Garden Detective

Gall wasps keep mystery ‘balls’ hopping

Oak galls  are produced by the tree in reaction to wasp larvae.
Oak galls are produced by the tree in reaction to wasp larvae. Bigstock

My husband and I live in a house we inherited from his grandparents 17 years ago. We have an oak tree in our yard that is at least 20 years old. For the first time, small, creamy-beige balls are dropping and then pop around for a few hours before lying still. I’ve looked them up on the Internet and believe they are gall wasp larvae. Are they? When they hatch, will our tree become infested? Do they only drop occasionally or has our tree reached maturity? We’ve also noticed them at the local pool’s oaks for the first time and my in-laws’ new backyard that is full of oaks. Should we treat the tree, clean them up, or just watch their action?

Jessica Jennings, Sacramento

According to UC master gardener Annie Kempees, most galls are caused by cynipid wasps or gall midge flies. Cynipid wasps do not sting humans. The adult female deposits eggs in or on plant tissue. Gall development is poorly understood, but galls generally form several weeks or months after eggs hatch. Larvae feed under the plant tissue; their secretions apparently induce abnormal growth of cells in the plant.

Many galls harbor a single, legless larva. Other galls may harbor several larvae, some of which may be different species that are predators or parasites of the gall maker. Among these unusual species is the jumping oak gall wasp, Neuropterus saltatorius, which causes discolored spots on the upper side, and seedlike deformations on the underside of valley oak leaves.

Galls drop in summer from the leaves. Huge numbers may be seen hopping an inch or more above the ground because of the movements of a tiny wasp larva inside each gall. Larvae wriggle to move their galls into cracks and crevices or beneath leaf litter to avoid being eaten by birds. Mature jumping gall wasps emerge the following spring to lay eggs on leaf buds.

Although healthy oak trees can withstand jumping oak gall infestations, the tree may suffer from partial defoliation when the galls are particularly numerous. Leaves may turn black or brown, twist and curl, and fall prematurely. Galls may appear on either side of the leaf, but are often more numerous on the undersides, with only a small, brown, blisterlike spot on the upper surface to signal its presence.

Guarding the tree’s health helps it withstand and recover from jumping oak gall infestations. Maintain good sanitation around the oak tree, especially from late summer (when the galls drop to the ground) through early spring (when mature wasps emerge from the galls).

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