California. Eliminate diseased or dying trees as soon as they are identified to prevent infestation. Pesticides are not typically effective at killing this pest.
Although not an insect, weeds are a common pest in most home landscapes that can overtake wisteria plants unless controlled. Weeds often surround the base of the plant and rob it of moisture and nutrients it needs to grow. Manually remove weeds by hand to prevent them from damaging your plants. Apply a thick layer of mulch, about 4 inches, on top of wet newspaper or other barrier, to thwart weed growth. Leave about 6 inches of space between the base of your wisteria plants and mulch to allow for air circulation and to prevent mildew growth.