Plant Pests: Scales

Scale insects infest a variety of ornamental plants, both indoors and out. The unusual appearance of scale insects can make them hard to spot, and control can be difficult.

Types

There are two types of scale insect. Soft scales secrete a waxy coating over their bodies and can move from place to place on the plant. They excrete a sticky substance called honeydew. Hard or armored scales form a shell made of molted skins and wax. They remain in a single place for their entire lives and do not produce honeydew. Small, soft-bodied "crawlers" represent the immature stage of both types.

Effects

Scale insects insert their long sucking mouthparts into plant tissue and drain it of sap. Plants afflicted by scales may show reduced vigor, stunted growth and dieback of leaves and branches. Severely affected plants may die. Gardeners often notice the sticky honeydew scales produce before seeing the bugs themselves. Honeydew can cause secondary infection of a black fungus called sooty mold.

Prevention/Solution

Scales become most susceptible to treatment during the crawler stage. Contact your local extension office to find out when crawlers emerge in your area. Horticultural oils, insecticidal soaps and insecticides such as malathion and permethrin kill crawlers and may have a limited effect on adults.

References

• University of Minnesota Extension: Scale Insects of Trees and Shrubs
• Virginia Cooperative Extension: Scale Insects
• University of Delaware Cooperative Extension: Scale Insect Control