Cynipid Wasp Galls: Making A Mess of White Oaks

The Extension office received a sample of damaged white oak leaves from the North Hills of Pittsburgh. Given the amount of rain we have had all year, anthracnose was a possible suspect.
Cynipid Wasp Galls: Making A Mess of White Oaks - News

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Overall picture of white oak impacted by cynipid wasp galls. Photo: Sandy Feather, Penn State

Closer inspection revealed white oak leaves that had been damaged by a small, non-stinging cynipid wasp, Neuroterus spp., possibly the jumping oak gall wasp. The leaves were badly off-color or entirely brown, and completely stippled with small brown “spots” about the size of poppy seeds. A look at the underside of the leaves revealed tiny little galls – each contains a cynipid wasp larva.

These wasps overwinter as mature larvae or pupae in the soil around host plants. The wasps that hatch in early spring are all female, and they promptly lay eggs in the opening buds of white oaks. Blister-like galls form on the leaves in response to this egg-laying activity. Both male and female wasps hatch from these eggs; they mate, and the females lay eggs in the leaves. The resulting galls develop in five to six weeks, appearing as small brown “seeds” on the undersides of the leaves. The tree produces galls in response to hormones released by the insects.

Upper Leaf Surface: Each tan spot has – or had – a cynipid wasp gall attached on the lower leaf surface. Photo: Sandy Feather, Penn State

Lower Leaf Surface: The black arrow points to one of the tiny galls. Each contains a cynipid wasp larva. Photo: Sandy Feather, Penn State

Lower Leaf Surface: above gall enlarged.

When the galls mature, the larvae drop to the ground. Once on the ground, larval activity causes the galls to jump around, hence their common name. Entomologists theorize that this movement allows the larvae to fall into crevices in the soil where they will be protected from cold winter temperatures. If you are extremely lucky, you might witness that phenomenon.

Although cynipid wasps are present every year, most of the time their numbers are low enough that the damage goes unnoticed. But periodically environmental conditions allow large numbers of them to survive and reproduce, resulting in very noticeable damage to affected trees. They may be bad for a year or two but then go back to their normal low profile.

Oak trees support many different gall-making insects. They generally do not cause life-threatening damage to trees, and chemical control is unnecessary. Jumping oak galls can cause premature defoliation, and that is stressful for affected trees. To protect affected trees from additional stress, water during hot, dry weather and mulch to conserve soil moisture, moderate soil temperatures, and to protect trees from lawn mowers and weed whackers. Remember that two to three inches of mulch is sufficient to achieve those benefits. More than that is too much.

Damage to Foliage: Close up look at the growing, curling and distortion caused by a large population of cynipid wasp galls on a white oak. Photo: Sandy Feather, Penn State

Raking and removing fallen leaves can reduce the number of overwintering larvae and may help prevent the problem next year. Chemical control is not generally recommended for jumping oak galls, despite what seems to be tremendous damage. And chemical control would have to be timed just as the trees are leafing out – there is nothing you can apply at this point that would make a difference.

Although white oaks are the primary host for jumping oak galls in our area, other species of oaks serve as hosts in different parts of the country.

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