Skip to main content

Gall Wasps Create Tumor-Like Growths on Trees

The thick summer forest foliage hides a wide variety of living creatures and plants in the rarely examined tree canopies. Wakulla County’s oaks, both deciduous and evergreen, are susceptible to a collection of knotty, hard tumor-like growths on leaves, twigs, and limbs.

These plump growths offer a stark contrast to the straight thin twigs and branches which shoot off at angles forming a porous maze for supporting leaves. The bulbous lumps may be few and scattered or come in heavy concentrations.

To the uninitiated these arboreal blisters may appear to be oversized and out-of-place acorns. In a thick stand of trees, it can be difficult to tell which tree species is producing these eruptions.   The culprit causing these sores is the gall wasp, Callirhytis. Locally, the two tiny members of this insect family use oaks as an integral part of their reproductive process.

The life cycle of these wasps is not completely understood as they have not been studied extensively. It has been verified the process begins when the female wasps emerge from twig galls in late spring to early summer.

These females quickly lay eggs on the underside of leaf veins. Small vein galls appear within a few weeks in early summer with male and female wasps quickly emerging from the vein galls. Mated females then oviposits eggs in young oak twigs. Twig galls appear in the spring of the following year. Two or more years are required for the immature gall wasps to complete development in the twig galls.

The galls provide shelter, protection, and food for the immature wasps. Inside a gall, the larvae are surrounded by nutrient rich tissues. As the larvae reach maturity small spines or horns become evident on the gall. An adult wasp emerges from each horn.

These wasps are barely visible at 2 millimeters in length, which is about the thickness of a half-dollar coin. The wasp’s petite size notwithstanding, the individual galls can be almost a foot in length.

The gall formation is a result of bark cell hypertrophy (over-growth) and hyperplasia (cell proliferation) after the eggs are laid.

Individual trees or small groups of trees usually experience a slight to moderate infestation of these wasp galls, depending on the annual environmental factors. Widespread infestations are not common.

However, severe gall wasp infestations have been regularly observed in several north and central Florida counties, including Wakulla County. Extremely high numbers of twig galls have occurred on thousands of laurel oaks in these locations.

These infestations have affected young to mature trees in woodlands and residential areas. The overabundance of twig galls has results in notable levels of branch dieback, crown thinning, and in rare cases, tree mortality.

Young galls are slight, tumor-like swellings of tree’s periderm tissue, the innermost living area of the bark. Bark color of young galls is a greenish-brown and bark surfaces are smooth except for protruding lenticels which act as pores for the tree.

Mature twig galls persist as distinct bulges or form massive compound galls more than two inched in diameter and may cover much of a branch. Older galls become very woody, dark and discolored.

The horns used for new wasp to emerge will deteriorate quickly as the new generations of wasps seek fresh branches. Most are never noticed while they remain hidden under Wakulla County’s tree canopy.

To learn more about gall wasp in Wakulla County, contact your UF/IFAS Wakulla Extension Office at 850-926-3931.

Les Harrison is the Wakulla County Extension Director. He can be reached by email at harrisog@ufl.edu or at (850) 926-3931.