Answer: What you're describing sounds like a common problem called mossy rose gall. What you're finding inside are probably not seeds, but eggs or larvae.
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. 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.
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