Checklist of UK Recorded Platygastridae [= Platygasteridae (Westwood 1840)]

Description & Statistics

There have been ca. 905 species of Platygastridae described from throughout the world. Important morphological characters include a geniculate female antenna, variable male antenna, but 8-10 segmented in both sexes; wings lacking stigma and venation, or only a short submarginal vein present in the forewing. The body is small (0.6-4.0 mm), glossy black or black with yellow or red markings; abdomen elongate to oval, with sharp or narrowly rounded lateral margins.

Masner (1993) placed Platygastridae in superfamily Platygastroidea. He characterized the species in having a body 1-2 mm long, rarely up to 4 mm, slender, usually black, rarely yellowish, with no metallic colors. Antennae are strongly elbowed, usually with 8 flagellar segments, rarely with fewer (5-7). Male flagellar segment 2 (rarely 1) is modified. The forewing is usually veinless or if submarginal vein is developed then only very rarely reaching the anterior margin of the wing, the stigmal and postmarginal veins are absent. Hind wings are at most with a short stub of the submarginal vein. Metasomal segment 2 is always the longest and widest. The female almost always has only 6 apparent tergites, exceptionally fewer. Metasomal tergum 7 (apical tergum) is internal, considerably reduced and depigmented, without cerci or sensory plates, hidden under tergum 6, and not extruded with the ovipositor.

Platygastridae parasitize gallicolous Cecidomyiidae, and also whiteflies and mealybugs. They are all endoparasitic; most are primary parasitoids, and both solitary and gregarious species are common. Polyembryony is common also. They are egg-larval or egg-pupal parasitoids. Only limited use has been made of Platygastridae in biological control against whiteflies and mealybugs, and against the cecidomyiid, Hessian fly.

Adults occur in most habitats, often high on vegetation, searching for hosts such as Cecidomyiidae (Diptera). Some Platygastridae are primarily solitary parasitoids in eggs of various insects (Coleoptera, Homoptera), or they parasitize egg-like hosts such as young larvae of Coccoidea or Aleyrodidae (Homoptera). The entire development is completed in one stage of the host (idiobionts). However, most Platygastridae are koinobionts, parasitizing the host egg (usually gall-forming Cecidomyiidae) but developing only after the host is nearly full grown (prepupa or pupa). Some of these species are polyembryonic, with 2 or more individuals developing from one fertilized egg.

The family has circa 1,100 described species worldwide, but several thousand species are thought to exist. Masner & Huggert (1989) recognized two subfamilies: Sceliotrachelinae and Platygastrinae; the former subfamily Inostemmatinae was shown to be a heterogeneous assemblage. Its species were reassigned to the two subfamilies, a discussion about which follows:

Sceliotrachelinae

includes mostly squat to plump species. The laterotergites are relatively wide, and the general structure of the metasoma is similar to that of Telenominae. In females the club is usually abrupt, with 3 clavomeres, or the clavomeres are partly to completely fused into a single solid clavomere. In males the antenna is often subclavate. The forewing in most species has a tubular submarginal vein, knobbed apically. Most members whose biology is known are idiobionts, parasitizing eggs of various insects such as Curculionidae and Cerambycidae (Coleoptera) and Flatidae (Homoptera), or parasitizing early stages of Pseudococcidae or Aleyrodidae (Homoptera). About 20 genera occur worldwide but in the Southern Hemisphere, especially Chile, Australia and southern Africa, there is found the greatest diversity (Masner 1993).

Platygastrinae

comprises mostly slender to very elongated species. The laterotergites are usually narrow and tightly appressed against the sternites, making the metasoma more compact than in Sceliotrachelinae. In females the cylindrical club usually has 4 or 5 clavomeres, with the clavomeres clearly separated. In males the flagellum is usually thread-like. The forewing submarginal vein is present in more primitive species but absent in the more numerous, derived species. Platygastrinae are biologically very cohesive, associated with Cecidomyiidae (Diptera) as koinobionts. The female parasitizes the egg or early instar larva, and the adult wasp emerges from the host prepupa or pupa. Different species seem also to be closely associated with particular host plants (or parts of the plant) where the host gall is located. There are circa 40 genera worldwide, with cool temperate zones apparently about as numerous in species as the tropics. But the dominance of Platygastrinae in Chile and New Zealand is thought to be remarkable (Masner 1993).

Masner & Huggert (1989) revised and keyed the genera of the former subfamily Inostemmatinae. Vlug (1985) keyed and redescribed some European species described by Walker & Haliday. Kozlov (1987) treated the species in the former USSR.

Further Description.

It was noted by Clausen (1940) that there is an exceptional uniformity in adult form, habits and host preferences and relationships in the family. Adults are relatively small and usually black in color. Females of the genus Inostemma are conspicuous because of the curved, tubular "horn" that arises from the dorsum of the 1st abdominal segment, extending over the thorax to the head. The exceptionally long ovipositor lies in this horn when not extruded for oviposition. The occurrence of polyembryony in the genus Platygaster has focused attention on the family.

Few species have been deployed in biological control. Miscocyclops marchali Kieff. was imported into New Zealand from Europe in 1925, and reportedly caused a significant reduction in density of the pear midge, Perrisia pyri Bouché (Clausen 1940/1962).

The majority of Platygastridae for which hosts are known are parasitic in larvae of Cecidomyiidae. Species of Amitus parasitize Aleyrodidae, and those of Allotropa are probably the dominant parasitoids of Pseudococcus in Asia. Occasional species were recorded from Hymenoptera, Coleoptera and Lepidoptera, but Clausen (1940) believed the records required further verification.

Kozlov (1978/1987), as translated from the Russian, noted that these were "Minute chalcids; body length 1.0 mm. Antennae usually with 10 segments, rarely with 7-9 segments. Fore wings, at most, with subcostal and basal veins; however, basal vein reduced in most platygastrids, subcostal vein reduced in the tribe Sceliotrachelini, and veins completely reduced in all members of the subfamily Platygastrinae. Apterous form very rare. Scutellum usually semicircular, but sometimes with an apical spine, or conical in shape. Abdominal petiole usually transverse, rarely fused with 2nd segment. Species of Inostemma unique in presence of a cornutus on abdominal petiole. In platygasterids 2nd sternite usually devoid of armature, but in some females a process is present on either the anterior or posterior end."

"Predominantly parasitoids of eggs and/or larvae of gall midges (Cecidomyiidae). usually, separation of subgeneric groups within the family is associated with the occupation of new ecological niches. The family is differentiated into tribes as a result of a changeover to a parasitic mode of life:
Platystasini - eggs of cicadids and weevils;
Aphanomerini - eggs of cicadids and bugs;
Pseudaphanomerini - larvae of sphecid wasps;
Allotropini - mealybugs;
Fidiobiini - eggs of leaf beetles;
Sceliotrachelini- apterous members of Aleyrodidae."

Biology and Behavior

Platygastridae are all internal parasitoids and are either consistently solitary or gregarious, very few being solitary in some host individuals and gregarious in others. Invariably hosts are attacked in very early stages, and death generally occurs in the pupal stage, though a few species kill before host pupation.

The egg of Platygastridae is always of minute size, ranging in length from 0.02 to 0.1 mm. The main body is usually lemon‑shaped, with a stalk at the anterior end which, in L. rhanis (Fig. 30A), is 3X the length of the egg body. In other species, this stalk is shorter, and the extreme is shown in P. herrickii (Fig. 30C) which has the posterior end somewhat attenuated and bears several short flagellum‑like processes at the anterior end. Several species that have a pronounced stalk also bear a blunt protuberance at the posterior end.

Oviposition.

Most platygasterids lay their eggs within the host egg, although a few, such as P. dryomiae and Trichacis remulus may also oviposit in the newly hatched larvae and Leptacis rhanis does so consistently. Oviposition in the host egg, with development and emergence occurring in later stages, was found by Herrick (1841) for P. hiemalis. However, previously Kirby (1800) presented evidence that this must be the habit of Isotasius inserens Kirby, a parasitoid of Phytophaga, although he did not actually witness oviposition. Herrick's observations were repeated and confirmed by several workers dealing with P. hiemalis and P. zosine,, but this manner of oviposition and development was not accepted until Marchal's extensive work from 1896-1906 on the biology of Ageniaspis and other Platygastridae.

Amitus hesperidum variipes Silv is a gregarious parasitoid of the aleyrodid, Aleurocanthus spiniferus Q. in tropical Asia. It oviposits in the young larvae just after hatching and largely before they become fixed to the leaf (Clausen 1934). When ovipositing in exposed host eggs, the female parasitoid usually stands over the egg, parallel with it, and inserts the ovipositor by a backward thrust. Concealed eggs or larvae are reached by probing with the long, extensible ovipositor. Females of some solitary species appear able to exercise a considerable discrimination and seldom deposit more than a single egg in one host. Hill (1923) working with P. hiemalis found that the female was able to recognize eggs which she had previously attacked and refrained from placing a second egg in them. But she was unable to recognize those attacked by another female. A similar behavior was noted in P. zosine. Most species are solitary in habit and thus deposit only a single egg at each insertion of the ovipositor, although P. hiemalis and P. variabilis Fouts usually lay 4-8 eggs.

In species that lay in host eggs, the location of the parasitoid egg in the host embryo is variable, though fairly constant for any given species. Eggs of Inostemma boscii and I. piricola are found in the brain, while that of Trichacis remulus is in the posterior portion of the nerve chain. However, Adler (1908) stated that the egg of I. boscii is found in the peduncle of the host egg, and some are even laid externally. Eggs deposited in the fluids between the chorion and the embryo cannot develop to maturity. P. zosine lays in the mid-intestine of the host and cannot develop elsewhere, while a number of species, such as Misocyclops marchali Kieff. (Dumbleton 1934), P. lineatus and P. ornatus, place the egg in the general body cavity. The exact placement of the egg in a particular portion of the embryonic mass requires and exceptional degree of accuracy on the part of the female at the time of oviposition. This is done in part by orientation of the body to conform to the long axis of the egg, which facilitates the location of the nerve chain or the intestine. The length of the ovipositor may also play a part, and it is probable that tactile organs at the tip of the ovipositor are of some aid. Freshly laid host eggs are not suitable for oviposition, for the lack of embryonic development precludes the possibility of the parasitoid egg being associated with any organ or cavity of the body (Clausen 1940/1962).

Mono- & Polyembryonic Development

Eggs that are placed in the brain or the nerve chain become enveloped by a cyst of host tissue (Clausen 1940), which often involves striking proliferations, which surrounds the developing embryo or embryos until the first larval stage, and they emerge into the host body for direct feeding. These cysts may be compared to certain animal galls produced as a result of the presence of various parasitic organisms (Clausen 1940). In most species that place the egg in the general body cavity, it becomes attached to some organ, such as a salivary gland or fat body, and a similar cyst is formed about the developing embryo. However, P. zosine lies in the intestine of the host and is free floating at all times until the intestinal wall is broken and massive feeding begins.

Polyembryonic reproduction occurs in a series of Platygaster species, all of which develop in Cecidomyiidae. In the genera Amitus and Allotropa, attacking Aleyrodidae and Pseudococcus, respectively, the general habits and the number of individual developing in each host suggest the possibility of this mode of reproduction. A number of species that are solitary show this mode of development in their early stages, thus following the development of Macrocentrus ancylivorus Roh. in the Braconidae.

Host species are all of relatively small size, and thus the enormous numbers of progeny produced in a single host by several Encyrtidae are not found in Platygastridae. The maximum recorded is an average of 18 in the case of Platygaster felti Fouts in Rhopalomyia sabinae Felt, but producing only 11 in Walshomyia texana Felt. P. variabilis in Rhopalomyia produces an average of 15, and P. zosine and P. hiemalis, parasitoids of hessian fly, average 7.9 and 6, respectively (Clausen 1940/1962).

P. hiemalis shows the simplest form of polyembryony, which results in the production of not more than two individuals from each egg. The female parasitoid lays a group of 5-8 eggs in each host egg and from these an average total of 6.3 individuals attain maturity. About 1/3rd of these eggs fail to develop beyond the cleavage nucleus stage, presumably because they have not become invested by host tissue and, thus, do not receive the food materials essential to their further development. An additional portion of the eggs develops monoembryonically in a highly specialized manner comparable, in mast ways, to the normal course of development of L. rhanis, T. remulus and P. ornatus (Marchal 1897, 1906). According to Leiby & Hill (1924) P. variabilis undergoes twinning in the same way as does P. hiemalis. The occurrence of both forms of embryonic development in the same species in unknown elsewhere in the Hymenoptera and is believed by Leiby & Hill to furnish a clue to the origin of polyembryony.

Although aborted eggs occur only in P. hiemalis, in other species that produce a large number of individuals from each egg, aborted germs, blastulae and larvae may be found. The number of these in any species of Platygaster is very small compared to polyembryonic Encyrtidae.

The trophamnion envelops the embryonic portion of the egg in the early stages of development. Later it serves as a sheath through which the food essential to development of the embryos to the larval stage are derived from the host blood. The growth of the trophamnion during embryonic development allows the germs to divide, and these later become invested individually b portions of it. This manner of obtaining nourishment is not peculiar to Platygastridae, being found in many other parasitic groups in which development is monoembryonic. Such feeding is necessary where little or no yolk is provided in the egg itself. The amount of food taken from the host body is relatively small, and thus the host is not much affected by the presence of the young parasitoids. The physical changes preparatory to pupation of the host provide a stimulus for the completion of embryonic development and for attaining the direct feeding larval stages (Clausen 1940/1962).

Mixed broods and their origin in polyembryonic Platygastridae has led to differences in opinion as to the manner in which they arise, in particular where the proportion of males is low. In P. hiemalis, 80% are mixed broods and 20% pure female broods. In 71% of rearings, females exceeded males. In P. zosine, the majority of broods are of a single sex. However, P. felti showed 90.5% of the broods mixed, more than half of these containing only a single male, and no pure male broods were found (Patterson 1919, 1921b). There is no case where males outnumber females in a mixed brood. In P. variabilis, 92% of the broods proved to be mixed, and only a single pure male brood was found in a total of 217. Mixed broods were thought to develop as a result of duplicate oviposition, in which at least one of the eggs has not been fertilized (Marchal 1898, 1906 Leiby & Hill 1924). However, Patterson (1919, 1921b) believed that they arose as a result of abnormal behavior of the two sex chromosomes during early cleavage of the egg.

Immature Stages of Platygastridae

Free-living 1st instar larvae feed directly on host body fluids, but a few species are known to ingest tissues as well. First instar larvae of the cyclops form are capable of considerable movement, accomplished by ventral or backward thrusts of the caudal processes and the posterior abdomen. Rather large falcate mandibles apparently are not used for feeding, but may be for the purpose of combat. Embryonic states require a long period of time for development, but the stage from emergence of 1st instar larvae from the trophamnion to host death is usually short. In gregarious species, the mature larvae form individual cells in the host body, which are distinctly evident externally. Each larva lines its cell with silk, forming a delicate cocoon (Clausen 1940/62),

The first instar larvae of the polyembryonic species is the form that frees itself from the enveloping trophamnion; it does not necessarily differ from those of the monoembryonic species. There are two types of larva of this instar, the first being hymenopteriform and the second cyclopoid. The hymenopteriform first instar larva is elongately oval to almost spherical in form and distinctly segmented, with the head small, the mandibles relatively large, no fleshy processes on the body, and three pairs of spiracles, situated on the second and third thoracic and the second abdominal segments. P. hiemalis (Fig. 3la) P. ornatus, and P. dryomiaee are. of this type. A large discoidal body replaces the spiracle on the first abdominal segment, and a spi­racular branch leads to it. In P. zosine (Fig. 31B), the body is more elongated, with a constriction between the head and thorax, but it lacks any further indication of segmentation. There are no spiracles.

The cyclopoid type of first instar larva is characterized by a cephalothorax usually larger than the remainder of the body and somewhat flattened dorsoventrally, which bears enormous falcate mandibles and conspicuous antennae. The abdominal seg­ments are narrowed and reduced in number. The mandibles are widely spaced, being set near the lateral margins, and they lie transversely. The body terminates in one or more fleshy processes of diverse form. This type was first studied and figured by Ganin (1869) (Fig. 32) for several species parasitic in cecidomyiid larvae; it was described as "cyclops like." Two of them described by him have a pair of large fleshy processes lateroventrally on the cephalothorax near the posterior margin, and the caudal segment bears two or more long spine like processes which themselves are armed with numerous spines. A similar larva has been described by Marshal for Leptacis rhanis (Fig. 33 A). A second form described by Ganin shows the caudal appendage bifurcate, with the inner margins serrate. That described by Marchal for Inostemma piricola (Fig. 33a) bears a close resemblance to it. In P. lineatus, P. herrickii, Misocyclops marchali (Fig. 33 B), and Sactogaster pisi Foerst (Kutter 1934), the caudal appendage is broad, terminating in a pair of lobes which, in the last named species bears short, heavy spines. In T. remulus (Fig. 34), the abdominal segments are very narrow and the caudal process is short and bilobed, with the tips rounded and curved inward. An undetermined species of Platygaster figured by Marchal (Fig. 33 A) has the last abdominal segment somewhat expanded, with the posterior margin serrated. In some species, such as M. marchali, the paired fleshy processes that generally occur ventrally on the cephalothorax are entirely lacking.

Spiracles are not known to occur upon any cyclopoid larvae, in contrast to the three pairs occurring upon most hymenopteriform larvae of the family.

There is considerable question as to the body parts that make up the so called cephalothorax of the cyclopoid larva. Some authors consider it to consist only of the head, and illustrations show the mandibular muscles attached near the posterior base, though Marchal considered it to include the thoracic segments, also. The fleshy paired ventral processes are presumably borne on the first thoracic segment. The fact that the visible segments following the cephalothorax number only 5-7 lends weight to this conclusion and accords with the reduced number of body segments found in the mature larva.

The consideration of the instars following the first is complicated by a variation in the number of molts recognized among the different species. According to Hill, P. hiemalis has only a single larval instar. It is of considerable size when separation is effected from the trophamnion, and a relatively slight growth brings it to larval maturity. In P. zosine, according to the same author, there is only one molt, and the mature form immediately succeeds the first instar.

The second instar, as here discussed, includes only those of species in which three larval instars have been noted. That described and figured by Ganin for Platygaster sp. is oval in form with no indication of segmentation; during its early period, it is enclosed within the distended skin of the preceding instar. There are no fleshy or cuticular process any sort, and no open spiracles are present. Other species that have been studied by various authors possess second instar larvae that likewise present no distinguishing characters. The mandibles are small in all species.

The mature larvae, which may be of the first, second, or third instar, depending on the species under consideration, present no characters by which they may be readily separated. The body is oval in form and distinctly segmented and comprises the head and 10-11 body segments. No cuticular spines or fleshy processes are to be found. The mandibles are small and widely spaced. The tracheal system possesses three pairs of spiracles, situated on the second and third thoracic and the second abdominal segments. The first abdominal segment bears the large discoidal body mentioned in the discussion of the first instar larva of P. hiemalis and others. This organ has been detected only in species of Platygaster but may occur in other genera also.

The parasitized host body inflates to some extent, but not as much as in polyembryonic Encyrtidae. Hosts that are killed as mature larvae become appreciably darkened in color so that they bear a resemblance to puparia. In some species parasitized puparia are smaller than normal and irregularly formed, indicating that the parasitoid larvae were advanced enough to interfere with the normal processes of pupation. The cecidomyiid host of P. instricator pupates normally if it contains only a single parasitoid, but it dies without pupating if several parasitoids are present (Kulagin 1892). In Kulagin's work it is possible that two species were represented, which could account for this difference, however (Clausen 1940/62).

Life Cycle.

Most Platygastridae have only a single generation per year, despite the fact that the host may have several. This is shown by species of Platygaster attacking hessian fly in North America. The single brood of adults of P. herrickii and of P. zosine usually emerges in springtime, while that of P. hiemalis emerges in autumn. However, the host has several generations each year. Thus, the parasitoids have to pass a considerable portion of the year in a slightly inactive condition. P. herrickii oviposits in springtime and takes almost the entire season for larval development. The adult stage is attained in autumn. P. zosine, with an annual cycle, also reaches the mature larval stage in June, but there is an extended period within the cocoon before adults appear. In P. hiemalis, eggs are laid in autumn, and embryos persist within the host larvae until the following season and do not attain the free larval stage until June or July, after which they pupate in August. Inostemma boscii has two generations per year which correspond to the cycle of the host, and Leptacis rhanis has a partial second, the main brood of adult emerging in May.

P. hiemalis, P. dryomiae and P. ornatus Kieff. pass winter as embryos within the body of the live host. L. rhanis and I. boscii hibernate as mature larvae in the cocoons, but P. lineatus, P. zosine, P. herrickii and I. piricola transform to the adult stage during autumn and do not emerge from the host puparium until the following spring.

Reproductive Capacity & Sex Ratio

Reproductive capacity of some species is very high, even without the multiplication brought about by polyembryonic reproduction (Clausen 1940/62). Gravid females of P. hiemalis were found to contain an average of 3,322 eggs in the ovaries which, on the basis of 1.5 individuals developing from each egg, gives 4,983 progeny per female (Hill 1923). P. zosine contained only 228 eggs, each of which may produce 8 parasitoids, or a total of 1,224. P. hiemalis oviposited 160 times in one day, which on the basis of 4.2 eggs per insertion gives a total of circa 675 eggs and finally >1,000 progeny.

Sex ratios usually show a preponderance of females. Approximately equal numbers of males and females are found in P. zosine and P. herrickii, while females predominate in P. hiemalis by 2:1 and in P. felti and P. variabilis by 6:1. There is no thelytoky in monoembryonic species.