Plant galls are caused by tiny wasps of the Cynipidae family. The wasps are the size of ants but can be identified to family level by their distinctive wing venation pattern (using a simple wing venation based key such as the FSC Aidgap Key to Bees, Ants and Wasps if you have a specimen). The most common host plants are oaks. Female wasps lay their eggs on the host plant and the plant reacts by swelling, creating a shelter for the young grubs to hatch in. The larvae eat the plant, but do not cause any real harm to their host. Some galls contain a single grub, others are multichambered and contain several larvae. The larvae pupate inside their gall and usually overwinter in this state before emerging as adult wasps. Some species of gall wasp are parthenogenic, with females who lay fertile eggs without the presence of males. The life cycles of many of these species are particularly complex, with alternating generations of bisexual and parthenogenic individuals. The different generations cause different types of galls. A few species do not produce their own galls, but parasitise the galls of others. Other parasitic wasps also use gall hosts, so you cannot guarantee that the occupants are the original gall makers.

Robin's Pincushion: a commonly encountered and easy to recognise gall, found on wild roses. It is also known as a bedeguar gall. It has multiple chambers and a woody exterior covered soft bristles. The gall matures in late autumn. It is the creation of Diploepis rosae, a parthenogenic species with a black head and thorax and red abdomen. Below, photographed in the Foret de Preuilly, June.

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