Many genera of Eurytomidae have been defined along biological lines which accounts for some of the difficulties encountered in trying to characterize genera morphologically. Almost all parasitic eurytomids feed on hosts confined in plant stems, galls, seeds, or in protected parts of plants. The subfamilies and their hosts in the Nearctic Region may be outlined as follows (behavior of extra-Nearctic forms is mentioned if unusual):

Rileyinae: Rileya- parasites of gall-forming cecidomyiids; Macrorileya- parasitic on tree cricket eggs (Oecanthus species); Neorileya- parasitic on eggs of Orthoptera and Heteroptera.

Heimbrinae: Nothing is known about the habits of this group. However, specimen label data in the National Collection indicate that the beetles Paragrilus (Buprestidae) and Eutinobothrus (Curculionidae) boring in stems of Sida acuta (Malvaceae) could be potential hosts.

Eurytominae: A few genera are strictly phytophagous: Tetramesa,Eurytomocahris, and Cathilaria are all reared as gall-formers on grasses (including bamboo). The genus Systole is phytophagous in the seeds of Apiaceae, Evoxystoma in grape seeds, and Bruchophagus in the seeds of Fabaceae. Bephratelloides is phytophagous in seeds of Annona. Sycophila, in the Nearctic, are mostly parasites of gall-forming Cynipidae, but a few attack other gall-forming Hymenoptera (e.g. Tetramesa Tanaeostigmatidae, and Hemadas (Pteromalidae). A single Nearctic species is recorded from figs where it may be phytophagous or, more likely, an inquiline or fig wasp parasite. In more tropical parts of the world, Sycophila is mostly associated with figs. The genus Eurytoma exhibits wide host preferences, parasitizing Lepidoptera, Coleoptera, Diptera, Hymenoptera, and Homoptera (Psyllidae) as well as being phytophagous in Anacardiaceae (Rhus), Poaceae (grass stems), Orchidaceae (Cattleya stems), Rhamnaceae (Ceanothus seeds), and Pinaceae (Pinus stems). Other Nearctic genera attack spider eggs (Desantisca), Cecidomyiidae, and Cynipidae (Tenuipetiolatus).

Most of the zoophagous species are ectoparasitic idiobionts of hosts concealed in plant tissue, although species of Neorileya are endoparasitic idiobionts in the exposed eggs of Hemiptera and Orthoptera (DeSantis 1979, 1989; Becker & Prato 1982). Alternatively, the Holarctic taxa Eurytoma obtusiventris Gahan, E. serratulae (F.) and E. tibialis Boheman (= curta) are koinobionts as they oviposit in young gall-forming tephritid larvae; the host and parasitoid develop concurrently, and eventually the parasitoid larva stimulates early pupation of the host (Varley 1937; Uhler 1951; Claridge 1961b). Larvae of phytophagous eurytomids generally feed in seeds but some are gall-formers on other parts of the plant. The biological diversity outlined for the family is represented in the largest and most common genus, Eurytoma. Most species in this genus are parasitoids, inquilines, or both in galls formed by other insects, but several are parasitoids of beetle larvae in seeds or stems (primarily Bruchidae, Curculionidae and Scolytidae), or other holometabolous larvae in plant tissues. A few are predators of spider eggs. Finally, several phytophagous species are also included in Eurytoma.

Ectoparasitic idiobiotic eurytomids usually attack the host larva or pupa (e.g. Taylor 1929). Although observations are scarce, this primitive type of parasitoid behaviour presumably occurs in Axima (host: solitary, stem-nesting bees), Bephratoides (host: xylophagous Coleoptera (e.g. Buprestidae)), Chryseida (host: Bruchidae in seeds, often of Fabaceae), and possibly in Rileya (host: gall midges (Cecidomyiidae)). Alternatively, Conoaxima parasitizes adult queens of Azteca ants living in stems of Cecropia (Brues 1922; Longino 1996) and other eurytomids (Isosomodes, Macrorileya) attack concealed eggs. Many of these egg "parasitoids" are technically predators since the parasitoid larva consumes several host eggs – for example, the North American Macrorileya oecanthi (Ashmead) on tettigoniid eggs (Smith 1930), the European Archirileya and Eurytoma oophaga Silvestri also on tettigoniid eggs (Silvestri 1920), and E. (Desantisca) arachnovora Hesse and E. (D.) latrodecti Fullaway on spider eggs (McMurty 1978).

The Asian Eurytoma monemae Ruschka has a very unusual biology. This species is a gregarious ectoparasitoid of mature lepidopteran larvae in their cocoons (Piel 1933). The female eurytomid is dependent upon another parasitoid to gain access to the host, i.e. she follows a species of Chrysis (Chrysididae) and uses the puncture of the latter through which to oviposit. The larvae develop, pupate within the host cocoon, adults emerge through the same puncture hole, and sometimes another generation of parasitoids develops on the same host. Dependency upon another parasitoid to gain access to the host is a form of cleptoparasitism and is known to occur in a few other eurytomids, such as the North American E. pini Bugbee, a parasitoid of the pine shoot moth (Arthur 1961).

Phytophagy appears to be restricted to certain members of the subfamily Eurytominae. Phytophagous genera in the Neotropics include: Bephratelloides in seeds of Annonaceae; Bruchophagus in seeds of herbaceous Fabaceae; Prodecatoma in seeds of Myrtaceae, Vitaceae and Rubiaceae where they are apparently phytophagous; Systole in seeds of Apiaceae; and Tetramesa, which form galls in stems or seeds of Poaceae. Eurytoma orchidearum (Westw.) has been found feeding in the buds and pseudobulbs of various species of Cattleya, Laelia, Epidendrum, Brassavola, and Cypripedium (Orchidaceae) (Swezey 1945; Tanada 1953; Noyes 1998). The Neotropical species, E. attiva Burks and E. cressoni are reported from seeds of Cordia (Boraginaceae) (Burks 1958; Williams 1960), while other Neotropical species of Eurytoma are associated with Podocarpaceae, Orchidaceae (including vanilla; presumably seed predators), Bromeliaceae (A. Costa, pers. comm.), Euphorbiaceae, Melastomataceae, and Rubiaceae (Hanson, unpublished), but the actual relationships require confirmation. Preliminary observations suggest that some Eurytoma induce galls on other parts of the plant: leaves of Zygia (Fabaceae) and Myrcia (Myrtaceae), and on stems of Eugenia (Myrtaceae). Finally, several extralimital species of Eurytoma are phytophagous in seeds of Juniperus (Cupressaceae), Ceanothus (Rhamnaceae) and Rhus (Anacardiaceae) (all Nearctic) (Burks in Krombein et al. 1979). Another species in the eastern Mediterranean is a serious pest in the seeds of almonds (Rosaceae) (Plaut 1971, 1972; Zerova & Fursov 1991).

The manner in which phytophagy evolved in the Eurytomidae can perhaps be seen in both Rileyinae (Rileya tegularis Gahan (Hawkins & Goeden 1984)) and Eurytominae (Eurytoma pachyneuron Girault (Phillips 1917); Eurytoma flavimana Boheman (Claridge 1961a); Eurytoma parva (Girault) (Phillips 1927)) as the taxa listed can switch from entomophagy to phytophagy during their larval existence. This mixed feeding habit is apparently an important component in parasitoid success (Hawkins & Goeden 1984). With respect to the phytophagous habit, Bugbee (1936) felt that evidence for ancestral host utilization, "...favours a plant feeding origin" while Nikol’skaya (1956) favored parasitic ancestry. Claridge (1961a) cited several examples that illuminate the possibility of multiple unique derivations of phytophagy within Eurytominae and discounted the arguments of Bugbee (1936) as subjective. Despite the arguments of Claridge (1961a), Malyshev (1968) regarded phytophagy as secondary (i.e. phytophagy redeveloped in the entomophagous Eurytomidae which had originally radiated as phytophages) in Chalcidoidea and as a reversal in dietary evolution in several taxa. He equated retention of phytophagy as plesiomorphic and equivalent to certain morphological features (elongated body, greater number of antennal segments) occurring in certain phytophagous taxa, but this does not hold true in all instances (e.g. Archirileya, Macrorileya). It may be that the structural constraints imposed upon eurytomids developing in stems (as phytophages or egg predators) are the driving force behind some of the gross morphological similarities between these taxa (Tetramesa compared to Isosomodes). Similarly, the constraints imposed by developing within spherical eggs versus spherical seeds (Neorileya compared to Systole).

Axima species

Bruchodape ignota Burks

Chryseida species