Which Organisms Live in Plant Galls?

The Gall Inducers Themselves

Gall inducers can be insects, mites and nematodes, as well as viruses, bacteria and fungi. Most gall inducers belong to the eukaryotes, i.e. organisms with a cell nucleus and a cell membrane. These include higher fungi (Ascomycetes, rusts and smut fungi), as well as invertebrates such as nematodes (roundworms) and arthropods, particularly mites (Acari) and insects (Insecta). Among fungal gall inducers, parasitic fungi are the principal gall-forming organisms. They are particularly numerous. These fungi were formerly included among the “plant gall inducers” (“phytocecidia”), a group that otherwise essentially comprised only parasitic plants such as mistletoe (Viscum album) and dodder (Cuscuta epilinum).

Inquilines (Synoekes)

Inquilines are organisms that share a particular dwelling with another species. They may live in plant galls as well as in leaf mines and galleries. Typical inquilines include certain species of gall midges, gall wasps, gall mites, leaf-mining flies and beetles. In many cases, neither the original occupants nor their dwelling are harmed. Similar forms of cohabitation also occur among other organisms, for example among mammals and birds.

Parasitoids

Parasitoids develop on or inside a host, which eventually dies after the parasitoid has completed its development. For example, around 200 species of parasitoid Hymenoptera living in cynipid galls on oaks (Quercus) are known from the Western Palaearctic (Nieves Aldrey 2001: 80–83; Melika 2006, I: 54–55).

Parasites

Parasites develop on or inside a host, while the host itself survives. More than 75 different insect species have been recorded from the galls of Biorhiza pallida alone, most of them parasites that fed on gall-wasp larvae (Storch & Alberti 1976). These included various moth caterpillars and the larvae of the weevil Curculio (Balaninus) villosus, which feed destructively within the branched, multilocular galls (Kieffer 1901: 591–596; Hellrigl 2008: 90).