Checklist of UK Recorded Eurytomidae

Description & Statistics

This is a moderately sized cosmopolitan family within Chalcidoidae with circa 1424 described species in 88 genera placed in 3 subfamilies as follows Eurytominae (73/1366), Heimbrinae (2/7), Rileyinae (12/50), unplaced (1/1). This is a common family, widespread throughout the world. It is perhaps better represented in temperate climates than tropical ones. Important morphological characters include head in frontal aspect round or semiquadrate; antenna less than 13-segmented with upto 8 funicular segments; pronotum quadrate, large; hind tibiae with 2 apical spurs. The forewing has a short marginal vein; head and pronotum with coarse, deep, thimble-like punctations, usually dull black, not shiny; head, thorax and antennae often a bit hairy.

Main diagnostic characters

Zerova (1978/1987), as translated from the Russian, discussed this family as "Chalcids of medium size, usually 3.0 to 4.0 mm, rarely 1.0 to 2.0 mm. Body black, rarely black with yellow tinge, yellow, or brownish; sometimes (in Nikanoria) with greenish metallic sheen. Head and thorax with alveolate, pitted, or rugulose sculpture, and abdomen usually more or less smooth. Antennae usually with 10 to 11 segments, with one ring and four- to seven-segmented funicle, and two- to three-segmented clava. Pronotum long and rectangular. Apterous forms rare. Abdomen of male often with long petiole. Phytophagous eurytomids are known, whose larvae develop in the stems of cereals, seeds, and galls, as well as entomophagous eurytomids, which develop at the cost of various insects that predominantly lead a hidden life inside the tissues of plants (gall-forming insects, bark beetles, and pests of cones and seeds). The larvae of many species, mostly species of the genus Eurytoma, are mixed feeders, destroying host larvae hidden in galls or stems of plants, and then consuming the plant. Exclusively phytophagous forms include species of the genera Tetramesa, Bruchophagus, Systole and Philachyra."

Most entomophagous Eurytomidae are primary, solitary ectoparasitoids of gallicolous or otherwise endophagous, dipterous, hymenopterous, or coleopterous larvae. Some hyperparasitoids are known as well as several species that are egg predators. Many eurytomids are phytophagous; some entomophagous species are facultatively phytophagous. Phytophagous species attack seeds (hence "seed chalcids"), and stems or are gallicolous. Eurytomids have not been used much for biological control.

"The genus Eurytoma comprises numerous phyto- and entomophagous forms. Only parasitic forms are known among species of Eudecatoma, Nikanoria and Archirileya. In world fauna 49 genera with more than 600 species of eurytomids have been described; 16 genera are known from the Palearctic, of which 10 have been recorded in Europe."

Further Description.

There are many phytophagous as well as entomophagous species among the Eurytomidae, the dominant genus Eurytoma including many species which develop in plant seeds, while Harmolita forms galls in the stems of grasses and other plants. Common phytophagous Eurytomidae are the wheat jointworm, Harmolita tritici Fitch, the clover seed chalcid, Bruchophagus gibbus Boh., and the wheat straw worm, H. grandis Riley. The plant feeding habit in Eurytomidae and related families was discussed by Gahan (1922), who also noted that some species are parasitic in their early stages and phytophagous later.

There is considerable information on Eurytoma spp., which show diverse host preferences. Some are external parasitoids of Hymenoptera larvae (principally Cynipoidea in galls), while others attack larvae of Coleoptera, Diptera and Lepidoptera in tunnels and galls. Other species are wholly hyperparasitic. Although most Eurytomidae are endoparasitic in larvae of gall-making tephritids, E. oophaga is predaceous on the eggs of Oecanthus, and species of Archirileya and Macrorileya have the same host preferences. Rileya spp. attack larvae of Cecidomyiidae, Conoaxima spp. are parasitic in or on adult queens of ants in the genus Azteca that inhabit the stems of Cecropia (Brues 1922), and Axima has been found in nests of small carpenter bees (Clausen 1940).

In Britain the Eurytomidae contains species which exhibit a wide range of biologies, but the majority seem to be endophytic, either as phytophages or as parasitoids of phytophagous insects. There are two large groups of phytophagous eurytomids in Britain, those that develop on endosperm in seeds, and those that feed in plant stems, especially stems of grasses. The seed-feeding group is represented by two taxa, Systole, species of which feed in the seeds of Umbelliferae (Claridge, 1959b), and Bruchophagus, many species of which develop in the seeds of Leguminosae. The stem-mining group is represented in Britain by the large genus Tetramesa.

Claridge (1961b) reviewed the biology of a number of British species. Several develop in the central cavity of grass stems, feeding above the nodes. They may be solitary or gregarious, but there is no external sign of their presence, and their effect on the flowering head is usually slight. Other species produce obvious stem-galls, and these often result in stunting of the flower head. In the United States several species of Tetramesa are pests of cereal crops, and a number of British seed-feeding eurytomids (eg Bruchophagus gibbus which attacks white clover, Trifolium repens) can assume pest status abroad (Valentine, 1970).

The majority of species of Eurytoma and Sycophila are entomophagous for at least part of their larval development, though several are known to complete their feeding on plant tissue (Varley, 1937). Most of these entomophagous species are idiobiont ectoparasitoids of insect larvae feeding within plant tissue. Hosts attacked include Coleoptera, gall-forming Hymenoptera (mostly Cynipinae), Diptera (especially Tephritidae) and Lepidoptera (Claridge, 1959a; Claridge & Askew, 1960). Fisher (1970) described the biology and life history of Eurytoma curculionum, a common larval parasitoid of an Apion weevil mining stems of docks (Rumex spp.). This eurytomid may develop either as a primary parasitoid of Apion, or as a facultative hyperparasitoid on other ectoparasitoids (eg Chlorocytus sp.) attacking the weevil larva. The eurytomid female paralyses a mature host larva before depositing an egg on or near it. The larva of another British species, Eurytoma rosae, is predaceous in multi-chambered galls on rose made by the cynipine genus Diplolepis rosae. The eurytomid chews its way from cell to cell consuming several cynipine larvae in succession (Blair, 1944). Considerable polyphagy is shown by another eurytomid which inhabits cynipid galls. This species, Eurytoma brunniventris, may parasitize a cynipid gall maker, its Synergus inquiline, other chalcid parasitoids, or even feed on the gall tissue (Askew, 1961a). Not all entomophagous species of Eurytoma are associated with phytophagous insects. For example, E. rubicola is a common parasitoid of sphecids nesting in bramble-stems, ovipositing through the stem and developing as an idiobiont ectoparasitoid of the prepupa (Danks, 1971).

A few eurytomids that attack cecidogenic (gall forming) insects are endoparasitic. Sycophila biguttata is found in Britain associated with oak galls. It develops as an endoparasitoid of the cecidogenic cynipid larva (Askew, 1984). Eurytoma serratulae develops as a koinobiont endoparasitoid of a tephritid that causes galls on thistles (Cirsium spp.). The eurytomid oviposits into an early instar larva, which continues to develop normally until late summer. Unparasitized tephritids overwinter as mature larvae, but parasitized tephritids attempt to pupate. The contents of the puparium are consumed by the parasitoid larva, which then overwinters in situ (Claridge, 1961a).

Most eurytomids are solitary parasitoids and, even though more than one egg may be laid on a host, larval competition ensures that only a single parasitoid develops. However, a few Eurytoma species are known which are gregarious ectoparasitoids of Lepidoptera (eg Piel, 1933). One of these, an Oriental species, is of particular interest as it is an obligate multiparasitoid (cleptoparasitoid). It can only oviposit on its host, a mature lepidopterous larva in a thick cocoon, if a chrysidid first bites an oviposition hole in the cocoon. The eurytomid oviposits through the plugged chrysidid hole.

A few eurytomids are recorded as predators of eggs of other insects (Clausen, 1940). Many of these belong to the non-British subfamily, the Rileyinae.

Eurytominae.

The following characters distinguish this subfamily: antennae with 12 segments (1 anellus, 5 funicle segments, 3 club segments), antennae are inserted at, below or above center of head, but always above level of ventral eye margin; genae posteriorly ridged; scape almost reaches level of anterior ocellus when raised; male with 4-5 funicle segments, some which are pedunculate anterior ocellus located above scrobe cavity; prepectus triangular or subcrescentric; scutellum normal; petiole variable but usually short; gaster slender and elongated to laterally compressed; gastral tergite 4 longest, except in Bruchophagus, with tergite 3 as long as tergite 4 or slightly longer than tergite 4; submarginal vein 5-6X as long as marginal vein except in Phylloxeroxenus, where submarginal vein is 3X as long as marginal; stigmal and postmarginal veins almost of equal length or shorter (Yoshimoto 1984).

Bugbee (1967) revised Eurytomocharis and Eurytoma; Zerova (1978) placed Systole in this subfamily.

Yoshimoto (1984) noted that "This group has diversified biology and behavior. The genus Tenuipetiolus is parasitic on either cecidomyiid (Diptera) or cynipid galls (Hymenoptera). Phylloxeroxenus is parasitic on gall-forming cecidomyiids. Eurytomocharis is phytophagous on grasses. Evoxysoma vitis is the only species of insect reared from seeds of grape; species of Bruchophagus are phytophagous on leguminious seeds; species of the large genus Eurytoma are generally either primary or secondary parasites but some are entirely phytophagous, whereas others are partly parasitic and partly phytophagous during larval development. Systole spp. have been reared from the seeds of Umbelliferae."

Heimbrinae.

Which is distinguished by an 11-segmented antenna (1 anellus, 7 funicle segments, and unsegmented club); antennae are inserted at level of ventral eye margin; genae are laterally ridged; anterior ocellus is located above the scrobe cavity; prepectus is small, rounded, 1/3rd as long as tegula; scutellum is produced apically into a long, blunt tubercle, projecting over gaster base; petiole is not visible; gastral tergite 1 is short, fused with tergite 2; gastral tergite 2 is longest, covering almost the entire gaster; the Apex of submarginal vein is thickened. (Yoshimoto 1984).

Rileyinae.

Not present in the UK, however Yoshimoto (1984) noted that "The subfamily can be separated from other groups by the following characters: Antenna 12- or 13-segmented (0-3 anelli, 6-8 funicle segments, 3 club segments); antennae inserted above or below center of head, but always slightly above level of ventral margin of eye; genae laterally carinate, or not carinate; scape not reaching level of vertex; anterior ocellus located above scrobe cavity. Prepectus reduced, triangular; scutellum normal. Gaster sessile; gastral tergite 4 or 5 longest. Submarginal vein 1.3-1.5 times a long as marginal vein; stigmal vein one-third to three-fourths as long as postmarginal vein, ore more less enlarged in Rileya."

Gahan (1918) revised the nearctic species of Rileya. Species of this subfamily parasitize gall-forming, cecidomyiids, and those of Macrorileya parasitize eggs of cricket, Oecanthus niveus (De Geer) (Gryllidae: Grylloptera). Subfamily Rileyinae was revised by Zerova (1976) for the Palearctic.

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Biology and Behavior

Eurytoma parva Gir. attacks hosts infesting stems and galls. Although mostly phytophagous, the habits and larval forms are typical of those which are wholly parasitic. It is a solitary external parasitoid of 1st or 2nd instar larva of H. tritici Fitch in the stems of wheat. Its parasitic phase occurs only over a short period, during which the Harmolita larva is completely consumed, and development is then completed on plant sap (Phillips 1927). Development to maturity may occur even on sap alone. The species is basically parasitic, because the stimuli for oviposition are given by the Harmolita larva rather than by the plant. Phillips (1927) stated, "The larva of Eurytoma parva is at present in a highly plastic or adaptive condition, a fact which indicates to the writer that it is in the process of changing over from parasitism upon animals to phytophagy, while as yet entirely dependent upon Harmolita tritici for the stimulus leading to oviposition."

There is only one generation each year, and winter is passed as mature larvae in a cell in the wheat stem. Adults emerge beginning the middle of May. Gestation is long for few or no eggs are present in the ovaries at emergence. Females are long lived and persist in the field for one month or more. During oviposition females insert their ovipositor through the wall of the wheat stem into the cavity occupied by the young Harmolita larva. The egg is placed only in the vicinity of the host larva and at times is outside the jointworm cell. Hatching takes place in 4-5 days, and the host larva is consumed by the 1st instar larva after which feeding begins on plant sap in the host cell. Plant tissue is lacerated during feeding, probably more than would have been done by the Harmolita larva, and a large amount of frass accumulates in the cell (Philipps 1927).

Eurytoma curta Wlk. is an internal parasitoid of gall-making tephritid Euribia jaceana Her. larvae in England. Very young maggots are attacked, and development of host and parasitoid ensues until premature host pupation in late summer, which is stimulated by parasitism (Varley 1937). Unparasitized hosts do not pupate until springtime. Hughes (1934) observed similar behavior in Eurytoma gigantea Walsh and E. obtusiventris Gahan, both internal parasitoids of tephritid gall makers in goldenrod. Clausen (1940) noted that parasitized host stunting in Diptera may be common as such is also found in Alysia and Brachymeria.

Mature larvae of E. curtia frequently feed on plant tissue after the host insect has been consumed (Varley 1937). Laceration of the cell wall is found in several other species that parasitize gall-inhabiting larvae, though the phytophagous habit has not reached so high a degree of development as in E. parva (Varley 1937, Clausen 1940).

Eurytoma appendigaster Dalm. in Europe is a secondary or tertiary parasitoid, through several Ichneumonidae and Braconidae, of a number of Lepidoptera and has also been found attacking tachinid puparia (Faure 1926, Rosenberg 1934). Hosts are paralyzed by the sting, and a feeding tube is formed through the cocoon wall, through which the body fluids are imbibed by the adult parasitoid female.

Eurytoma monemae Ruschka of China is a gregarious external parasitoid of the mature larva of the oriental moth, Monema flavescens Wlk., in its cocoon (Piel 1933a, Piel & Covillard 1933). Because the female is not able to penetrate the thick, hard cocoon wall with the ovipositor, she has devised another way to access the host larva. This is achieved through the intervention of another parasitoid, Chrysis shanghaiensis Smith. The latter is also parasitic on the same larvae. In early broods, the Eurytoma female pays not attention to the Monema cocoons themselves but is attracted to the Chrysis female which she follows persistently until her egg is laid. While the Chrysis is engaged in cutting an opening in the cocoon and laying her egg, a process that may take 15-60 minutes, the Eurytoma waits patiently. After the oviposition puncture has been closed, she approaches it, thrusts the ovipositor through the spongy plug and deposits her own eggs over a period of several hours. A maximum of 83 individuals have been found developing on a single host larva, and all adults emerge through the Chrysis oviposition puncture. The third generation of Eurytoma has no counterpart in Monema or Chrysis, which have only two generations, thus this brood reproduces on the same host brood as the preceding generation and females are easily able to locate the parasitized cocoons and the plugged oviposition holes (Piel 1933a, Piel & Covillard 1933).

Although E. monemae is basically a primary parasitoid of Monema larvae, it is still able to develop as a secondary parasitoid through Chrysis. As a primary of Monema, the destruction of the egg or young Chrysis larva is incidental. This relationship in which one parasitoid has to utilize the efforts of another was noted by Clausen (1940) as being unique to this family.

Eurytoma oophaga Silv. develops as a predator on eggs of Oecanthus pellucens Scop. in Italy (Silvestri 1920). The egg is laid near the anterior end of one of the host eggs in the egg chamber, and the first four larval stages derive their food from a single egg.

Archilileya inopinata Silv. is a predator on the eggs of Oecanthus and other Orthoptera and Homoptera that place their eggs in twig incisions. Oviposition and early larval development are the same as in E. oophaga. However, the 4th and 5th instar larvae are very active. They may consume all the eggs contained in the Oecanthus chamber and may burrow 1 cm or more through the pith of the plant stem to attack another egg cluster. The first generation develops on eggs of Tettigia and Cicada and the second on Oecanthus (Clausen 1940/1962).

Macrorileya oecanthi Ashm. develops on the eggs of O. niveus DeG. in North America. Its habits are different from A. inopinata (Smith 1930). Host eggs are laid in individual incisions in plant stems, and the parasitoid places its egg alongside the base of the host egg, with the long stalk extending the full length of the latter. After consuming the contents of one egg, the parasitoid larva burrows through the pith until another is found, and continues this procedure until 10-15 are located and consumed. The burrow constructed by the larva is smooth and free of frass. its length depends on the spacing of Oecanthus eggs, in one case being found to extend in a winding course for 37.5 cm. (Smith 1930).

The life cycle is short in all species except those which develop internally in tephritid larvae, although the inactive portion of the last larval stage may be prolonged. Archirileya attains the mature larval stage 24 days after oviposition, of which 3 days is required for egg incubation. Summer broods of E. monemae develop from egg to adult in 15-16 days. All species of Eurytomidae seem to pass the winter in the mature larval stage, and they may persist in that stage until the middle of the following summer. The time of emergence of the first brood of adults of E. monemae is usually during July, which is true also with the single brood of E. pissodis Gir and Macrorileya oecanthi. The number of generations varies with the host, and thus the majority of species have only a single generation. However, E. monemae has three generations to two of the host, and E. oophaga and A. inopinata may show a partial second generation, which in A. inopinata is on a different host (Clausen 1940/1962).

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Immature Stages of Eurytomidae

There is a considerable degree of uniformity in the eggs of the known species of the Eurytomidae. In the majority of cases, the egg is oblong in form with a somewhat col­lapsed stalk at the anterior end and a flagellum of varying length at the opposite end. The stalk may range in length from half that of the egg body to five or 6X its length, as in Macrorileya oecanthi. In this species, the stalk is much more slender than usual, being virtually a filament. In some species, the posterior flagellum is greatly reduced or lacking. In the externally deposited eggs of Eurytoma, the chorion exhibits a distinct sculpturing which, in E. pissodis Gir., is described as a black pubescence, while in others it is stated to consist of short but strong "spines" (Fig. 57A). These spines are densely placed and give the egg a color ranging from brownish to black, and the form of these spines has been useful for separating sibling species (Claridge & Askew, 1960; Fisher, 1965). The stalk and flagellum lack this sculpturing. The ovarian egg of E. oophaga Silv. has reticulate markings on the chorion; after deposition, this sculpturing is in the form of fine spines. The egg of E. curta is cylindrical and mea­sures 0.4 by 0.07 mm.; the stalk is 1.1 mm. in length, and the chorion is unsculptured. Eggs of Macrorileya and Archirileya likewise lack the surface sculpturing.

The first instar larvae of the family are broadly oval to elongated in form, with 13 distinct body segments and a relatively large, hemispherical or conical head. The head bears a number of sensory setae which, in E. appendigaster, are very large. The sensory setae, of which there are four pairs on each thoracic segment and three on the abdomen, may be minute, as in E. curta, or may exceed the length of a segment, as in E. robusta and E. parva. The integumentary setae are abundant in Eurytoma and may completely clothe the body or occur as a band on each segment. The larva of A. inopinata is apparently devoid of setae. In E. rosae Nees (Fig. 57B) and E. parva, a pair of small, sclerotized processes of unknown function is found on the venter near the median line of the first thoracic segment. The normal equipment of spiracles in the family is four pairs, situated on the mesothorax and the first three abdominal segments. E. parva has five pairs, the additional one being on the metathorax. E. curta is provided with 10 pairs, on the second and third thoracic and the first eight abdominal segments. A. inapinata has eight pairs, situated on the mesothorax and the first seven abdominal segments.

The second instar larva has been described for only a few species; it differs from the preceding instar mainly in the reduction of the sensory setae. E. oophaga still has the four pairs of spiracles situated as in the first instar; in E. parva the number is increased from five to nine and in A. inopinata from eight to nine. They are situated on the last two thoracic and the first seven abdominal segments. E. dentata Mayr has eight pairs, that on the second thoracic segment being absent.

Five larval instars have been distinguished in a number of species, and this is presumably the normal number for the family, though only four are indicated for E. oophaga. In this species the full complement of nine pairs of spiracles appears first in the third instar.

The mature larva of Eurytoma is more robust than the preceding instars. The sensory setae are usually small, though they are relatively long in E. dentata and E. masii Russo (Fig. 57C). Cuticular spines are minute or lacking. The larvae of Archirileya (Fig. 57D) and Macrorileya differ markedly from those of Eurytoma, being cylindrical, with the caudal segments broad and the last one broader than those pre­ceding it and forming a disk or sucker. There are 12 apparent body segments rather than the usual 13, and the anterior ventral region of the abdomen is appreciably distended. Intersegmental welts occur dorsally from the first thoracic to the sixth abdominal segments. The larvae of Axima and Conoaxima have large median dorsal welts on the thoracic and the anterior abdominal segments. The nine pairs of spiracles, on the second and third thoracic and the first seven abdominal segments, occur in all genera, though E. curta is stated to have an additional vestigial pair on the eighth abdominal segment.

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