Mossy rose gall, Diplolepis rosea; Spiny rose gall, Diplolepis bicolor:

1. Insect pests of roses (Kevin Stroom, Jody Fetzer,  and Vera Krischik, U Minnsota Extension Service)
 

2. Insecticide Suggestions to Manage Landscape Tree and Shrub Insects (U of Minnesota Extension Service)
 

3. Roses for the North (Kathy Zuzek, Marcia Richards, Steve McNamara, and Harold Pellett, U Minnsota Extension Service)

Rose Stem Borer

The rose stem borer, Agrilus aurichalceus, damages rose canes when larvae tunnel in a spiral fashion beneath the bark, girdling and killing the canes (figures below). Their presence is indicated when a cane dies above the point of borer tunneling. Leaves on the infected cane turn brown as they die, creating a “flag” among healthy, green-leaved canes. On close observation, a swelling or gall on the infected cane can be seen below the dead tissue, indicating where the borer’s tunneling occurred. The gall formation weakens canes and it is common to see infected canes broken off by wind. The quickest and most effective control is removal and disposal of infected canes in fall.

Mossy Rose Gall

Mossy rose galls are caused by Diplolepis spinosa, a cynipid gall wasp. These galls are common on wild roses of North America, from Ontario to Alberta in Canada and throughout most of the northern United States. They are becoming common on Rugosa cultivars.17 The presence of these insects is indicated by the formation of spherical, golf ball-size, spiny galls on the canes of host plants.

The development of these galls is stimulated in the spring by newly hatched larvae. The galls encase the larvae until adult wasps emerge the following spring. The galls are unsightly and alter the plant’s shape. They also stress the host plant, behaving like nutrient sinks, drawing nutrients away from the rest of the plant. Large numbers of galls on a plant can kill the plant.

Insecticides have no effect on the wasp that causes mossy rose gall. The most effective control is physical removal and disposal of galls in autumn after leaves have dropped and galls are visible. It is important to dispose of all galls since even a single missed gall can produce and reintroduce 30 to 40 mature wasps to the garden the following spring.

Mossy rose galls caused by Diplolepis spinosa on rose canes. Hybrid Rugosas are particularly susceptible to this disfiguring gall.

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