The gall wasp causes swollen lumps in the young stems and branches. The female wasp lays her eggs in the young, soft twigs and as the wasp larvae develop, the plant grows extra cells around the wound. The result is a distorted lump in the branch. The wasps lay their eggs from spring to summer, but the galls are generally not noticeable until summer or autumn and gradually grow larger. Gall wasp will gradually weaken the tree, eventually making it un- productive.
To control the gall wasp, simply cut off the stems below the galls using sharp secateurs. Do this before winter as the larvae will be due to emerge as an adult in winter. Destroy the removed gall affected stems by burning them to make sure the larvae are killed. If you are unable to burn them, place the cutoff sections into a black plastic bag with a small amount of water. Close and seal the bag and then leave it in the sun for 3 to 4 weeks. The heat produced inside the bag will kill the larvae and prevent it from developing into a new wasp.