Cherry Galls

 Cherry Galls (Fr. Galles-cerise du chêne) are caused on Oak trees by the tiny gall wasp Cynips quercusfolii. I've never seen the wasp, which is only 3 mm long, but I regularly encounter the galls on oak leaves in the forest.

The wasps develop on the oak trees, where they are responsable for the formation of spherical galls on the underside of leaves. 

This abundant gall was appears each year in two forms, one which reproduces sexually and one which reproduces asexually ie by parthenogenesis. In the summer, after mating, the female wasps lay their eggs on the oak leaves. Then their larvae develop in the galls on the underside of the leaves, a single 2 mm larva in each gall. The galls start off as yellow-green and transform into red-brown. 

 

Spangle and Cherry galls on oak leaves.

The adult parthenogenic female wasps emerge from the galls in winter, and in the spring they lay eggs on the new leaf buds on oak trees. A gall forms, but it is very different, being only a few millimetres across and covered in red filaments. The sexual wasps emerge from these galls in May and June. In the past it was believed that the parthenogenic and sexual wasps represented two different species.