Tallahassee Democrat
April 10, 2015
By Les Harrison
The knots on this oak tree are caused by a tiny wasp. Photo by Les Harrison.
Mothers want what is best for their offspring. Surprisingly, one of the greatest mothers of all may be the tiny wasp, Callirhytis. Often mistaken as a fly, the gall wasp provides her young with all of the food and comfort any loving mother would give. Strange as they may be, the galls found on oak trees are a source of shelter and sustenance for baby wasps.
Locally, the two tiny members of this insect family use oaks as part of their reproductive process. The life cycle of these wasps is a bit of a mystery as they have not been studied extensively.
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 veins of leaves. 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. Fresh twig galls appear in the spring of the following year among older galls. 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 tissues rich in nutrients. 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 observed in several north and central Florida counties, especially on droughty and infertile soils. Extremely high numbers of twig galls have occurred on thousands of laurel oaks locally.
These infestations have affected young to mature trees in woodlands and residential areas. The overabundance of twig galls has resulted 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 a majority of branches. Older galls become very woody and discolored. The horns which are used for new wasp to emerge will have deteriorated and be missing on the old galls.
Les Harrison is the UF/IFAS Wakulla County Extension Director, Agriculture and Natural Resources. For gardening questions, email us at Ask-A-Mastergardener@leoncountyfl.gov
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