Scelionidae

Description & Statistics

Scelionidae are a moderately sized family of circa 1,010 species known as of 1993. They are widespread and especially abundant in the tropics. Important morphological characters include a elbowed antenna, 8-12 segmented; wings usually without a stigma and venation considerably reduced; gaster broadly or elongate oval, with sharp, or "keeled" lateral margins. The body is small (<5 mm), dark colored; female antenna often with a club, but males lacking the club.

All are primary, solitary, endoparasitoids of eggs of insects from most major orders and occasionally of spider eggs. Phoresy is involved in the life cycles of several species. Scelionids are relatively important to biological control as several species have been imported to control Lepidoptera and Hemiptera.

Masner (1993) placed Selionidae in the superfamily Platygastroidea. He characterized the species in having a body 1-2.5 mm long, rarely as small as 0.5 mm or as large as 10 mm, and predominantly black, sometimes yellow or multicolored, often distinctly sculptured, rarely with metallic colors. Antennae usually have 9-10 flagellar segments, occasionally as few as 4 or as many as 12. In males, flagellar segment 3 is modified. The forewing has the submarginal vein usually reaching the anterior margin of the wing and continuing as the marginal vein (sometimes thickened into a darker spot. The stigma and often postmarginal veins are present. The hind wing in most genera has a complete submarginal vein reaching to the hamuli. Wings are rarely without veins. The metasoma in most genera is moderately to strongly depressed dorsoventrally. Primarily, metasomal segments are subequal in length, but secondarily, one of the segments may be much larger than others. If segment 2 is longest then the submarginal vein reaches the anterior margin of the wing to continue as the marginal, stigmal, and often also the postmarginal veins. In females, the apparent metasomal segment 7 is either external or internal, with cerci or sensory plates, and may be extruded with the ovipositor during oviposition or attached to apparent tergum 6.

Members of this large family are surprisingly diverse in appearance, depending on the shape and size of the host egg from which they emerged: cylindrical to depressed, elongated and spindle-shaped to short, squat and stocky (Masner 1993).

Solitary endoparasitism is usual in eggs of insects and spiders (Araneae), with hyperparasitism and superparasitism strongly avoided. Unsegmented teleaform 1st instar larvae kill the host embryo. All subsequent development of the parasitoid is completed within the single host egg (idiobiont development)). Adults occur mostly in more open, sunny habitats such as grasslands, but they are also frequently found in deserts, forests, soil, marshes and water.

There are circa 150 genera and 3,000 described species worldwide, but the total fauna is thought to be circa 7,000 species. Three subfamilies are usually recognized: Scelioninae, Teleasinae and Telenominae. The first two are closely related and should form one group, but the latter is very distinct (Masner 1993).

Scelioninae is the largest and most polytypic subfamily, containing >90% of scelionid genera, classified in 16 tribes. The primitive stage of the metasoma with the segments subequal in length occurs in Sparasionini, Mantibariini, Scelionini and most Calliscelionini. The advanced stage, with segment 2 or 3 distinctly the longest, occurs in Gryonini, Baeini and Embidobiini. Laterosternites are well defined and together with the sharply flexed laterotergites, they form an acute lateral margin to the metasoma. Females of Scelioninae parasitize eggs of various insects and spiders (Araneae). The geological age of each group is reflected by choice of appropriate host. The very primitive Sparasionini parasitize archaic Tettigonioidea (Orthoptera) or Grylloidea (Grylloptera). Females of the more advanced Scelioninae parasitize more advanced Tettigonioidea or Grylloidea; members of Scelionini are parasitoids of Acrididae (Orthoptera). Members of some highly derived tribes parasitize nonorthopteroid hosts such as Heteroptera or Embioptera. The tribe Baeini apparently coevolved with araneomorph spiders (Araneae) (Austin 1985). Members of Thoronini parasitize Heteroptera under water (Masner 1972), and other scelionines parasitize Grylloidea in caves. Several highly specialized phoretic species in the tribes Mantibariini, Scelionini and Gryonini are associated with Mantidae (Dictuoptera: Mantodea), Acrididae (Orthoptera) or Hemiptera, respectively. It is surprising that no Scelioninae evolved as associates of Formicidae or Isoptera. Maximum diversity and number of species in all groups of this subfamily occur in the tropics, where only a small fraction of the species have been described. Members of Scelioninae also flourish in dry habitats including deserts (Masner 1993).

Teleasinae ought to be regarded as a tribe of Scelioninae (Masner 1993). This subfamily is very homogenous with relatively few genera, often difficult to distinguish from one another. Adults of Teleasinae are distinguished from those of Scelioninae by the apomorphic wing venation with long marginal vein, reduced palpi and large metasomal tergite 3. In females, apparent tergite 7 is not extruded with the ovipositor during oviposition. Although little is known about their biology, all species of the subfamily probably parasitize eggs of Carabidae (Coleoptera). A high degree of wing reduction occurs in many species along with possible wing polymorphy in at least some of them, including the males. Most species occur in temperate climates.

Telenominae may be distinguished by the absence of laterosternites and therefore the entire structure of the metasoma, which is not held so rigidly together as in the other subfamilies. The wide laterotergites overlap the sterna relatively loosely, and metasomal segment 2 is the largest. In females, apparent tergum 7 is external, not extruded with the ovipositor during oviposition, and the cerci are transformed into sensory plates studded with long hairs. Males usually have the antenna with 10 flagellar segments and females with 9, with only a few apomorphic exceptions. The subfamily is very homogeneous, with few genera but with a large number of described species and many more undescribed. During the evolution of the subfamily a host shift from Heteroptera (more primitive genera) to Lepidoptera (most Telenomus) occurred, with only a few species parasitizing such diverse hosts as Neuroptera, Diptera and Homoptera. The largest genus, Telenomus, is important in biological control. The species are distributed equally in both temperate and tropical climates (Masner 1993).

Further Description.

The family contains a large number of species, most of which are solitary and of small size and show exceptional uniformity in host preferences, behavior and in morphological characters of immature stages. All are parasitic in eggs of other insects, mainly Hemiptera, Lepidoptera, Orthoptera, Diptera (Tabanidae) and of Arachnida. A few species attack eggs of Coleoptera and Neuroptera (Chrysopidae). Common genera are Phanurus, parasitic in eggs of several orders, Telenomus, mainly in those of Hemiptera and Lepidoptera, Scelio in eggs of Orthoptera, and Rielia in mantid eggs. A few species of Microphanurus were reared from eggs of Coccidae. Members of Tiphodytes and Limnodytes are aquatic, developing in eggs of water boatmen (Gerris spp.) (Clausen 1940/62).

Species that have been successfully used in biological control are Aphanomerus puscillus Perk., introduced to Hawaii from Australia in 1904 for control of torpedo bug, Siphanta acuta Wlk. This resulted in complete control of the pest. Scelio pembertoni Timb. was introduced to Hawaii from Malaya during 1930-31, and is credited with the reduction of Oxya chinensis Thunb. Microphanurus basalis Woll. was imported into New South Wales, Australia from Egypt in 1934, resulting in a high parasitization and significant field control (Clausen 1940/62).

It has been possible to increase the effectiveness of native Scelionidae. Phanurus emersoni Gir. was liberated in Texas where it previously was absent (Parman 1928), which resulted in a marked reduction of horsefly, Tabanus hyalinipennis Hine. Okada (1934) reported a considerable increase in parasitization of eggs of the rice borer, Chilo simplex Butl., in Japan, through early colonizations of P. beneficiens Zehnt.

Kozlov (1978/1987), as translated from the Russian, noted that "Scelionids with compact bodies predominate in Europe. All tibiae usually with one short spur; rarely middle and hind tibiae with two spurs. Forewings in most Palearctic scelionids with subcostal, marginal, postmarginal, stigmal, and sometimes basal veins; sometimes only the subcostal vein preserved in forewings, or veins entirely absent. Tendency toward reduction of wings evident. Usually, antennae of females clavate and male antennae filamentous. Rarely, sexual dimorphism evident in that the abdominal petiole of females has a cornutus. Part of the ovipositor extends into this cornutus when not in use. Abdominal petiole not well developed, often transverse."

"Endoparasitoids of eggs of arthropods, mostly insects. Generally, eggs laid in clusters are parasitized and the females come into contact with the hosts only at the time of invasion. Phoresy constitutes an exception wherein the host is a comparatively large and strong female, capable of carrying its specific parasitoid to the place of oviposition. The egg parasitoid Mantibaria manticida lives on the body of the praying mantis (Mantis religiosa). Sloughing its wings, it settles underneath the base of the wings of the host and becomes an ectoparasitoid, feeding on the hemolymph of the praying mantis. When the praying mantis is ready to oviposit, the Mantibaria manticida females migrate to between the genital plates of the host. The moment oviposition occurs the egg parasitoids exit from the body of the praying mantis and attack its eggs before the viscous mass of the ootheca can harden. The parasitoids then return to their earlier place on the body of the host. Phoresy has also been observed in Telenomus tetratomus (gracilis), which is mainly a parasitoid of eggs of the Siberian moth and pin moth. This species congregates in those placed where the number of pupae of the Siberian moth is maximum. When the larvae of the moths emerge, the egg parasitoids adhere to the thorax of the host near the base of the fore wings and in the fold formed by the anal vein. Up to 40 female egg parasitoids have been recovered from a single moth. Moths in flight transport the parasitoid as much as several kilometers to the oviposition site."

Kozlov (1978/1987) noted that they were predominantly mesophilic, but also were found in dry habitats, steppes and desert. Scelionidae have not been adequately studied in European USSR.

Biology & Behavior

Oviposition.

There is a high degree of oviposition selectivity shown among Scelionidae. Female T. ashmeadi Morrill, after ovipositing, scrapes the host egg surface with her ovipositor, making several circular lines around the point of penetration (Morrill 1907), which undoubtedly serves to deter oviposition by other parasitoids. The same behavior was observed in T. farioi (Costa Lima 1928). Further discussion of this behavior was made by Voukassovitch (1925a) for Trissolcus simoni Mayr, developing in eggs of Eurydema. He noted that the females carefully examine all eggs encountered and will not attack those previously parasitized. However, Eumicrosoma benefica shows a complete lack of host discrimination for females are observed to attempt oviposition in host eggs containing immature parasitoids of their own species, which frequently results in hyperparasitization. These females may even insert their ovipositor in empty eggshells (Clausen 1940/1962).

Phoresy is known in the Scelionidae, where adult females attach themselves to the female of the host species and cling tightly to her until ovipositing. Then the parasitoid leaves the body and attacks the eggs. Such behavior was discussed by Rabaud (1922) and Chopard (1920, 1923) in Rielia manticida, developing in mantid eggs. Winged females of Rielia attach by the mandibles near the base of the wings or at the extremity of the abdomen of adult mantids, after which the wings are discarded. Parasitoids may be found both on male and female mantids, but are most common on the latter. Host oviposition occurs in autumn. At the time the egg mass is formed and before the frothy covering has hardened, the parasitoid descends into it and lays her eggs singly in those of the host. After oviposition she attempts to regain her position on the mantid. Parasitoids attach themselves to mantids shortly after their own emergence and thus may remain inactive in that position for several months before host oviposition takes place. Clausen (1940) considered these adult females to be true parasites of mantid adults because they feed on the body fluids during the waiting period.

A similar habit was recorded for Lepidoscelio viatrix Brues, developing in egg pods of Colemania locusts in India (Brues 1917). Females were found attached by the mandibles to intersegmental membranes between the abdominal plates. Several field-collected females of the plague grasshopper had females of Scelio fulgidus that clung to the abdomen (Noble 1935, 1936).

A detailed account of phoresy was given by van Vuuren (1935) for Phanurus beneficiens, a solitary parasitoid of rice borer eggs, Schoenobius bipunctifer Wlk., and other rice borers in the Asia. Adult moths collected in traps at twilight showed that 15% had Phanurus females on their bodies. About 1/3rd of these parasitoids were found on the wings, with the scales being rubbed off at the point of attachment, forming a distinct figure 8 shape. Male moths did not bear parasitoids. Host eggs were attacked as soon as they were laid. However, this relationship is not obligatory for other researchers have made extensive life history studies on this species in areas other than Java where direct oviposition was observed (Clausen 1940/1962).

Most Scelionidae show a preference for freshly laid host eggs, which is also common to some extent in Trichogrammatidae. Oviposition in R. manticida and P. beneficiens takes place only right after egg deposition by the host. Females of T. ulyetti Nixon show a preference for Heliothis eggs from a few hours to 1 day old, but will out of necessity oviposit in those up to 2 days old, the latter representing nearly half the incubation period of the host egg (5-5.5 days) (Jones 1937).

Eumicrosoma benefica, in its attack on the eggs of the chinchbug, Blissus leucopterus Say, in North America, chooses eggs that are 1-3 days old, but successful parasitization becomes less frequent in those which are older. Hatching takes place in less than one day, often in a few hours, and larvae complete feeding in 3 days. P. emersoni will not oviposit in Tabanus and Chrysops eggs that are older than 6 hrs. (Clausen 1940/1962). Exceptions to the above pattern regarding the stage of development of the host eggs at time of attack are found in T. farioi which is able to develop successfully in Triatoma eggs even though embryos may be well-advanced at the time. Scelio pembertoni oviposits in Oxya eggs of all developmental stages.

Scelio parasitoids of eggs of Orthoptera which are laid in masses in the soil show some interesting behavior. Female S. pembertoni penetrates the loose soil above the egg mass, after which she may bite out a hole in the egg pod and then turn abound and insert her ovipositor. The latter may be extruded to a length twice that of her body, and thus all eggs in the mass come within reach. From 3-5 hrs may be spent upon one egg mass without withdrawing the ovipositor. Females of S. fulgidus are present in the field at the time the grasshoppers are ovipositing, and they may even make their way down to the egg mass before it is fully formed. Once the grasshopper female departs, the parasitoid bites out a tunnel through the secretions covering the mass and along the side, stopping periodically to oviposit. Although the host eggs are available for a large portion of the year, the greatest parasitoid activity occurs during the 24 hrs immediately following host oviposition.

Immature Stages of Scelionidae

The eggs of the species of Scelionidae that have been described are of a uniform type, all being stalked, with the main body ovate to spindle shaped and the tapering or tubular anterior stalk ranging in length from 1/2 to 1 & 1/2 X that of the main body. The eggs of Scelio are slender, with the line of demarcation between the stalk and main body not distinct, and that of S. pembertoni has a small pedicel at the posterior end, also. There is an increase in size during incubation, as a result of which the stalk disappears.

In Telenomus ulyett on Heliothis eggs, the parasitoid egg is found floating free in the yolk between the amnion and serosa, and the young larva lies in the outer yolk layer where it is attached by its mouth parts to the serosa. Host embryo development is unaffected by the 1st instar parasitoid larva, because it is not dependent on that part of the yolk material on which the parasitoid feeds. Second instar larvae attack the body of the embryo, and eventually the entire egg contents are consumed. Thus the parasitoid larva in order to complete its development successfully, must reach the 2nd instar before the body wall of the embryo has hardened so as to become invulnerable to the parasitoid's mandibles. If hardening has occurred, the parasitoid larva dies from starvation, the host embryo completes development, and normal hatching occurs. Of interest is this parasitoid' inability to develop in infertile host eggs. However, Microphanurus basalis develops readily in dead Nezara with no embryological development at the time of death as in live eggs in an advanced developmental stage. This seems to suggest that either development of the early stage larva takes place within the embryo or that the larva itself is able to penetrate it at any stage of development (Clausen 1940/1962).

Development of Larvae.

The first instar larva of the family is "teleaform," so called from the larva of Tiphodytes (Teleas sp.) described and figured by Ganin (1869). It is characterized by a complete lack of segmentation but with the body divided by a sharp constriction into two almost equal portions. There is a difference of opinion as to the parts that constitute the anterior portion. Henriksen, Bakkendorf (1934), and Pagden (1934) considered that it represents the head alone, while Noble & Kamal termed it the cephalothorax, made up of the head and the three thoracic segments. This latter interpretation is more probably correct. The mandibles of all species are external, widely spaced, exceedingly large, curved, and sharply pointed and may be either heavily sclerotized or fleshy and unsclerotized. In Phanurus sp. dissected from eggs of Chrysopa in Japan, there are no other evident structures or organs on the cephalothorax, while in several other species of the family various head structures are well­ developed. The antennal processes of Scelio fulgidus (Fig. 28E) and S. pembertoni are large and conical, are widely spaced, and arise immediately above the bases of the mandibles. In a number of species, there is a large, fleshy lobe or process on the median ventral line of the cephalothorax, below or behind the mandibles. This is highly developed in the genus Scelio (Fig. 28I) and has been considered the labium by several authors.

The abdomen is almost globular in form and terminates in a caudoventral horn, or tail, which may be fleshy and of irregular form or heavily sclerotized, sickle­like, and terminating in a sharp point. In some species, there are one or two supple­mentary lobes at the base of the tail. The fleshy type of tail is usually spined dorsally and on the distal portion and is occasionally bifurcate. at the tip.

In Eumicrosoma, Telenomus, Scelio, Phanurus and Microphanurus, and probably in other genera also, there is a partial or complete transverse ring of long hairs near the anterior margin of the abdomen. These hairs vary considerably in number and distribution. In E. benefica (Fig. 28B), they occur upon the sides only, whereas in others the ring is complete and it is double in several species of Phanurus. The abdominal hairs of Teleas sp. (Fig. 28C, D) figured by Ayers (1884), Limnodytes, and Tiphodytes are in distinct tufts upon the summits of a pair of fleshy lobes situated at the lateroventral margins on the anterior portion of the abdomen. Marchal (l900) illustrated them in that arrangement in T. gerriphagus Mnrchal, Martin (1928), dealing with the same species, shows the hairs in a transverse row.

Chopard (1923) figured several supposed developmental phases of the first instar larva of Rielia manticida in the eggs of Mantidae. The first (Fig. 28F), secured from host eggs in April, is of simple form, with the abdomen lacking the band of hairs and the tail. Those found in May show a lateroventral tuft of short hairs, and the tip of the abdomen is produced into a broadly conical tail. The form found in June and July (Fig. 28G) has the lateral abdominal hairs well developed and the tail further enlarged (Fig. 28H). The author is inclined to consider the latter to be the second instar. It is however, identical in general characters with the first instar larva of various other species.

The second instar larva had been described in only a few species as of 1940 (Clausen 1940). That of T. gerriphagus figured by Martin is irregularly ovoid in form, with the mandibles still large and a small hook like caudal horn. Immediately above the mandibles are two plate ­like thickenings of the integument, separated by a median depression. The abdominal hairs are present in groups of 5-6 in a band across the dorsum and sides. None of these characters was found in all of the specimens examined, and the true form of the second instar is thus doubtful. That of M. basalis is very robust, with the segmentation indistinct; the mandibles are small and simple. There are no integumentary spines or setae, and the caudal horn is lacking. The second instar larvae of T. ulyetti and Phanurus angustatus Thom. (Fig. 28J) are cylindrical and distinctly segmented, but otherwise similar to that of M. basalis.. Neither the first nor second instar larva of any species has been found to possess a tracheal system or spiracles.

The third instar larva of T. ulyetti is similar in form to the second but may be readily distinguished by the presence of nine pairs of spiracles, situated on the last two thoracic and the first seven abdominal segments. Kamal mentioned that only the two pairs of thoracic spiracles are functional in M. basalis, the following seven being minute and closed. The integument of the abdomen bears numerous small tubercles at the lateroventral margins, which extend across the venter on the posterior segments. This species and S. fulgidus are grayish green in color.

The mature larva of Cacus oecanthi Riley bears a pair of rounded protuberances Internally on each body segment except the last, and the second to seventh segments also have a pair of prominent tubercles dorsally (Parrott & Fulton 1914).

In n number of species, only two larval instars are mentioned, this being due presumably to the marked similarity of the second and third. The larva of P. angustatus (described and figured as the second and last instar (Bakkendorf 1934) is probably the true second, as judged by the lack of a respiratory system.

Changes in coloration of parasitized host eggs are distinct and contrast with changes by healthy eggs. Within 5-6 days after parasitization, eggs of most species become gray or grayish-brown, and in a few cases black. Eggs parasitized by Scelionidae are usually distinguished from those containing Trichogrammatidae by this means, the latter generally becoming black.

The sac-like first instar larva is capable of considerable movement. The caudal horn, or tail, can be moved in a wide arc in the vertical plane. This movement has been variously considered as a means of locomotion, bringing food materials to the mouth, or of disorganizing the host egg contents. In M. basalis, the ring of long hairs on the abdomen, which normally lie flat on the surface, can be raised perpendicularly, and a locomotory function is apparent. Parasitized host eggs having a transparent chorion show a distinct undulatory movement of the fluid contents, which is similar to that seen in leafhopper eggs bearing mymarid larvae.

At pupation the parasitoid head usually is at the anterior end of the host egg; and emergence in parasites of hemipterous eggs, is through an irregular hole in the operculum. Most adults of P. benficiens emerge in the early morning, whereas those of E. benefica do so during late afternoon and early evening, which is the reverse of would normally be expected because adults of P. beneficiens are nocturnal and those of Eumicrosoma are diurnal.

In species of Limnodytes and Tiphodytes which are parasitic in Gerris eggs, the eggs are deposited upon foliage beneath the water surface. Thus, it is necessary for the parasitoids to be capable of locomotion in the water medium, and they swim readily, using both wings and legs. This is in contrast to other groups of aquatic Hymenoptera which use only the legs and find the host by crawling down plant stems, and other submerged surfaces.

Reproductive Capacity.

Scelionidae have a low reproductive capacity, the maximum indicated for Scelio fulgidus, in which an average of 234 eggs was found in the ovaries. In rearing experiments, Phanurus beneficiens produced an average of 143 progeny, with a maximum of 275 from a single female. An average of only 22 eggs was found in ovaries of Eumicrosoma benefica and 54 was the greatest number deposited by a single female. The ovaries of Telenomus ulyetti have 4 ovarioles each, indicating a low capacity. At emergence, 6-14 fully mature eggs are found either in the uterus or ready to descend from the oviducts, but the individual egg production was found to average only 55. Mating does not affect reproductive capacity in Scelionidae, with the exception of Eumicrosoma benefica, where virgin females deposit fewer eggs than do mated females (Clausen 1940/1962).

The initial batch of eggs if fully developed in the reproductive system of the female and ready for deposition at emergence from the host egg. This seems true in the great majority of true egg parasitoids among Chalcidoidea and Serphidae, etc. and their complete food requirements appear to be fully met by the contents of the host egg. There is no host feeding known at the ovipositor puncture hole, and some species are believed not to feed at all during adult life (Clausen 1940/1962).

Although most Scelionidae are solitary parasitoids, 5-6 T. nigrocoxalis Ash. are produced in each egg of Brassolis saphorae L., and a maximum of 16 was recorded for T. farioi in Triatoma. While solitary species lay only a single egg at each insertion of the ovipositor, gregarious species lay the full complement at one insertion.

There is almost always a preponderance of females, ranging to 10:1 in P. benficiens in Java, but only 2:1 in Japan. There is no thelytoky known as of 1940. Costa Lima (1928) found that males of T. farioi that are produced parthenogenetically from unmated females differ markedly in size from those from mated females, which suggests that the latter may be polyploid males.

Life Cycle.

The minimum duration of the life cycle is relatively short, ranging in most species from 8-15 days under optimum conditions. One half to 2/3rds of this time is spent in the pupal stage. There are a considerable number of generations per year. In E. benefica there are 4-9 during the season, resulting in the development of 4-5 successive generations in the eggs of a single host brood. Rielia, attacking single-brooded hosts, has only a single generation, and this is also true of some species of several other genera. P. beneficiens passes winter as adults in clumps of dry grass and otherwise sheltered niches. Most species attacking hosts that overwinter as eggs appear to hibernate as first instar larvae within them. S. fulgidus in Australia is able to complete its cycle in 4-5 weeks, and may persist for long periods in the adult stage within the egg. This diapause of aestivation is induced by arid conditions, and emergence takes place as soon as the soil is moistened.