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HYMENOPTERA, Pteromalidae (Walker 1835) - (Chalcidoidea) -- <Images> & <Juveniles>

Please refer also to the following links for details on this group:

Pteromalidae = Link 1, Link 2

Pteromalidae. -- The PteromaIids are a large group of about 400 North American species. They are tiny black or metallic-green or bronze wasps . They are parasitic and attack a wide variety of hosts with many being valuable in the biological control of crop pests. The adults of many species feed on the body fluids of the host, which exude from punctures made by the parasite's ovipositor (see Photo #3 to the right below).

In Habrocytus cerealellae (Ashmead) that attacks larvae of the angoumois grain moth and where the host larvae are in the seed and out of reach of the adult pteromalid, a viscous fluid is secreted from the ovipositor that is then formed into a feeding tube extending down to the host larva. The adult sucks up the body fluids of the host through this tube.

Pteromalidae now also includes the former separate families, Cleonymidae, Miscogasteridae and Spalangiidae, which have been designated subfamilies Cleoneminae, Miscogasterinae and Spalangiinae, respectively. For the present, discussions of the various subfamilies will be separate because of considerable distinctness among them. The families Asaphinae, Austroterobiinae, Austrosystasinae, Brachyscelidiphaginae, Ceinae, Cerocephalinae, Chromeurytominae, Cleonyminae, Coelocybinae, Colotrechinae, Cratominae, Diparinae, Ditropinotellinae, Eunotinae, Erotolepsiinae, Eunotinae, Eutrichosomatinae, Herbertinae, Keiraninae, Leptofoeninae, Louriciinae, Macromesinae, Miscogasterinae, Neodiparinae, Nefoeninae, Ormocerinae, Panstenoninae, Parasaphodinae, Pireninae, Pteromalinae, Spalantiinae and Storeyinae are treated in the respective files, <ASAPHINA.TXT>, <AUSTROTE.TXT>, <AUSTROSY.TXT>, <BRACHYSC.TXT>, <CEINAE.TXT>, <CEROCEPH.TXT>, <CHROMEUR.TXT>, <CLEONYMI.TXT>, <COELOCY.TXT>, <COLOTREC.TXT>, <CRATOMIN.TXT>, DIPARINA.TXT>, <DITROPIN.TXT>, <ELATOIDI.TXT>, <EROTOLEP.TXT>, <EUNOTINA.TXT>, <EUTRICHO.TXT>, <HERBERT.TXT>, <KEIRANI.TXT>, <LEPTOFOE.TXT>, <LOURICI.TXT>, <MACROMES.TXT>, <MISCOGAS.TXT>, <NEODIPAR.TXT>, <NEFOEN.TXT>, <ORMOCER.TXT>, <PANSTENO.TXT>, <PARASAPH.TXT>, <PIRENIN.TXT>, <PTERONAE.TXT>, <SPALANGI.TXT> and <STOREYIN.TXT> [see Taxnames for more detail].

Among Chalcidoidea, the Pteromalidae are one of the most common families containing many genera and species of frequent encounter as parasitoids or hyperparasitoids of various insect pests. Dominant genera include Pteromalus, Habrocytus, Dibrachys and Pachyneuron. The family Spalangiidae is frequently included under Pteromalidae and is represented by many species of Spalangia (see Bou…ek 1963). Most species are external gregarious parasitoids of larvae and pupae of Lepidoptera and Coleoptera, but some also attack pupae of Diptera and larvae of Hymenoptera. Genera such as Spintherus, Enargopelte, and Peridesmia are egg predators. External and internal parasitism within a genus is found in Dibrachys, Pteromalus and Stenomalus. Ophelosia crawfordi Riley occurs as a predator on the eggs of Pulvinaria, Pseudococcus and Icerya (Smith & Compere 1931), and as a hyperparasitoid of these genera and sometimes of larvae of Coccinellidae. It has been reared from Icerya females where hyperparasitism seemed possible through Cryptochaetum. Some species of Asaphes are hyperparasitoids of Aphididae through various braconids, aphelinids and encyrtids that behave as primary parasitoids (Griswold 1929). Dibrachys cavus Wlk. (= boucheanus Ratz.) attacks a wide range of hosts with over 45 host species comprising 2 Coleoptera, 2 Diptera, 27 Hymenoptera and 14 Lepidoptera (Faure & Zolstorewsky 1925).

Most Pteromalidae are primary parasitoids, but hyperparasitic species are common. Most species are ectoparasitic, but endoparasitic species are common also. Solitary and gregarious species and races are common. Generally, this family has a wide host range. Most species are gregarious ectoparasitoids of larvae and pupae of Lepidoptera and Coleoptera, but a number of species attack larvae and pupae of Diptera as well. Some are predaceous on eggs of Coccidae. There are no phytophagous species. Considerable importance has been placed on pteromalids for biological control of Lepidoptera, Coleoptera and synanthropic Diptera. A few species have also been used for the biological control of Coccidae.

Pteromalus puparum L., a gregarious internal parasitoid of cabbage butterfly pupae and Ascia rapae L. in New Zealand have been credited with marked reductions in host population densities in biological control efforts. Species of Muscidifurax, Urolepis and Pachycrepoideus have been deployed successfully against synanthropic Diptera (Legner et al. 1976, Rueda & Axtell 1985).

Biology & Behavior

Extensive studies have been made on species in the genera Dibrachys, Dibrachoides, Pteromalus, Eupteromalus, Habrocytus, Muscidifurax, and Nasonia (Mormoniella).

Variation within a genus and species is aptly demonstrated in the genus Muscidifurax, which is a group of closely related species (superspecies) attacking puparia of synanthropic Diptera. The five described species occur in geographic isolation in the Nearctic and Neotropical regions, except two species which are sympatric in the western Nearctic (Legner 1969, 1983, Kogan & Legner 1970, Kawooya 1983). The suspected ancestor of this apparent clade, M. raptor Girault & Sanders, is widely distributed in Europe, Africa, North America, and portions of the Pacific area (Legner & Olton 1988, 1971; Legner et al. 1976, Rutz & Axtell 1980, Legner 1987, Smith et al. 1987), but it has not been found in the Neotropics. There are no known clinal patterns. The genus has not been reported from Asia and is poorly represented in equatorial regions (Legner & Olton 1968, Legner & Greathead 1969, Legner et al. 1976, Legner 1983).

Muscidifurax are most prevalent in or near accumulated decaying organic material deposited by humans and livestock, where they parasitize host Diptera that also breed selectively in this habitat. Therefore, they fit the endophilous synanthropic category (Povolny 1971, Legner et al. 1974), and their existence depends largely on herdsmen. This has led to the suggestion that the four species currently confined wholly to the Americas could have evolved within the recent time period of European settlement, or during the past 400 yr (Kogan & Legner 1970, van den Assem & Povel 1973).

The only known South American member of the genus, M. raptorellus Kogan & Legner, occurs as three or more separate populations. One population from coastal Peru is solitary, whereas the others show various degrees of gregarious oviposition and development (two or more eggs laid at one insertion) (Legner 1967, 1988, Kogan & Legner 1970, Kawooya 1983). One of the gregarious populations from central Chile oviposits more than one egg in >60% of the hosts it attacks, with subsequent successful gregarious development. The Chilean population compensates for a lower host-searching capacity with the gregarious behavior, which results in a greater number of progeny per host (Legner 1967, 1987).

Adult Habits.--The time between emergence of adults and oviposition is short. Females of Nasonia vitripennis Wlk. (= Nasonia brevicornis Ashm., Mormoniella vitripennis Wlk.) deposit eggs within three hours of emergence, and Habrocytus cerealellae Ashm. does so the following day. Two to three days are required by Dibrachoides dynastes Foerst. and Homoporus braconidis Ferr. for gestation, but Pirene graminea Hal. lays its first eggs 7-8 days after adult emergence (Clausen 1940/1962).

As is typical for Hymenoptera, adults feed extensively on honeydew, and plant secretions. Such food is sufficient to maintain life, but in many species it is not sufficient to meet the optimal nutritional requirements for egg production. It was demonstrated that a protein diet is essential for some species before normal oögenesis can proceed. The body fluids of the host provide a suitable food of this type, and the females of Pteromalidae more generally than of any other family, have developed the habit of feeding on fluids that exude from punctures made with the ovipositor. Roubaud (1917) seems to be the first to realize the significance of host-feeding, as his work with Nasonia vitripennis showed that such feeding was essential before normal oviposition could take place. Such feeding may be associated with and immediately precede oviposition, or the two acts may be entirely independent and upon different host individuals (Clausen 1940/1962, Legner & Gerling 1967). Feeding on exposed hosts presents minimal difficulties for the parasitoid, but where hosts are sequestered in a burrow, cocoon, cell or puparium, direct feeding is not possible. Therefore, it is necessary to provide some means by which the body fluids of the host can be brought within reach of the parasitoid. This is accomplished by the construction of a feeding tube which extends from the oviposition puncture in the host's body to the outside wall of the cell, etc. Fluids rise to the top of the tube by capillary action and are there lapped up by the parasitoid. In some species, withdrawal of the body fluids is so great as to suggest the deployment of actual suction to bring it to the surface.

This adaptation for feeding was first discovered by Lichtenstein (1921) while studying Habrocytus cionicida Licht. and external parasitoid of larvae and pupae of the weevil, Cionus thapsi F., in their cocoons. The habit has since been found in many other genera of Pteromalidae and occasionally in other Chalcidoidea as well. It also occurs among the Braconidae, particularly in the genus Microbracon. A detailed account of the manner of tube formation was given for Habrocytus cerealellae, parasitic on the angoumois grain moth Sitotroga cerealella Oliv. (Fulton 1933). This parasitoid normally attacks larval stages in cells in the seed, although parasitization and development are also possible on exposed larvae. A feeding tube is constructed only when the host is deeply embedded in the seed, however. After stinging, the ovipositor is withdrawn until only its tip penetrates through the hole in the cell wall. A clear, viscid fluid begins to ooze from it, most abundantly from near the tip. This material is spread by a twisting and vertical movement of the ovipositor, which serves as a spatula. It is worked downward gradually, and fresh material is added continuously until the body of the host is finally reached. The ovipositor tip is then slowly moved about until the original puncture in the skin is found. It is then reinserted in the puncture and held in that position for several minutes, during which time the tube is completed. The ovipositor is withdrawn very slowly in order not to damage the delicate tube. In the meantime, the stylets move alternately up and down. A small extension of the tube appears above the surface of the opening in the cell wall. The female then turns about and begins feeding on the fluids from the tube, which may continue without interruption for almost an hour. At the completion of feeding, the female reinserts the ovipositor in the tube, seemingly for the purpose of breaking it. Some researchers have noted this reinsertion of the ovipositor by different species and considered it to be for the purpose of inducing a renewal of body fluids flow and that breaking the tube was only accidental.

Faure & Zolstorewsky described an identical manner of tube formation in Dibrachys cavus. H. D. Smith (1930) observed in Dibrachoides dynastes Foerst., that the chalky fluid flows down the full length of the ovipositor while its tip is still inserted in the wound, with hardening taking place quickly after which the ovipositor was cautiously withdrawn. Flanders (1935b) noted tube formation among egg predators. Female Spintherus sp. punctured one of a cluster of eggs of the alfalfa weevil, Hypera variabilis Hbst., that were embedded in a plant stem. A tube was formed and the entire egg contents was sucked out. Colleterial glands are thought to be the source of the tube substance.

Feeding is not always limited to host fluids but to tissues as well. Noble (1932) reported that when the ovipositor of Habrocytus cerealellae was withdrawn from the wound the barbs of the sheath draw up strands of solid matter and then the parasitoid chews vigorously upon them. The same habit was found in Dibrachys clisiocampae Fitch, attacking mature larvae of the bee moth in its cocoon (Graham 1918).

Detailed studies of the relation of the feeding habit of Peridesmia and Spintherus to egg development were made by Flanders. Sufficient protein was not stored up during the larval period to provide for egg production, which must be accounted for later. The preference for host fluids is shown only during oögenesis and maturation. This reaction first appears about six days before egg deposition. If environmental conditions were adverse, the ovarian follicles were absorbed, feeding hon host fluids ceased, and a long period of reproductive inactivity followed. The degeneration of the ovaries under such conditions was considered to be a form of phasic castration. The induced inactivity corresponded in time to periods of low numerical status of the host stage that was subject to attack. It may have a direct effect on the ability of a parasitoid species to survive where the host undergoes long periods of aestivation.

The female is able to develop and deposit eggs for a certain period as a result of feeding on protein substances, but carbohydrates are necessary to provide the energy required for extended activity. Females of Pteromalus puparum deposited eggs for a period of three weeks when confined with fresh host pupae, to which they were limited for food, while with the addition of honey they continued oviposition for two months (Doten 1911).

Most species that attack larvae of Lepidoptera and Coleoptera permanently paralyze the host prior to oviposition. This is especially true in Habrocytus, Dibrachys and Dibrachoides and some other less common genera. In H. cerealellae the parasitoid pumps several droplets of a paralyzing fluid into the host body, which can be readily seen as it flows down the ovipositor. This organ is thrust deeply into the host to ensure thorough distribution of the fluid. The stinging act requires up to 10 min., and the host is completely paralyzed before the ovipositor is withdrawn. In Drabrachoides dynastes the host may be stung 3-100 times before complete immobility, which may take up to 8hrs (Clausen 1940/1962). The larvae of the bee moth, Galleria mellonella L., die from the effects of the sting of Dibrachys clisiocampae, and the eggs of the latter are not deposited until death occurs. Doten reported that larvae of the codling moth and pupae of the cabbage butterfly were usually killed by thrusts of the Meraporus sp. ovipositor. In a large number of species hosts are consistently killed by the sting, and decay ensues very soon thereafter. In such cases the larvae are scavengerous in habit rather than parasitic. A few species do not paralyze the host, among which are Trichomalus fasciatus Thoms., an external parasitoid of Ceutorrhynchus assimilis Payk. larvae. Pirene graminea paralyzes its host larva Contarinia pisi Winn. temporarily (Clausen 1940/1962).

Some species that are parasitic in dipterous, seal the puncture in the puparial wall with a drop of fluid after oviposition. Clausen (1940) noted the behavior of Scymnophagus townsendi Ashm., an external parasitoid of Scymnus sp. pupae, a coccinellid predator on aphids in Japan. During oviposition the female stands on the exposed portion of the dorsum of the pupa and makes a preliminary insertion with the ovipositor to form a feeding puncture. Several hours may be spent in lapping up the body fluids exuding from the wound, after which the ovipositor is reinserted, usually in the original puncture, and forced completely through the body. The egg is laid on the ventral surface of the abdomen or under a wing pad or leg. Feeding and oviposition may be repeated, the single original puncture serving for this purpose. A compact cluster of 4-6 eggs is usually found when the host is finally abandoned.

Fulton (1933) described the mechanics of oviposition in H. cerealellae, noting the presence of a remarkable adaptation for the passage of a large object through a relatively minute tube such as the ovipositor. The small end of the egg approaches the ovipositor first; the spicules of the chorion, in conjunction with the backward directed spines and ridges on the walls of the vagina and stylets, prevent any backward movement once the surfaces are engaged. The greatly compressed egg tip is drawn down through the ovipositor. The chorion, being stretched, passes down the ovipositor by a pulling action rather than any pressure exerted through the abdomen. Therefore, the portion of the chorion being drawn through the tube is almost devoid of liquid content, and little lateral pressure is exerted on the ovipositor. At this time the stylets, moving together rather than alternately, work the tip of the egg downward until it reaches the end of the ovipositor. Thereafter the egg contents flow down the tube into the released portion, and by this action the remainder of the egg is pulled through the ovipositor. The reduction of egg diameter during its passage down the ovipositor is shown by a comparison of egg size with ovipositor width. The egg averages 0.16 mm in width, while the outside diameter of the ovipositor is ca. 1/5th of this, or 0.03 mm.

The habit of adult swarming, which is rate in Chalcidoidea, was recorded for Pteromalus deplanatus Nees (Scott 1919). During 1916-1919 in some localities in England this species was present in huge swarms in buildings during late July and August. Clausen (1940) believed this to be a search for suitable quarters in which to pass the winter.

Reproductive capacity of Pteromalidae is high. An egg deposition of 676 in 78 days has been recorded by Fulton for H. cerealellae, and P. puparum has been found to lay as many as 697 eggs. A single female Nasonia vitripennis produced 557 progeny (Cousin 1933), suggesting an egg capacity much in excess of that number. A series of Eupteromalus nidulans Thoms. females laid an average of 251 eggs, with a maximum of 583 (Proper 1931). Aplastomorpha calandrae How. lays ca. 250 and Dibrachoides dynastes a maximum of 122 during a single month.

Development of Larvae.--First instar larvae of species that feed externally are usually very active, moving about readily on the body of the host and in the cell containing it. However, Smith (1930) found that larvae of D. dynastes which hatch from eggs not placed directly on the host, usually die without reaching it. In solitary species, there is a pronounced cannibalistic tendency, and the larva that hatches first often destroys any remaining eggs. In cases where a number are able to hatch, the youngest, due to its greater mobility, is usually victorious in the combat for the host. Such an elimination of surplus individuals seems necessary because of the indiscriminate oviposition of the parent female, which is apparently unable to recognize hosts that are already parasitized.

Many individuals of Pteromalus puparum develop within a single host pupa, which results in the colony being too large for the available food supply. Numerous dead larvae are often found at the extremities of the pupa, these having died from starvation (Hardy 1933). It is evident that the food material at these points is exhausted more quickly than at the middle of the body, as Faure (1926) found that development of individuals in the extremities of the host is retarded. But Voukassovitch (1926) concluded that the retardation in emergence of a portion of the brood is independent of nutrition. It was found that emergence from pupae that had been parasitized on known dates often extended over several months. This appears to be a larval diapause of uncertain duration and affects a varying portion of each colony. Clausen (1940) regarded it unusual for the individuals comprising a single colony of an internal parasitoid to emerge so irregularly, and noted that it contrasts sharply with the synchronous development and emergence of polyembryonic Encyrtidae, which encounter the same adverse conditions through overcrowding, etc. However, Legner (1969) found that a spread of emergence was characteristic of a number of parasitic Hymenoptera in several families even though oviposition was limited to a 24h period. The spread was the result of differential rates of development in different stages; and which stage differed varied with the species.

Some solitary ectophagous species show a considerable adaptability with regard to the size of host individuals upon which development can be successfully completed. Noble found that Habrocytus cerealellae, when developing on small Sitotroga larvae, is able to pass through the final larval stage without feeding and to attain the adult stage. In such cases the larval stage was prolonged with resulting adults of minute size.

Stenomalus micans Ol., 4th instar larvae, which are parasitic in larvae of Chlorops taeniopus Meig., bears a specialized boring apparatus on its head that is used to break up the internal organs of the host and to effect emergence through the hardened shell of the host larva, which has died just as it was undergoing pupation (Kearns 1931). No feeding occurs during the 5th larval stage, which is very short in duration. Parasitism by Stenomalus results not only in appearance changes of the host larva, but in its activity. Healthy larvae move downward in a barley stem and, prior to pupation, turn about and ascent to a point just below where the leaf leaves the stem. Reddish-brown puparia are then formed. Parasitized individuals do not make this position reversal, and partly formed puparia are colorless.

Normally pupation in Pteromalidae occurs in the cell, cocoon or other cavity in which the host resided. However, Eupteromalus nidulans forms a naked pupa in the web of its lepidopterous host. Nasonia vitripennis pupae retain the larval exuviae about the posterior portion of the abdomen, and this, adhering to the meconium, attaches the pupa to the host puparium wall. Enargopelte ovivora Ishii is one of the few Chalcidoidea showing a tendency toward normal cocoon formation (Ishii 1928). Mature larvae, of which there may be ca. 10 in the egg chamber of Lecanium sp., spin individual, yellowish-brown cocoons.

In some species many individuals are able to develop on a single host. Martelli (1907) recorded 165 adults of Pteromalus puparum from a single pupa of the cabbage butterfly, and Picard (1922) reared 212 males from the same host and 47 Tritneptis klugii Ratz. (= P. nematicidus Pack.) have been secured from a cocoon of Lygaeonematus erichsonii Htg. (Hewett 1912). Roubaud reared 105 N. vitripennis from a single dipterous puparium. Such figures undoubtedly represent maximums for which food material was available. However, in most gregarious species development to maturity is possible even if only a portion of the available food material is utilized. The different species of Dibrachys usually develop in numbers of <10 on each host, and all the recorded species of Habrocytus are solitary. Muscidifurax raptorellus K. & L. has both solitary and gregarious races, the habit being under the control of polygenic loci (Legner 1987d, 1988a, 1989b, 1991). The gregarious races produce individuals of a characteristic size (Kogan & Legner 1970).

Life Cycle

Pteromalids usually have short life cycles, averaging ca. 3 weeks from egg to adult at room temperature. There was a minimum of 10 days recorded for Habrocytus cerealellae and Nasonia vitripennis. The females of many species require 1-2 days longer for development than do the males. The incubation of the egg requires from less than 1 day to 3 days, the larval stage 4-10 days and the pupal stage 4-14 days. A notable exception is E. ovivora, in which the egg, larval, and pupal stages take 7 days, 20 days and ca. 11 months, respectively (Clausen 1940/1962).

The availability of suitable host stages influences the number of generations per year. Most species produce generation after generation as long as hosts are available, but some species are limited to a fixed number. E. ovivora has only one generation per year which corresponds to the host cycle. Pirene graminea and Stenomalus micans have two generations, as do their respective hosts. However, Aplastomorpha calandrae (Cotton 1923) and H. cerealellae are able to produce several generations to each one of the host. In these species there is no need for synchronization of the cycles of parasitoid and host, for they attack insects infesting stored grains which have all stages present continuously. Trineptis klugii has ca. 6 generations each year on one brood of the host.

Most species that hibernate do so in the mature larval stage within the host cocoon, puparium or cell. But, Eupteromalus nidulans is found in the hibernation webs of the satin and brown-tail moths. E. ovivora, Rhopalicus suspensus Ratz., and Merisus febriculosus Gir. pass the winter in the pupal stage, while Dibrachoides dynastes and Pseudocatolaccus asphondyiiae Masi persist through the winter as adults. Other may pass winter as either mature larvae or adults.

A number of Pteromalidae are able to undergo long periods of inactivity as either larvae or adults when conditions are unfavorable. The relation of food to reproduction in Spintherus and Peridesmia was already noted, and it was shown that phasic castration in females may continue for a long time. This is one way of maintaining a species during periods of adverse conditions; another is larval diapause, such as is found in H. medicaginis Gahan and Nasonia vitripennis. In the former individuals have been observed to pass almost two years in the larval stage, as compared to the normal two weeks. Nasonia vitripennis may even pass several years in dipterous puparia when conditions are unfavorable (Clausen 1940/1962).

Adults live much longer than many other chalcidoid parasitoids. Species which normally pass the winter as adults are exceptionally hardy, and D. dynastes has been kept alive for >8 months at temperatures of 5-13°C. Species without diapause usually live 6-8 weeks.

Sex Ratio & Parthenogenesis

Females predominate in ratios up to 30:1 in Habrocytus medicaginis. Clausen (1940) noted that ratios for N. vitripennis vary from 1:1 to 10:1. The normal ratio for P. puparum is ca. 2:1; but George (1927) noted a seasonal variation, the ratio being 2.8:1 in springtime and 1.1:1 in autumn. In H. cerealellae, the field sex ratio is ca. 3:2. Experimental determinations give an increasing proportion of male progeny toward the end of the life of females (Clausen 1940/1962; Legner & Gerling 1967), even in thelytokous populations (Legner 1987c). Griswold (1929) recorded an excess of males in the ratio of ca. 3:1 in some rearings of Asaphes americana from aphids collected in glasshouses.

Extended studies in thelytoky have been done on Muscidifurax uniraptor Kogan & Legner, showing the presence of extranuclear influences (Legner 1985a,b; 1987b,c; 1988c). Certain bacteria have been found in the male and female reproductive tracts, which are capable of inducing endomitosis in, unfertilized eggs, thereby causing them to be diploid and female (E. F. Legner unpub. data, R. Stouthamer unpub. data).

For detailed descriptions of immature stages of Pteromalidae, please see Clausen (1940/1962).

References: Please refer to <biology.ref.htm>, [Additional references may be found at: MELVYL Library]

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