Mossy Growth on a Rose Bush

Cynipid Gall Wasp

• This wasp species (Diplolepis spinosa) consists almost entirely of females that can reproduce without fertilization from a male wasp. The females measure less than an inch long, with reddish heads and yellowish red coloration on parts of their abdomen and legs. These wasps lay their eggs on the rose bush in the spring. The plant tissue then changes its normal growth pattern to swell around the eggs and protect the larvae from weather and natural predators. Adults emerge from the galls in the summer or the following spring and the cycle repeats again.

Gall Identification

• Mossy rose galls develop on leaves and stems of rose bushes and look like a green to bright crimson, mosslike sphere or swelling. The gall eventually fades to a rusty brown color and has a filamentous texture, like moss. Mossy rose galls may be very small, or they can grow to the size of a golf ball, and they're rich in nutrients that the wasp larvae need to grow into adults.

Damage

• Most of the damage caused by mossy rose galls is cosmetic. Galls on the stem of a rose bush can damage the parts of the plant above the location of the gall, but they don’t typically affect the vigor of otherwise healthy rose bushes. Mossy rose galls on the leaves of your rose bush can give the bush a scraggly appearance if they're large or numerous enough, but they are not usually fatal to your plant. A severe or prolonged infestation of galls, however, can weaken or kill parts of the bush because they steal nutrients that the rose bush needs.

Control

• Insecticides are not effective in eliminating the wasps that cause mossy rose galls, so the best way to control infestations is by ensuring your rose bush has adequate water and nutrients to ensure it is healthy and vigorous. To prevent severe infestations, remove the galls from stems in the fall, after the rose bush has dropped its leaves and the galls are easy to see. Cut below the gall and above a bud. Make sure to prune out every gall. Missing just one can result in the reintroduction of 30 to 40 mature wasps the following spring. Pruning in the fall also removes the galls before the larvae have time to emerge as adults. In addition to pruning out stem galls, clean up fallen leaves from the soil, and remove galled leaves from your rose bush as soon as you see them.