Aphelinidae are among the smallest and most unique chalcid wasps. Many of the 1300+ species in the family specialize on agriculturally impactful insects including whiteflies, scale insects, mealybugs and aphids. In many species the males and females undergo development in different host organisms, with the male sometimes developing on females of the same species. Most aphelinids are less than 1.5 mm in length, brown to yellow in color and incredibly soft bodied, making them a challenge to house in museums due to their tendency to shrivel.
While most Aphelinidae lays their eggs in the bodies of scale insects or juvenile whiteflies, some species use alternate hosts and parasitic strategies. Eutrichosomella blattophaga is a parasite of cockroach eggs, many species of Aphytis lay their eggs on the outside of armored scale, Centradora xiphidii develops on locust eggs, and species of Marietta are parasites of other parasitic insects (hyperparasites). Males of Coccophagus and Encarsia are frequently parasites of females of their own species (autoparasites). While parasitizing one’s conspecifics might seem like an evolutionary dead-end, these two genera are the most biodiverse in the entire family. The reproductive behavior and physiology of some species of Aphelinidae is influenced by the presence of endosymbiotic bacteria. Wolbachia is known to induce clonal reproduction (parthenogenesis), resulting in completely female populations, and Cardinium cause infected and uninfected individuals to be reproductively incompatible.
Species of Aphelinidae are widely used in classical biological control of major agricultural pests. For example, Aphelinus abdominalis provides control of potato aphid across Europe, Encarsia noyesi was released in Southern California to control giant white fly (a pest of hibiscus and other decorative garden plants), and many species of Eretmocerus have been released to control populations of Bemisia tabaci. Several species of Aphelinidae are available commerically for augmentative release in greenhouses and gardens, including Encarsia formosa the primary parasite of the greenhouse whitefly, Trialeurodis vaporariorum.
Aphelinidae contains 1385 species in 5 subfamlies: Coccophaginae (938 species in 17 genera), Aphelininae (358 species in 21 genera), Eretmocerinae (85 species in one genus), Calesinae (14 species in one genus) and Eriaphytinae (two species in one genus). Recent advances in the phylogenetics of Aphelinidae have resulted in the exclusion of several genera from the family (Ablerus, Euryischia, Euryischomyia, Myiocnema, Eunotiscus, and Promuscidea). Ongoing analysis also indicates that Calesinae should also be excluded from Aphelinidae.