Sometimes camouflage is the best option for making inroads toward one’s objective. This ruse can be quite effective for going unnoticed, until an event exposes the truth of the situation.
Such has been the case with many defoliated or partially defoliated plants and trees as the autumn progresses. Any resident of north Florida may stare into the boughs of recently denuded trees and wonder about the recently exposed structures growing within.
In the case of local oaks, several species of gall wasps use these trees as part of their reproductive process. The byproduct, galls, is quite noticeable when suddenly revealed for all to view.
Evergreen and deciduous oaks both 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.
Usually hidden in oak trees, gall wasps incubate their young in the tree’s branches and twigs. Autumn will reveal many galls, and they can be the site of limb and twig breaks.
To the uninitiated these arboreal blisters may appear to be oversized and out-of-place acorns. If in a thick stand 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. The oak galls it creates are the most common locally and are 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 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.
Severe gall wasp infestations have been regularly observed in several north and central Florida counties. Extremely high numbers of twig galls have occurred on laurel oaks which are common to these locations.
Contrary to urban legend, this gall wasp is not an exotic invasive species or a purposely introduced insect used to eradicate certain oak species from the area.
Other less common 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.
Only time and the flush of leaf growth will return the galls to their obscured existence as the incubation site for next year’s gall wasp population. All things considered it will be good to have them out of sight.
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