Pest Corner
Galls

Gall formed by the cypress twig gall midge on bald cypress (Taxodium distichum). The gall pictured here is about 8 mm wide. Albert (Bud) Mayfield, Florida Department of Agriculture and Consumer Services, www.forestryimages.org.

In the last few weeks, we have seen many homeowners taking notice of insects attacking their trees and shrubs. Two of the most common insects to attack your trees and shrubs are galls and bagworms. We talked about bagworms last month, so this month we will discuss galls. Trees attacked by these insects can be very unappealing to homeowners.

Galls are abnormal growths of plant tissue that can be found on almost all parts of a plant. The most common are leaf, stem, and flower galls. The majority of galls form when certain insects or mites feed or lay eggs on the plants and the cells are stimulated to grow abnormally. Insect and mite galls usually result from chemical secretions that are produced during the feeding or egg laying process. The chemicals act like natural plant growth hormones. Galls can also form when a plant is injured from a feeding insect. The galls are typically strong and protein-rich, thus providing food and protection for the developing insects or mites. As the insects or mites continue to grow and mature so does the gall.

Gall on oak. James Solomon, USDA Forest Service, www.forestryimages.org.

Galls appear in late spring and early summer when plant growth is rapidly occurring. Once gall development begins, it will continue even if the insect dies or leaves the gall. Galls form in various shapes, sizes, colors and textures. Many times, the insect or mite can be identified by the shape or color of the gall and the surrounding area, or by the type of plant it is found on. Some common examples of insect and mite galls around the Columbia area include maple bladder gall, maple gouty vein gall, oak flake gall, gouty oak gall and the marginal fold gall.

Galls are unattractive, but they are rarely harmful to the host plant. Chemical applications are very costly and usually ineffective as the insect is very well protected in the gall, therefore, chemicals are not recommended in most situations. Large infestations or infestations that last for several seasons may require spraying. Sprays will only be effective if timed properly for application to coincide with the egg-laying period.

For more information on gall-producing insects, refer to MU Extension Guide G7272 ( http://muextension.missouri.edu/explore/agguides/pests/g07272.htm). Mary can be reached at (573) 882-9633

Mary Kroening
University of Missouri
Horticulture Specialist
KroeningM@missouri.edu