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 so-called eukaryotes, i.e. organisms with a cell nucleus and cell membrane. These include higher fungi (Ascomycetes, rusts and smut fungi) and invertebrates such as nematodes (roundworms) and arthropods, particularly mites (Acari) and insects (Insecta). Among fungal inducers, parasitic fungi are the principal gall-forming organisms. Their number is considerable. 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, or organisms that share a particular dwelling with another species, 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, including mammals and birds.

Parasitoids

Parasitoids develop on or inside a host, which eventually dies, but only after the parasitoid’s development has been secured. For example, around 200 species of parasitoid Hymenoptera 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 emerged from galls of Biorhiza pallida alone, most of them parasites that had fed on the gall-wasp larvae (Storch & Alberti 1976). These included various moth caterpillars and the larvae of the weevil Curculio (Balaninus) villosus, which feed destructively in the multicellular branch galls (Kieffer 1901: 591–596; Hellrigl 2008: 90).