Gall Wasps Associated with Oak
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Gall Wasps Associated with Oak
Of all the insects that produce galls on woody plants, none are so numerous and diverse as the various gall wasps (Cynipidae) associated with oak. In the United States and Canada, over 550 different types of galls have been recorded from oak (76 percent of all types of galls produced by insects and mites).
Most galls on oak are produced on leaves or twigs. Common leaf galls found in Colorado take the form of small pale-colored balls, reddish cup-like swellings, bright-red raised areas along the veins, or woolly patches. The woody galls found on oak twigs include 'bullet' gall forms, that resemble a chocolate drop, or generally round balls. Galls on oak leaves are generally innocuous, although they can be a considerable curiosity. However, some of the twig galls are associated with twig dieback.
The life cycles of gall wasps can be very complex, with alternating forms and types of galls produced by some species. For example, many of the twig galling species spend the winter in the woody galls, and only females are found in these galls. They emerge in spring and lay eggs in young leaves. Galls along the leaf veins are formed by this stage, which involve both males and females. These emerge in midsummer and females insert eggs into the twigs. The developing gall wasps feed on the twigs, producing the gall. However, gall development is slow and it may take almost two years for the wasp to complete this development cycle, emerging in spring of the second year.
Intensity of attacks by gall wasps is highly variable. Much of this is related to how much susceptible new growth is present when the insects are laying eggs. Gall wasps are also heavily parasitized by other wasp species.
Effective controls have not been developed for gall wasps. Insecticide applications, if attempted, should be synchronized with periods when adult wasps emerge from galls and are laying eggs. This may occur in late fall or winter, several months before symptoms of gall formation are first observed.
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