Along with Encyrtidae, this "family" provides most of the biocontrol agents which DeBach (1964) characterized as providing complete or substantial biocontrol. Each family has contributed to about 13 such cases of control. As has been previously discussed under the families Encyrtidae and Eulophidae, what constitutes the family Aphelinidae is somewhat of a mystery. Recent studies indicate that even some of the included subfamilies may be unnatural assemblages or may be misplaced in this family (e.g., Eriaporinae). The relationship of this family to the other familes of Chalcidoidea remains a subject of debate. A brief summary of the problem was given by Hayat (1994).

The classification of aphelinids is in a state of flux. Scarcely any author agrees on how many subfamilies are present or where genera should be placed. In 1976 Yasnosh reviewed the family and split it into 7 subfamilies (without providing a key). Rosen and DeBach (1979) stated that "so much splitting" was not justified and recognized only 3 subfamilies: Aphelininae, Coccophaginae, and Calesinae. Hayat (1983), in his key to world genera, noted the confusion and refused to follow any tribal or subfamily scheme.

Generic limits are still open to interpretation due to an absence of morphological characters that reliably differentiate taxa. For example, the genera Prospaltella, Encarsia, and Aspidiotiphagus have been synonymized by Viggiani and Mazzone (1979) under Encarsia causing several well known names (with hundreds of references in the economic literature) to be lost from use. This synonymy is not acceptable to everyone, and, in fact, DeBach and Rose (1981) have already removed Aspidiotiphagus from synonymy. Hayat (l983) and Yasnosh (1983) independently produced world keys to genera of the family, and this is certain to help stabilize generic names. However, other workers have not agreed entirely with their assessments. Flanders (1953) gave a key to genera based upon hosts, endo- or ectoparasitism, primary or hyperparasitism, and a little morphology. In general this key is not very practical. Woolley (in Gibson, et al. 1997) has produced a key to North American genera.

At the specific level, problems become even more intense due to the presence of sibling or semispecies in at least one genus and the possibility that this phenomenon occurs more often than we suspect. Aphytis was monographed at the world level, in a classic biosystematic study, by Rosen and DeBach (1979). To identify species in this genus, it becomes necessary to know factors such as pupal coloration, reproductive biology of adults (i.e. uniparental or biparental), and host preferences in addition to fairly detailed, minute morphological differences. Among the few keys to Nearctic species is that of Schauff et al. (1996) to the species of Encarsia parasitic on whiteflies.

In general, the aphelinids are rarely longer than 1 mm, so that handling and studying them is no easy matter. Like many eulophids they shrivel badly when air dried. Many genera need to be mounted on slides for critical study.

STATISTICS: Number of world species: about 1000 (120 Nearctic); number of world genera: 38 (15 Nearctic).

BIOLOGY: Taxa of the Aphelinidae have been delimited as much on host preferences as on morphology. This is an interesting approach, but one might question the validity of using biological rather than morphological characters when one examines the host records. Yashnosh (1976) defines the hosts of her 6 subfamilies (hosts not known for 1 subfamily) as follows: Coccophaginae --endoparasitic on Coccidae, Asterolecaniidae, Pseudococcidae and Acleridae; Physcinae -- endoparasites of Diaspididae; Prospaltellinae -- endoparasites of Diaspididae and Aleyrodidae; Aphelininae -- endoparasites of Aphididae; Aphytinae -- endo- and ectoparasites of Diaspididae and Aleyrodidae, secondary parasites of Coccidae and Psyllidae, primary parasites of eggs of Orthoptera and Heteroptera, puparia of Diptera (Cecidomyiidae), and larvae of Dryinidae; Azotinae -- secondary parasites of Coccidae and Aleyrodidae, primary parasites in eggs of Cicadellidae and Lepidoptera.

There is some overlap of hosts between subfamilies as well as some diversity of hosts within subfamilies. Additionally, the diversity of biology and host within a single species might be considered somewhat disconcerting to any host related scheme of relationship. For example, one species of Encarsia is known which has a scale host in the female, but males develop on lepidopterous eggs. Rosen and DeBach (1979:104) provide a good example of how some genera may be closely related, yet have divergent host requirements. For example, Aphytis are primary ectoparasites of scales, Marietta are secondary ectoparasites of encyrtids and aphelinids on scales, and Centrodora and Tumidiscapus are primary parasites presumably ectoparasitic on full-grown embryos within eggs as well as dipterous puparia. The "common denominator" of all these habits is that the parasitic larvae develop "in a very similar ecological situation, as ectoparasites on a soft-bodied host enclosed in an empty 'shell'." This would seems to indicate that host data alone is not without limitations as far as classification schemes are concerned.

One final biological oddity may be noted in Aphelinidae, and it is quite complex in its many manifestations. This is the development of adelphoparasitism, where males develop as hyperparasites of females of their own species (obligate adelphoparasitism or autoparasitism) or of females of different species (facultative adelphoparasitism). Much of this behavior is dependent upon the mated or unmated condition of the female and is very complex. Flanders (1967) gave a complete assessment of the situation.

DISTINGUISHING CHARACTERS: As has been mentioned previously, aphelinids are not always easily placed to family. The characters generally given to separate the family are: small size (1 mm or less), generally 5 segmented tarsi (thus different from the 4 segmented tarsi of Eulophidae except that some aphelinids have 4 segmented tarsi), and antennae with eight or less segments (thus different from the usually 11-segmented antennae of Encrytidae). A number of exceptions occur, however, so that while the above characters are "generally" good, they are not always correct. A problem arises in that for such small specimens, a count of antennal or tarsal segments is not always easy. Under the Eulophidae, we suggested ways (i.e. by size and host association) to separate the few numbers which might be confused. Size is also useful for separating aphelinids from encyrtids, but host is not, because both families are found in much the same habitat. In general, the fewer segmented antennae (8 or less in aphelinids, about 11 in encyrtids, although a very few genera have 8 or less) is nearly always useful. Also in the encyrtids the transverse axillae meeting medially are very characteristic, but in aphelinids the axillae rarely meet. In addition, most aphelinids have the notauli present, while this condition is quite rare in encyrtids.

COLLECTING: Aphelinids are easily reared from scale and whitefly material and may be swept from plants infested with such hosts.

DISTRIBUTION: Apparently well distributed throughout the world, and distributed by man through citriculture and biocontrol programs.