Knopper gall wasp (Andricus quercuscalicis) is a gall wasp species that infests two types of oak trees: common oak, and turkey oak. The word knopper is derived from the German word ‘knoppe’, which is defined as a swelling or protuberance. It also refers to a type of helmet worn during the 17th century.
Knopper galls form on common oak when the adults deposit eggs into the developing buds and acorns. Once the larvae hatch, they grow within the galls, causing a partial or complete distortion of the buds and acorns. While knopper galls are generally an aesthetic nuisance, severely infested trees may experience a drastic reduction in seed yield, inhibiting plant reproduction.
Distribution & Habitat
Knopper gall wasp has become widely established in the United Kingdom, and North America. It may be found wherever susceptible plants are present. The insect is particularly abundant in the United Kingdom, where it has garnered notoriety for the distinct shape of the galls it induces.
Knopper gall wasp infests Quercus robur, often referred to as common oak, English oak, European oak, or pedunculate oak, and turkey oak, also known as Austrian oak.
Description of Knopper Galls
The galls induced by knopper gall wasp initally appear as a sticky mass of green to yellow-green plant tissue arising from the infested plant buds or acorns. The galls become ridged as they enlargen. The degree of ridging that occurs on the galls is variable, depending on the number of grubs present within the gall. If several grubs are competing for space within the gall, it may assume a more malformed appearance. Two to three galls can form on the same bud.
Knopper gall wasp produces two generations per year: a sexual generation, and a parthenogenetic generation. Common and turkey oak are required for the insect to complete its life cycle. In spring, a generation of females wriggle through vents located in the tops of the previous year’s galls. After emerging, the females use their ovipositors to insert eggs in small conical galls that form on the male catkins of turkey oaks. Within these galls, a sexual generation of male and female wasps becomes established. Once mature, the wasps vacate the galls, and mate. Upon mating, the males expire, and the females migrate to common oaks, where they inject eggs into susceptible plant buds and acorns.
Once the second generation of grubs have hatched, they secrete chemicals that cause the buds and acorns to be converted into galls. Most knopper galls will contain a single grub. Some may harbor several grubs. At first, the buds and acorns appear to develop normally until summer, when they are partially or wholly displaced by the galls. The galls thicken in fall, turning woody and brown. They are cast from the tree shortly thereafter. The grubs overwinter, and pupate within the galls. They emerge as adult females the following spring.
Symptoms of Infestation
The most visible symptom of infestation is the formation of the knopper galls on the buds and acorns of common oak. The galls are green to yellow-green at first, and sticky to the touch. The galls often function as a habitat for other microorganisms. Cynipid wasps are one of the most frequent occupants. The adult cynipid wasps lay their eggs in the galls. Once the larvae hatch, they feast on the surrounding woody tissue.
- Knopper gall wasp is not considered a significant nuisance. However, severe infestations can be treated with a registered insecticide.
- Applications should be administered in spring, when the adults first appear.
- Several parasitoids, particularly chalcid and ichneumon wasps, inject their eggs into the grubs. The eggs hatch, and the larvae devour the grubs from within.
Photo courtesy of Lairich Rig CC-by-2.0