Evidence of the activities of wasps can be found even when the insects themselves are long gone. Their mud or paper nests remain affixed to branches, and beneath the eaves of our homes. Sometimes, however, those nests are not recognizable as such. Plant galls are one example of this.
Many types of organisms can create galls. Flies, aphids, adelgids, mites, nematode worms, bacteria, viruses, and even fungi can stimulate abnormal plant growth, but the gall wasps of the family Cynipidae are perhaps the most common culprits. There are nearly 2,500 species worldwide, more than 750 of those occurring north of Mexico. Here in North America, the overwhelming majority of gall wasps are associated with just two groups of plants: the oak family and the rose family.
Nothing about the biology of cynipids seems to be very straightforward. Most species present an “alternation of generations” life cycle as figured in the diagram below from Some Plant Galls of Illinois. The spring population is normal, with winged adults of both genders emerging and mating to create the next generation. There is also an “agamic” generation represented only by females. These insects, which typically emerge in late autumn or winter, are wingless, and capable of reproducing without insemination by males. This kind of asexual reproduction is termed “parthenogenesis,” and many insects are capable of it. I found this example of just such an agamic female gall wasp on Thanksgiving day at the Cheyenne Mountain Zoo in Colorado Springs.
The large, hard galls most obvious to us are the ones produced *by* the spring bisexual generation, and the ones from which the asexual winter generation emerges. The spring galls are soft, small, and inconspicuous, usually on buds, flowers, or young leaves of the host plant.
Chemical compounds introduced by the adult female wasp in the course of laying her eggs, and/or by the larval wasp in the course of feeding, are probably responsible for the form and size of the gall produced, stimulating the expansion of cells in the host plant’s tissues as well as the type of growth in the vicinity of the insect. One can often determine at least the genus of the wasp by examining the gall it forms and knowing the exact host plant. The gall, below, for example, is a “Mossy Rose Gall,” produced by Diplolepis rosae, a species probably introduced from Europe in the course of cultivating roses. This specimen was also imaged at the Cheyenne Mountain Zoo.
The rough oak bullet galls pictured below are solid. Here on the Front Range, three species of wasps in the genus Disholcaspis create these kinds of galls on one host plant: Gambel’s Oak (aka “Shin Oak”), Quercus gambelii. I have observed that oaks are either gall-free or nearly so, or very heavily infested.
Ironically, galls rarely cause more than mere cosmetic “damage” to the host plant. An affected tree or shrub won’t win a beauty contest, but neither is it likely to keel over.
Much remains to be learned about galls and their makers, perhaps because so many other insects exploit galls and/or parasitize gall wasp larvae. Collect a large number of galls and you are as likely to raise an assortment of parasitic wasps as you are the gall wasps you would expect. Further complicating matters, still other insects lay their eggs in galls because of the abundance of food that galls present, and their nutritional value. These “houseguests” are called “inquilines” that live off the work of the gall wasp mother, but that do not affect the survival of her offspring.
Should you decide to take up the study of gall wasps, you will be in good company. Dr. Alfred C. Kinsey (yes, that Alfred Kinsey of human sexuality fame) began his career by spending two decades studying cynipids at Indiana University. I’ll be grateful if I can identify two other types of galls that appear on Gambel’s Oak. One is a leaf gall (top image in this post), and the other is a more linear twig gall.
Sources: Winterringer, Glen S. 1961. Some Plant Galls of Illinois. Springfield: Illinois State Museum, Story of Illinois No. 12. 51 pp.