Galls and Bagworms

Leaf Gall on Red Oak

In the last few weeks, we have seen the rapid emergence of 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. 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.

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 and are therefore 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.

The bagworm’s tough, silken bag offers protection for the over-wintering eggs

Another unsightly insect, the bagworm, produces a protective silken bag around its body, and many homeowners confuse this bag as a part of the tree or a pinecone. During the insect’s life on the tree, it rapidly consumes needles and leaves, defoliating entire plants before your very eyes. Plants that are attacked become weakened or unsightly, and smaller trees may not recover from a heavy bagworm infestation. It is not uncommon for complete defoliation to occur with heavy infestations. Bagworms primarily attack needle-leafed evergreens such as juniper, spruce, and arborvitae, but also deciduous trees such as honey locust and bald cypress. In late May to mid-June, bagworm larvae (caterpillars) emerge from the previous year’s bags and immediately start producing their own protective bags around their bodies. A larva produces its bag using bark, leaves and twigs woven together with silk for strength and camouflage. The larva’s head and legs are free, allowing it to move about the plant and feed on the foliage. The larva will spend its entire life in the bag and complete its development by mid-September. If a tree is completely defoliated, the larvae will crawl off the tree with their bags and search for a new plant to feed on.

The best control for bagworm is to remove and destroy the bags as soon as they appear. Any bags left on the tree will provide a source of insects for subsequent years. Some species of birds are able to open the bags and feed on the larvae. Bagworms can be controlled chemically, but the treatment must occur in the spring as soon as the eggs hatch and the larvae begin emerging from the over-wintering bag. Chemical control becomes much less effective once the larvae get larger and more protected in the bag. For more information on web producing insects, refer to MU Extension Guide G7250 (muextension.missouri.edu/xplor/agguides/pests/g07250.htm). For gall producing insects refer to MU Extension Guide G7272 (muextension.missouri.edu/xplor/agguides/pests/g07272.htm).

Mary Kroening, Missouri Master Gardener Coordinator and UMC Horticulturalist, (573) 882-9633