Oak Galls Are Integral Part of Gall Wasp Life Cycle

oak galls

Hidden in the oak trees of Wakulla County, gall wasps incubate their young in the tree’s branches and twigs. This apple oak gall will hatch a new generation of wasps later this year.

By Les Harrison
Wakulla County Extension Service

Sometimes a disguise is the best option for survival. This ruse can be used to blend in or to appear to be something not worth the trouble of encountering.

Mimicry is used by insects which need the appearance of being an unpleasant menu option to the creatures which prey upon them.

Viceroy butterflies, for example, are a scaled down version of the foul tasting, but similar appearing, Monarch butterflies.

Another of Wakulla County’s native insect residents is quite effective at using this technique at a different stage of its life cycle.

Several species of gall wasp use oak trees as part of their reproductive process.

Evergreen and deciduous oaks are susceptible to being inflicted with the galls. The most common are 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.

If in thick stands of trees, it can be difficult to tell which tree species is producing these eruptions.

The culprit causing these particular sores is the gall wasp species, Callirhytis cornigera. It is the most common oak gall locally and an integral part of their reproductive process.

The life cycle of these wasps is not completely understood and can vary depending on a range of environmental factors. 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 oviposit 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 hyperplasias (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.

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 laurel oaks which are common to these locations.

Other oak galls caused by wasps in the Callirhytis genus can appear on the underside of leaves, or on the end of stems.

These are sometimes referred to as oak apples.

The resemblance to apples is very general, but it is enough to deceive predators.

When it comes to survival, this facsimile is all that is needed.

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