Oak gall wasps and other cynipids
Distorted, sometimes colorful swellings in branches, flowers, leaves, or twigs caused by gall wasps (Cynipidae) are especially abundant and varied on oaks. Cynipid gall wasps also can infest rose and certain other hosts. The size, shape, and color of the galls depends on the species of wasp and host and the plant part infested.
Adult cynipids are usually purple or black, small, stout insects that have clear wings with few veins. The appearance of galls and wasps and the part of the plant attacked often differ between generations. The abundance of cynipids and their new galls varies greatly from year to year.
Certain gall wasps contribute to annoying problems, such as leaf or twig dieback or messy dripping. Certain Andricus, Disholcaspis, and Dryocosmus spp. gall wasps induce plants to secrete copious sticky nectar.
Oak gall wasps alternate between one sexual and one asexual generation each year. Several weeks or months after egg laying by the female wasp, a gall forms as one or more white larvae feed inside. Galls on woody parts and evergreen foliage can persist long after the wasp larvae have completed feeding and emerged from the galls.
Provide oaks with proper environmental conditions and good cultural care. No other management is recommended because gall wasps can be very difficult to control, and usually they do not seriously harm oaks.
Gall wasps are naturally killed by a complex of fungi, parasites, predators, and competing insects (primarily moth larvae and other wasps) that live within galls. Cynipids are also preyed upon by various small insectivorous birds, woodpeckers, and small mammals.
If nectar-producing gall wasps are the problem, controlling ants can gradually (over several years) help to dramatically reduce nectar-inducing gall wasp populations by allowing gall wasp competitors and parasites to become more abundant.
In most situations, no controls have been shown to be effective. Certain leaf-galling species may be reduced by insecticides with systemic or translaminar (leaf-penetrating) activity applied in late winter to spring, or by a broad-spectrum, residual insecticide foliar spray timed to coincide with leaf bud break or the early expansion of new leaves. However, gall wasps are unlikely to be well controlled by any single treatment; sometimes insecticide provides no control at all. Avoid using residual, broad-spectrum foliar sprays because they can induce outbreaks of other oak pests and wash off and pollute surface water.