Plant Galls: Insect-Caused Growths That Look Alarming But Are Harmless

S.N. described a strange sight: "For two springs, small red or reddish-brown mounds on Chinese witchhazel leaves. Could this be lady bugs or praying mantids? If beneficial we'll leave them, but we don't know what they are."

I knew immediately: witchhazel horn gall. A particularly good year for gall-making insects, says Nina Eckberg, Cobb County Extension agent. "Folks bring in wooly, ball, and spiky ones. Galls and huge numbers of aphids and lady beetles are our biggest call source."

What Are Galls?

Galls on leaves and stems are common now. Caused by insects—usually aphids, wasps, mites, beetles, or midges. They deposit eggs in plant tissue, which swells as larvae hatch. A chemical irritant released by the female at egg-laying or by developing insects causes gall formation around eggs/larvae. Each gall insect species has a favorite host and forms characteristic shapes—large and woody, hollow, silky, or spiny. Common on oak, elm, hickory, rose, goldenrod, witchhazel.

HORN GALL

Witchhazel horn gall is caused by aphid Hormaphis hamamelidis. In early spring, aphid females hatch from overwintered eggs, crawl onto leaf buds. As leaves expand, aphids inject chemicals causing hollow, cone-shaped gall. Aphids live inside, protected, until maturity. Three generations per season; only third includes males and females. Late summer final generation lays eggs on witchhazel branches. Cycle repeats next spring.

Other galls house immature insects for only weeks. As larvae mature, they escape; gall tissue left behind is only a plant disfigurement and gardener curiosity. The relationship is not symbiotic—plant doesn't depend on gall for health. Insects completely depend on plant for part of life cycle.

CONTROL NOT NECESSARY

Gall-forming insect populations fluctuate widely year to year. A plant with few galls this year might be covered next year. Once a gall is noticed, chemical control is ineffectual. Gall insects are prey for many other insects; spraying insecticide only thwarts natural control. If you notice a gall, carefully break it open and marvel at this fascinating plant-insect relationship.

MORE INFORMATION:
Witchhazel Galls