David Malone, Graduate Student, and David Keith, Extension Entomology Specialist August 29, 2003
Pest Identification & Damage Common Name(s): Gall Wasps
Scientific Name: Superfamily Cynipoidea; Order Hymenoptera
Identifying Characteristics for Damaging Stage(s): Gall wasps are very small and all but a few species are less than 5mm (1/4 inch) in length. There is a high degree of color variation between species. Some are black, others are red, yellow or amber. The larvae are legless and both larvae and pupae are white in color.
Damage/Nature and Symptoms: Description of Damage:
Galls occur on a wide variety of woody plants. There are many organisms that may cause gall formation including fungi, bacteria, nematodes or mites, but insects are the prime cause. Gall-forming insects include aphids, phylloxerans, psyllids, midges (gall gnats) and cynipid wasps (gall wasps). Each gall-forming insect produces a gall that is characteristic of that particular insect. Some galls may be as much as 50mm (2 inches) in diameter while others are so small they are rarely noticed.The gall wasps are the most important insects that induce plant galls. Galls primarily occur on trees. A few occur on woody ornamental shrubs and flowers and they are virtually non-existent on bedding and foliage plants. Galls are found most commonly on the stems and leaves but also occur on trunks, flowers, leaf-shoot terminals, petioles and roots. They occur in a wide range of colors and shapes. They may occur as spheres to tubes with a surface that may be smooth, hairy or even covered with spines. Gall production itself is believed to result from the reaction of cambium and other meristematic tissues to stimuli produced by the larvae to cause the abnormal growths.About 80% of the gall wasps produce galls specifically on oak trees while about 60% of all
known insect galls occur in the oak family and 30% occur in the daisy, rose and willow families.
Distribution and Life CycleDistribution: The gall wasp family, Cynipidae has about 1,400 species. The tribe Cynipini, with about 970 known species in 40 genera, is the most diverse of the six recognized tribes within the Cynipidae. The Cynipini of North America are especially diverse, with about 640 species in 35 genera, of which 26 are endemic, north of Mexico.
Woolly Oak Gall
Summary of Life Cycle: Typically adult gall wasps emerge from (for example ) twig galls during the winter. Eggs are laid in the spring and may be deposited in buds, in the leaf or stem. Adults die soon after egg laying. Once eggs hatch, salivary secretions of the gall wasp grub act as powerful plant growth regulators and force the tree to form the gall.Gall wasp galls typically have an outer wall, a spongy fiber layer and a hard, seed-like structure inside of which the gall wasp grub develops. It is believed that larval gall wasps obtain their nutrients from nutrients secreted by the gall itself. Many gall wasps take 2 to 3 years to complete development, and small holes on the outside of the gall indicate that the adult insects have emerged.
Management Methods:Inspection/Survey Methods: Larvae, pupae and adult gall wasps may be observed if galls are cut open as long as they have not previously emerged. For gouty oak gall wasps emergence from stem galls usually occurs in winter while many of the wasps from leaves and succulent stem galls emerge in late spring or early summer.If insecticidal treatments are to be carried out it is important to sample galls and observe them until emergence of adults occurs. Galls may be collected and stored in plastic bags in the shade. The time to make spray application is when the small dark wasps emerge. If after a few weeks no wasps have emerged, then the process can be repeated until adults emerge.
Non-Chemical Management Strategies: Trees or ornamental plants that are unhealthy or under stress are more likely to suffer damage due to gall formation than healthy ones. As a result practices such as appropriate use of fertilizers, irrigation and pruning should be employed to keep plants healthy. Where galls form it may be possible to remove and destroy them by pruning.
Chemical Management Strategies: Insecticidal treatments for gall wasps is usually not practical because:
most of the time, injury caused by gall formation is not serious
for application to be effective against adult gall wasps it must be made application before the gall is formed
there are considerations of environmental contamination and expense involved, especially on larger trees.
Thorough treatments of trees may be made for the control of adult gall wasps as long as they are applied at the time when adult wasps have emerged from the galls and are active. The insecticide Sevin (carbaryl) is labeled for gall wasp control on shade trees and park trees.
For the latest information on available chemical controls, consult the
Pesticide Selection Guide.
References:
Davidson R H, and W F Lyon, 1987. "Insect Pests of farm, garden and orchard" I Wiley & sons, pg 61
"Galls on Oaks" J. R. Baker and S. B. Bambara, Extension Entomologists, NC State University http://www.ces.ncsu.edu/depts/ent/notes/O&T/trees/noteO5/noteO5.html
Insect Galls - University of Florida (http://entnemdept.ifas.ufl.edu/fasulo/woodypest/galls.htm)
