What Are Small, Brown Cottony Balls on a Tree's Leaves?

Galls

• When plants are attacked by insects, mites, nematodes, fungi, viruses or bacteria, they sometimes produce vegetative growths known as galls. In the case of feeding pests, the galls often form around the pest, providing them with cover for part or all of their lives. In the case of feeding insects or other pests, leaves form galls in direct response to egg-laying or feeding. Gallmaker pests include more than 2,000 species of wasps, aphids, mites, psyillids and midges in the United States.

Gall Wasps

• Galls vary in shape, size, color and texture, but brown, cottonball-like growths on tree leaves could be a symptom of gall wasps. Most soft-textured, brown galls on leaves are the result of gall wasps. Jumping oak leaf galls are small, brown galls produced by wasps. When a wasp larva is big enough, its gall falls to the ground and appears to jump around due to the wasp's activity inside the gall. Woolly oak leaf galls are small, dense, brown galls that look like soft, brown wool. Hedgehog galls also are brown, although they don't look soft; instead, they appear spiny, much like the back of a hedgehog.

Other Explanations

• Galls aren't the only explanation for small, brown, cottony balls on leaves. Many insects spin cocoons on the underside of leaves for their pupa stage. They include caterpillars and beetles of various species. Egg sacs are typically whitish, but some have a brownish hue. Many spiders, including common garden-dwelling spiders such as orb weavers, attach their egg sacs to leaves. Sac spiders spin small sacs, as opposed to webs, in which to hide.

Control

• Galls are more an aesthetic concern than a health concern for trees and other plants. Also, once they're formed, simply spraying them with an insecticide doesn't get rid of them. Treatment for galls' specific pest is required before galls form, and that isn't always practical because gall-formation times can vary from year to year. As for cocoons or egg sacs, the affected leaves can be removed and placed somewhere else or destroyed, particularly if you discover the cocoons or egg sacs are from a pest or potentially dangerous species.