GALL WASP

(subfamily Cynipinae), any of a group of insects (family Cynipidae, order Hymenoptera), notable for their ability to stimulate the growth of galls (tissue swellings) on plants. Some gall wasp species are gall inquilines; i.e., they do not cause the formation of galls but inhabit those made by other insects. The overgrowth of tissue, or gall, presumably is caused by a substance secreted by the immature insect living within it. Most of the approximately 600 species of gall wasps that occur in North America are about 6 to 8 mm (about 0.25 to 0.30 inch) long and black. The shiny abdomen is oval, and the thorax has a sculptured appearance. A given species of gall wasp will cause a characteristic type of gall to form on a certain part of a particular species of plant; many of the wasps attack oak trees or rose plants. Male gall wasps are rare. Reproduction is usually by parthenogenesis; i.e., larvae develop from unfertilized eggs. The egg passes through the long ovipositor of the female and into the plant tissue. After the egg hatches into a larva, the plant tissues around it begin to grow faster than normal. The gall grows with the larva, which feeds on the plant tissue and pupates into an adult within the gall. The so-called oak apple, a round, spongy, fruitlike object about 2.5 to 5 cm (1 to 2 inches) in diameter, is caused by the larvae of the gall wasp Biorhiza pallida. About 30 such larvae may develop in a single apple, or gall. The marble gall, a green or brown growth about 2.5 cm in diameter, is caused by Andricus kollari. The bedeguar gall (also called moss gall, or robin's pincushion) is commonly seen on rose bushes and is caused by the gall wasp Diplolepis rosae. One gall may contain about 50 or more larvae. Most gall wasps are not economically important. The galls of some species, however, have been used as a source of tannic acid or in the manufacture of inks or dyes.