Rose Galls by George Giltner, Louisiana Master Gardener
If you cut inside of plant galls, chances are that you will discover an insect larva. The bugs have found a unique way to provide a protective habitat and food for the developing larvae. The exact mechanisms for gall formation are variable, but plant growth chemicals released by larval salivary secretions cause these distortions. Some galls are smooth while others are spikes, or fibrous like the mossy rose gall. Other solid galls are produced by mites, nematodes, bacteria, fungi, and viruses.
The mossy rose gall is the common golf ball sized growth caused by the 0.2 inch cynipid wasp (Diplolepis rosae). Chances are that you will never see these small wasps, but you will notice the galls. Each one of these growths is capable of producing about 30 additional wasps which will be seeking other Moss Roses (Rosa centifolia mucosa), R. rubiginosa, R.dumalis, and R. rubifolia. Therefore it is wise to prune off these unsightly growths before they mature.
Most insecticides are not effective in controlling these wasps. Since the typical infection is usually one to two galls, it is much easier to just prune and destroy the gall. Make a slightly angled cut about ¼ inch above a bud in pruning.
Birds, especially woodpeckers enjoy feeding on the emerging wasps from the Mossy Rose gall which occurs from May to August. Mice and other small mammals recover the larvae or pupae from the galls. Also parasitoid insects are natural predators that attack the galls.
An interesting fact is that nearly all of the cynipid wasps produce asexually. A bacterium, Wolbackia, causes the infected females to produce only female progeny. When an antibiotic are used to kill the bacteria, females then produce normal quantities of both males and females.