A wasp did this to a rose bush. The sample showed up this week in the Ada County Master Gardener lab, so be on the lookout for this in your home roses. The gall is caused by Diplolepis spinosa, a cynipid gall wasp. The story is a bit disgusting. The wasps lay their eggs in the stem and the newly-hatched larvae grow inside until they are adults. The plant is tricked into growing the gall to house the babies, stealing nutrients from the plant. A lot of galls can kill the rose. Insecticides aren't an option because they don't work. The galls have to be picked off and disposed of. Check for galls that may have fallen from the plant in autumn.