UK Checklist of Trichogrammatidae (Westwood 1833)

Description & Statistics

This is a moderately sized cosmopolitan family with circa 71 valid genera and over 340 described species known as of 1993. They are recognized by their 5-9 segmented antennae which lack a funicle. The forewings either are wide with a short marginal fringe and distinct setal tracts, or are narrow with a longer marginal fringe. The tarsi are always 3-segmented and the family is readily identified by this character. Trichogrammatidae are among the smallest insects, adults of Megaphragma being <0.18 mm long.

All species of primary, endoparasitic egg parasitoids. There is a wide host range, many species parasitizing the eggs of Lepidoptera, but representatives of many other insect also orders are attacked. Both solitary and gregarious species exist, in the latter the number of individuals developing per host egg varying directly with the volume of the egg. Parasitized eggs usually darken. The family is important to biological control, several species having been deployed in periodic mass releases.

Gibson (1993) described this family as having a body usually 1 mm or less in length, lightly sclerotized, without metallic luster. The body is usually shriveled when air dried. The antennae are distinctly shorter than the combined length of the head and mesosoma, with 3-7 antennal segments (which includes at most 2 ring-like and 2 nonring-like funicular segments) and with a club of 1-5 segments. Some of the club segments or flagellomeres, may be confused with additional funicular segments in males that have a noncompact club. Individuals are usually fully winged. The fore wing varies from very wide (with apical margin almost truncate) to very narrow (and then with long marginal setae). The setae on the membrane are often partially aligned in longitudinal or radiating rows. The forewing marginal vein, stigmal vein, and postmarginal veins are variable, but postmarginal vein is often lacking. The marginal vein is short and the there is a relatively long stigmal vein, in which case the venation is somewhat S-shaped. If the postmarginal vein is present it is very short and does not extend past the point in line with the apex of the stigmal vein. The hind wing is usually with the membrane extending to the base, rarely stalked. The protibial spur is short and straight. The tarsi have 3 tarsomeres. The metasoma is widely attached to the mesosoma.

About 75 genera and 675 nominal species were recognized by Gibson (1993) as valid. The family was believed to be monophyletic based on the number of tarsomeres. Two subfamilies were recognized by Viggiani (1971): Trichogrammatinae and Oligosetinae. This was based on the male genitalia. Differences in male genitalia were also used to help delimit genera, and species in various genera, which requires good quality slide mounts for study.

Trichogrammatidae are all egg parasitoids of the large holometabolous orders and of Homoptera, Hemiptera, Orthoptera and Thysanoptera (Gibson 1993). Species seem more habitat specific than host specific, and females of some species attempt to oviposit into almost any egg of an acceptable size and shape. Species of Trichogramma are frequently used in biological control of particularly Lepidoptera, which has produced an enormous economic literature. Gibson (1993) believed that this literature was of largely of questionable value because of taxonomic confusion in species identification.

Key references are Doutt & Viggiani (1968), who revised and keyed the world genera, and Viggiani (1971) who presented a key based on male genitalia for 27 of the 72 genera that were recognized. Nagarkatti & Nagaraja (1977) reviewed the biosystematic literature on Trichogramma and Trichogrammatoidea, and Yousuf & Shafee (1986) provided a checklist and bibliography for the world species of the family.

Further Description.

This cosmopolitan family comprises a relatively small number of genera, the most common being Trichogramma. All species are primary internal parasitoids of the eggs of other insects. The greatest diversity in hosts is found in Trichogramma, where in T. evanescens Wstw. (minutum Riley) alone there are more than 150 host species, representing seven orders; Megaloptera, Lepidoptera, Hymenoptera, Coleoptera, Neuroptera, Diptera and Hemiptera, the majority of which are Lepidoptera (Martin 1928). Although several species are known to be represented under one name, an exceptional range in host preferences is still illustrated, with Schread & Garman (1933) referring to more than 215 host of the genus.

Flanders (1935c, 1938b) grouped trichogrammatids into three species, largely on the basis of the environment that they inhabit. Because of this difference in habitat, he found that each species had different host complexes, and the three are rarely found associated. T. embryophagum Htg. (pretiosa Riley) occurs in arboreal habitats, T. evanescens Westw. (minutum Riley) under field conditions, and T. semblidis Aur. in marsh habitats. Other less common genera are Chaetotricha, recorded from the eggs of Lepidoptera, Hymenoptera, Coleoptera and Hemiptera, Oligosita from coleopterous and hemipterous eggs, Ophioneurus and Poropoea in coleopterous eggs, Ufens in cicadellid eggs, and lathromeris in eggs of Orthoptera. The genus Trichogrammatoidea seems unique to the Australian continent.

Trichogrammatids, particularly the genus Trichogramma, are of considerable value in the natural control of their hosts, and are used extensively in inundation. In many cases a field parasitization of >90% is recorded, although this usually occurs relatively late in the season and consequently its value against multibrooded species, or those which cause their greatest injury during springtime and early summer, is reduced. Trichogrammatids suffer a very high winter mortality due primarily to a lack of host eggs in which to develop, and therefore they require a number of generations to build up their population to an effective level the following season (Clausen 1940/1962). Also a high percentage parasitization can be attained only in a high host egg population, due largely to the random searching of females. A factor operating against the attainment of a higher parasitization in eggs of pest species which deposit them in masses is that the parasitoid is usually limited to attack upon the topmost layer. Trichogrammatids are not known to search for host eggs in sequestered positions, such as those of pink bollworm, Pectinophora gossypiella Saunders.

Mass production of Trichogramma on a large scale was first attempted by Radetzky (1912), who imported T. semblidis (Pentarthron carpocapsae Ashm.) into Taschkent for the control of codling moth. The parasitoid was reared in the laboratory on eggs of the browntail moth. Since then, extended work in the biological control of a number of crop pests, particularly sugarcane moth borer, certain lepidopterous rice borers, and the codling moth, has been done in many countries. Most early attempts resulted negatively, but modern attempts have been more encouraging.

Specific Geographic Areas

NEARCTIC (CANADA).-- Yoshimoto (1984) noted that "Members of this family are small, ranging from 0.17 to 1.6 mm long. They are distinguished from all other chalcidoids by the following characters: Tarsi 3-segmented. Fore wings frequently with longitudinally radiating rows of setae (as in some Euderinae). Fore tibial spur short, straight, without strigil. Gaster broadly joined to thorax, penetrated by large muscle. Flagellum short, no more than 7-segmented (at most 2 funicle segments, 1 or 2 anelli, 1-5 club segments)."

"The family is distinct and widely separated from the nearest relative, Eulophidae. It is represented in North America by 17 genera, which can be identified by means of the key to world genera and subgenera by Doutt & Viggiani (1968). Only six genera are known from Canada: Trichogramma Westwood, Hydrophylita Ghesquière, Oligosita Walker, Aphelinoidea Girault, Paracentrobia Howard and Trichogrammatomyia Girault. The North American species of Trichogramma can be identified by means of the keys provided by Nagarkatti and Nagaraja (1971) and Nagaraja and Nagarkatti (1973). Pinto et al. (1978) re-examined the types and made re-descriptions of species of the common trichogrammatids in North America."

"Unlike the other families of Chalcidoidea, the Trichogrammatidae are classified largely on the male genitalia, especially in members of the genus Trichogramma, on which more papers have been published than any other group of chalcidoids. Doutt & Viggiani (1968) published a monograph of the 70 world genera of Trichogrammatidae with a key and a list of all known species. Viggiani (1971) divided the family into two subfamilies, with two tribes under each subfamily. Trichogrammatinae (Trichogrammatini, Paracentrobiini) and Oligositinae (Chaetostrichini, Oligostini), based on characters of the male genitalia. He has provided a key to 27 of the 70 known genera. Because of the minuteness and the degree of specialized work needed for this group, it seems best to place the entire group under one family for the purpose of this manual."

"Among the trichogrammatids, as in other groups of chalcidoids, some species show a certain degree of host specificity, whereas others show very little specificity. All members of this family, however, ad parasitic on eggs of other insects. Examples are Oligosita Walker and Paracentrobia Howard on Cicadellidae (Homoptera) and Lestidae (Odonata); Trichogramma Westwood on many different hosts; Trichogramma tomyia Girault on Lepidoptera; Aphelinoidea Girault on Cicadellidae (Homoptera)."

PALEARCTIC (EUROPEAN former USSR).-- Nikol'skaya & Trjapitcyn (1978/1988), as translated from the Russian, described this family as "Very minute chalcids; body length 0.18 to 1.2 mm. Antennal funicle usually with two segments, with one to two rings; sometimes segments of funicle annular or reduced. Parapsidal grooves of shield of mesonotum complete. Forewings often with rows of discal hairs, and often with long marginal fimbria. Tarsi always with three segments. Body yellow, brown, or black, without metallic sheen. Endoparasites of eggs of various insects, mainly of members of Lepidoptera, some beetles (Coleoptera), and cicadids (Auchenorrhyncha). More than 70 genera with 360 species; 8 genera with 15 species known in the European part of the USSR."

AFRICA.-- Prinsloo (1980) noted that like the mymarids, the species of Trichogrammatidae are usually very small and fragile, including some of the smallest insects known, many being only a fraction of a millimeter in length. Prinsloo (1980) elaborated on this family as follows:

Relationships & Diagnosis.-- "The Trichogrammatidae is probably most closely related to the Eulophidae and Aphelinidae, having in common a fragile body and reduced number of antennal segments. The trichogrammatids are however separated from all other chalcidoid groups by the tarsi which only have three segments."

"Small to minute chalcidoids, less than 1 mm in length; fragile, weakly sclerotized, the body usually pale, never with a metallic lustre; antennal sockets placed close to mouth margin; antenna short, usually similar in the sexes, with at most nine segments; funicle sometimes absent, the flagellum then only with one or two small ring-segments and a large club; if funicle distinct, then at most with two segments; club with one to five segments; thoracic dorsum sparsely setose, the axillae projected forwards; fore wing usually distinctly shaped: often broad with long marginal cilia, the wing disc with setae arranged in radiating rows; venation with postmarginal vein usually, and stigmal sometimes, absent; legs with tarsi three-segmented, the fore tarsus without a strigil; abdomen with gaster broadly sessile, its base broadly connected to propodeum."

Biology.-- "Like the mymarids, the species of Trichogrammatidae are all primary endoparasitoids of the eggs of other insects, showing a great diversity in host preference, sometimes even within a given species. Species representing all the more common insect orders have been recorded as hosts, and especially those belonging to the Lepidoptera, Hemiptera and Coleoptera, many of which are of economic importance. Many species of trichogrammatids are thus used in biological control programmes. The indigenous Trichogramma lutea Girault attacks the eggs of a wide range of injurious moths, including American bollworm, codling moth, false codling moth and apple leaf roller. Species of Trichogramma are also well known enemies of injurious moths and a wide range of other insects. Megaphragma is unique in that its species are probably parasitic in the eggs of thrips. it is interesting to note that some trichogrammatids, such as Prestwichia and Hydrophylita (neither yet recorded from Africa) have become aquatic and parasitize the eggs of winter inhabiting insects, such as water beetles and certain Odonata."

African Trichogrammatidae.-- "Doutt & Viggiani (1968), in their review on the genera of this family, record 17 genera from the Ethiopian region. The well known cosmopolitan Trichogramma is one of the most common genera found in this region. This genus is characterized by the fore wing which has the marginal and stigmal veins in the shape of an S. The female antenna has two ring-segments, two well defined funicle segments, and a large club, whereas the male has a long undivided, subcylindrical club with long setae; Trichogrammatoidea is much like the latter genus, but the male has a three-segmented club. Another large cosmopolitan genus is Oligosita, in which the antenna has one ring-segment, a single funicle segment, and a three-segmented club. In this genus the wing disc is almost devoid of setae. Megaphragma, comprising only a few species, probably includes some of the smallest insect species, measuring about 0.18 mm in length. In this genus the fore wing is long and slender with long marginal cilia, much as found in the species of Mymaridae."

INDIA & ENVIRONS.-- Hayat & Subba-Rao (1988) noted that "The family Trichogrammatidae consists of small to minute wasps that develop in the eggs of other insects. Trichogrammatids are of great importance in the maintenance of balance between the noxious insect pests and their parasitoids in nature. Being of minute size, the adults are easily carried by wind to new areas and this easy dispersal ensures their continued procreation. Species of Trichogramma have been extensively used in the biological control of pests with varied success. Several species are known to exhibit phoresy."

"Trichogrammatidae is comparatively a medium-sized family comprising about 75 genera and 500 species. They seem to be of recent origin though closely resembling species have been found in the Oligocene amber from Mexico. The recent forms appear to have been derived from Eulophidae or some ancestor resembling eulophids."

History.-- Girault (1911, 1912a,b, 1913) was perhaps the only chalcidologist who made great strides in the study of trichogrammatids, though Ashmead (1904) laid the foundation for subsequent taxonomists to work further. However, the need for a reliable system for classifying trichogrammatids was fulfilled by Doutt & Viggiani (1968). These authors studied all the available types and presented for the first time a workable key to the genera known from the world and gave notes on their distribution and synonymies."

"The family is represented in the Indian fauna by 25 genera and a further 5 or 6 genera are known from the neighbouring countries. Mani (1938) included 6 genera (three valid) in his catalogue. Later, Subba Rao, Hayat, Hayat & Viggiani, Nagarkatti & Nagaraja and Nagaraja made noteworthy contributions on the systematics of the family. Recently Hayat & Viggiani (1985) published a catalogue of the Oriental fauna of the family."

Classification.-- "Ashmead (1904) divided Trichogrammatidae into two subfamilies, viz. Oligositinae and Trichogrammatinae; the former with 5 and the latter with 9 genera. His classification was based on the discal ciliation of the forewing. Girault (1912) rejected this classification as discal ciliation of the forewing was found to be more variable. He found the venation more reliable and divided the family into two subfamilies, Chaetostrichinae and Trichogrammatinae, and further recognized two Tribes in each; Chaetostrichini and Lathromerini in Chaetostrichinae and Trichogrammatini and Poropoeini in Trichogrammatini and Paracdentrobiini, and Oligositinae into Oligositini and Chaetostrichini."

"A persual of the literature and the advances made in the taxonomy of this family lead us to believe that the family cannot be divided into subfamilies and tribes with any certainty as many morphological characters intergrade.

Biology and Behavior

Adult Behavior.

Adults bite a hole in the egg chorion in order to emerge. Faure (1926) found that all individuals of Trichogramma developing in the egg of Ascia left it through a single emergence hole. Emergence occurs during the early morning,and females are able to oviposit the same day. During oviposition in large host eggs, the parasitoid stands on the egg and inserts the ovipositor perpendicularly, while with small eggs this is accomplished by a backward thrust. In Poropoea, attacking curculionid eggs contained in leaf rolls, the ovipositor is inserted through one end of the roll. The female of several species of Trichogramma feed on the fluids exuding from the ovipositor puncture in the host egg.

Phoresy was reported in the family by Ferriere (1926a). Fourteen females of Oligosita xiphidii Ferr. were found clinging to the hind wings of a tettigoniid, Xiphidion longipenne de Haan, in Java. The host preferences of O. xiphidii are not clear, but it is assumed that the females attached themselves to Xiphidion in order to gain access to their eggs immediately after deposition. The ability of Trichogramma females to penetrate the chorion of different eggs increases with an increase in size of the females themselves. Large females are able to attack large host eggs with thick chorions successfully, while small individuals are restricted to eggs with light chorions. This physical limitation explains the different results obtained by various researchers using the same parasitoid species or race, but eggs of different host species in which to rear them. Tothill et al. (1930) found that where a choice of hosts is presented, the determining factor in selection is the relative toughness of the egg chorion.

Trichogramma are usually able to oviposit successfully in host eggs of almost any stage of development; but T. evanescens prefers freshly laid eggs of the gypsy moth and the cabbage butterfly, although those containing well developed embryos and even dead eggs may be successfully attacked. Several researchers have found that oviposition and successful development, is possible in host eggs in which the embryo is fully mature and on the point of hatching. Tothill et al. (1930) found that in Chilo, Schoenobius and other rice borers the embryo was killed by the insertion of the ovipositor of T. nana Zehnt., whether or not an egg was deposited. Martin 91928) and Peterson (1930) found that oviposition by T. evanescens inhibited further embryonic development. Several eggs may be deposited at one insertion of the ovipositor.

Simple stimuli operate on the female at the time of oviposition which leads her into numerous errors of instinct, resulting in oviposition or attempted oviposition in false hosts where development cannot take place. Holloway (1912) described oviposition by T. evanescens females in globules of partly dried sap of okra which had formed a surface film, which gave the globules a superficial resemblance to host eggs. Detailed examination of objects with the antennae seems to be for the purpose of locating a suitable point for ovipositor insertion, and Salt (1935) described attempted insertion into objects such as bits of glass, globules of mercury, plant seeds, etc., and in insect eggs in which development to maturity was impossible. Salt concluded that the principal criterion used by the female in her choice of hosts is size. Marchal (1936) similarly recorded oviposition of the same species in globules of sap of Hibiscus. He disagreed with Salt as to the dominant influence of size governing host selection, for surrounding conditions in conjunction with the senses of the female confound the issue. Clausen (1940) thought that probably the so-called errors of instinct occur much less frequently under natural conditions than in confined laboratory circumstances.

Oviposition seems random, the female being incapable of determining prior attack upon the host egg or at least she disregards it in oviposition. However, Salt (1934) found that this conclusion was erroneous for T. evanescens and that the female avoids oviposition in host eggs already parasitized either by herself or by another female. In cases where adequate supplies of unparasitized eggs are not available, the parasitoid will withhold deposition for a time rather than oviposit in hosts already attacked. When forced to deposit more than the normal number of eggs in a limited number of host eggs, the parasitoid chooses the larger hosts to receive surplus eggs. Superparasitization occurs in a much smaller percentage of eggs than would be expected from random oviposition. To verify this, a series of 10 females were confined for eight hours with 10 host eggs each, and a total of 138 eggs were deposited during this period. By chance, only 34.7% of these hosts should have received only one egg, while actually 69% contained only one. Restraint in oviposition was also indicated, for a control series of an equal number of females provided with an ample supply of hosts deposited 225 eggs during the same time period. Later Salt (1937a) studied the senses involved in selective oviposition, showing that sight, hearing and touch play no part in the choice of host eggs, but that a chemical sense was employed. The female is able to recognize the external odor of another parasitoid of her own species that previously had walked on or stung the egg. If the egg were washed, she would attack it without hesitation, though the ovipositor was quickly withdrawn if parasitization had previously been effected.

Reproductive capacity of Trichogramma varies. T. embryophagum was found to deposit a maximum of 29 eggs, while 50 were secured from T. evanescens (Schread & Garman 1933). The former is thought to be less prolific in small eggs and more so in large eggs than T. evanescens. Peterson (1930) found a much higher rate of reproduction, with one female of the latter species producing 131 progeny. Salt got a total of 225 eggs from a series of 10 females that were provisioned with an adequate supply of host eggs. T. evanescens and T. embryophagum had a mean reproductive capacity of 37 and 62, respectively (Bowen 1936).

Lund (1938) found an average length of adult life to be circa 6 days at a temperature of 25°C. There were 40% of eggs deposited within 12hr after adult parasitoid emergence, and 76% within two days. Withholding host eggs until 2-3 days after emergence materially reduced female productivity, although it did not affect their longevity. Females remaining unmated throughout the oviposition period produced many more eggs than did mated females, which is in opposition of that known for most other parasitic Hymenoptera.

Adult Dimorphism

Seasonal dimorphism occurs in T. cacoeciae Marchal, parasitic in eggs of Cacoecia roseana L. in France (Marchal 1927, 1936). This host has a single generation each year, with the eggs deposited during July and persisting in this stage until the following spring. Two generations of T. cacoeciae develop in these eggs. Adults of the overwintering generations, which are rather dark in color and possess vestigial wings, emerge in late March or April. The second generation, being on these same overwintering eggs, must develop in those in which embryonic development is considerably advanced. The adults resulting from this generation are light colored and possess normal wings. Adults from the overwintering generation in Mamestra eggs, after a number of generations of the normal winged form in that host during the one season, similarly show a marked tendency toward micropterism. Salt (1937b) in a study of the dimorphic T. semblidis, parasitic in the eggs of Sialis in England, used females that were mated with these apterous males, and the following generation was produced in grain moth eggs. He found the males of this brood fully winged. Such an influence of the host on the form of the parasitoid attacking it was unexpected. The two forms of male are distinct, and intermediates do not occur. Other parts of the body as well as the wings were changed, and this dimorphism of the males was considered to be comparable in some respects to that of the females in the Cynipoidae and in some social Hymenoptera by Clausen (1940). The changes brought about likewise have a nutritional basis, but in quality rather than quantity. Winged males from Sitotroga and Ephestia eggs are smaller than the apterous males from Sialis.

The appearance of vestigial winged forms of T. semblidis (T. minutum) developing in eggs of Sialis infumata Newm. was noted by Martin (1928). All males and a portion of females were of this form, and the males of this species are regarded to be commonly dimorphic (Clausen 1940/1962). Flanders (1931) studying races of T. evanescens, found that temperature influenced the coloration of adults. The general yellow body color was obscured by dark pigments when development took place at lower temperatures.

Life Cycle

Immature Stages of Trichogrammatidae

The egg forms of very few species of the family were known by 1940 (Clausen 1940). Those of several Trichogramma are somewhat elongate, with the middle portion distinctly expanded, and both ends are smoothly rounded (Fig. 42A). The egg of Oligosita utilis is of similar form, though with a short, heavy peduncle at one end. In Poropoea stollwercki (Fig. 43A), the main body is 0.5 mm. in length, elongate and irregularly curved, and it bears a slender peduncle at the anterior end. The narrower posterior portion represents a somewhat ringed appearance. The egg of P. defilippii Rond. is similar, although the peduncle is shorter and more delicate. In Chaetostricha pulchra, figured hy Bakkendorf (1934), the main egg body is cylindrical, and the anterior peduncle, which is half as long, is set at an angle with the main axis of the egg.

There is a considerable increase in volume of the eggs during incubation in Trichogramma. Larvae lie immobile in egg fluids, gradually consuming them completely, and they pupate in situ. In Poropoea and other genera having mymariform larvae, the younger stages are more active and are thought to be able to disorganize the egg contents by lashing movements of the tail. Oligosita utilis Kow. differs from other species of the family in voiding the meconium at the end of the larval period rather than retaining it until after adult emergence. Flanders found that the pupal skin and the last larval exuviae also are scraped off by the adult of Trichogramma as it emerges. In Oligosita that attacks flat eggs, the pupal normally lies upon its back, a position that facilitates adult emergence.

Parasitism by Trichogramma is quickly apparent and produces noticeable physiological changes than that of most other groups of egg parasitoids. In T. nana (Tothill et al. 1930), the area surrounding the point of puncture in the Artona egg becomes blackened in 2-3 days after attack. Thereafter, black streaks and patches appear in the same general area and extend down the side of the egg. In the end this color change becomes uniform and complete over the entire egg, with dissections showing that it is due to a deposit of minute, dark granules on the chorion's innersides. Faure (1926) studying T. evanescens as a parasitoid of Ascia eggs, found that this color change occurred in the vitelline membrane. Flanders found that the discoloration of the vitelline membrane took place during the parasitoid's prepupal stage. In the case of eggs having a translucent chorion, it serves as a means to distinguish parasitism by Trichogramma from that of other parasitic groups. Coleopterous eggs parasitized by Poropoea become reddish rather than black.

The volume of the egg determines the number of individuals that can be produced from it. A sufficient number of parasitoid eggs is usually deposited to guarantee complete consumption of the contents by the developing larvae. Trichogramma is usually solitary in Sitotroga eggs. Flanders (1935a) found that 1-10 T. evanescens develop in each egg of Estigmene acraea Drury and that, irrespective of the number of individuals involved, the total mass of adults from one egg is nearly constant. He also recorded the development of 50-75 individuals in each egg of Pachysphinx, and up to 69 of Trichogramma sp. from the Philippines have been secured by Shibuya & Yamashita (Clausen 1940/1962) from eggs of Dendrolimus spectabilis Butl. in Japan. Studying several thousand adults of T. evanescens emerging from eggs of the corn earworm, Barber (1937) found that the population in each egg ranged from 1-5. In relative volume the smallest males were only 1/25th the size of the largest, and the smallest females 1/40th the size of the largest female. Thus, there is a wide range in the quantitative food requirements of the individuals of that species.

There are two distinct types of first instar larva, the first of which is sacciform almost globular or cylindrical, and lacks sensory and integumentary setae and other external characters. The mandibles are minute but distinct. Those of Trichogramma (Fig. 42B), Chaetostricha, and Oligosita are of this type. The second is much more highly specialized, being mymariform, with the head and thorax appreciably larger than the abdomen, the segmentation distinct, the caudal segment drawn out into a tail, and the thorax and abdominal segments bear long setae. This type occurs in Poropoea and Ophioneurus. The first instar larva of P. stollwercki (Fig. 43B), as described by Silvestri, is 0.28 mm. in length, with the head and thorax exceptionally large, and the abdomen consists of six ring like segments and a seventh that is curved ventrally and extended into a point. There are 16-18 long, slender setae in a transverse row at the posterior margin of the thorax, situated dorsally and extending to the lateral margins. All abdominal segments except the last bear a smaller number of these setae dorsally near the anterior margins. The larva of Ophioneurus signatus Ratz. (Fig. 44), described by Bakkendorf (1934), has a distinct head; but the thorax and abdomen are unsegmented, almost spherical, and the last segment is produced into a slender, curved tail which bears a tooth at the mid‑ventral margin. In addition, there is a long, heavy process or spine arising dorsally slightly in front of the base of the tail.

There has been considerable disagreement as to the number of larval instars in the family. Taylor stated that there is only one in Oligosita utilis, and Bakkendorf was unable to find evidence of intermediate molts in Chaetostricha pulchra. Flanders described three instars in Tricho­gramma, and Silvestri recorded five for Poropoea stollwercki, though here, also, the evidence of a corresponding number of molts is incomplete.

The second instar of Trichogramma (Fig. 42C) is somewhat elongate and tapering anteri­orly, and the segmentation is indicated only on the anterior half of the body. The man­dibles are extruded and only slightly curved. The presumed second instar larva of P. stollwercki (Fig. 43C) is still mymariform, with the abdomen further reduced, and the tail exceeding the body in length. It appears more probable that this larva is still of the first instar rather than a distinct second. Even the third instar as figured (Fig. 43D) is identical with the first except for the elimination of the segmental lines, possibly due to an increase in volume through feeding.

The mature larvae of all genera are of similar form, being robust, more or less distinctly segmented, and without spines or setae. The mandibles are elongate and extruded and lie parallel to each other. They are immovable and consequently not used in feeding, though they may serve to lacerate the remaining embryonic tissues in the host egg. Several authors emphasized the complete lack of a tracheal system in the larvae of Trichogramma, and it has not been mentioned in other genera.

The life cycle of most species of trichogrammatids is very short. For Trichogramma this is 7-10 days at summer temperatures. Flanders found that in T. evanescens, under conditions that yielded adults 8 days after oviposition, incubation of the egg was complete in 22hrs and larval feeding in 26hrs. Others have reported 3-4 days as the duration of the larval feeding period, and Lund (1934) found that the optimum conditions for development of T. evanescens was 32°C and 100% RH. Salt believed that the feeding period involves mainly ingestion, and that digestion followed later.

Generations are continuous as long as suitable host eggs are available and environmental conditions are favorable. An exception is the Trichogramma reared by Marchal from the eggs of Cacoecia in France. This species produced only two generations per year, a limitation imposed by the annual cycle of the host. This seasonal cycle is not a fixed attribute of the species itself, as was demonstrated by the rearing of 7-9 generations in the eggs of mamestra during one season in the laboratory. Embryonic development of mamestra is completed in a short time, and the cycle of the parasitoid is thus expedited. Prolongation of the cycle in fresh eggs of Cacoecia was correlated with the less rapid development of the host embryo. Larval diapause is induced by physicochemical influences, in particular those affecting enveloping membranes. Poropoea stollwercki, also has only two generations in the eggs of Attelabus in Italy (Clausen 1940/1962).

Life cycles of genera other than Trichogramma are usually much longer. Oligosita utilis had a minimum cycle of 42 days at 29.4°C in Fiji, of which circa 33hrs were required for incubation of the egg, 3-5 weeks for larval development and 3 weeks for the pupal period. The actual feeding period of the larva covered only a few days, however. Silvestri (1916) found that Poropoea stollwercki Foerst. required 14-15 days for the early summer generation. Chaetostricha pulchra Kryg. had only a single annual generation in eggs of Tettigonia and a larval development was completed in one month (Clausen 1940/1962). A short cycle of 7 days was recorded for C. mukerjii Mani in eggs of Bruchus quadrimaculatus F. in India (Mukerji & Bhuya 1936).

Hibernation has not been demonstrated in any species of Trichogramma, although it is reasonably certain that adults do not persist through the winter. There is no obligatory diapause, except such as may be imposed by the host, as in T. cacoeciae, and the development of immature stages, as well as activity of the adults, takes place whenever temperature is suitable. This can be a very low levels in some species. At the latitude of tokyo, Japan, adults were found to emerge and oviposit during February; all immature stages could consequently be found throughout the winter, but under more severe conditions, only mature larvae could be found (Clausen 1940/1962). In P. stollwercki, with only two generations per year, mature larvae persist in the host egg within the leaf roll from June until the following spring. Chaetostricha and Ophioneurus, which have only 1-3 generations in temperate regions, have the same hibernation habit, and there appears to be a true diapause in the mature larval stage.

Sex Ratio & Parthenogenesis

Female/male sex ratios in American races of T. evanescens and T. embryophagum are circa 2:1 and 4:1, respectively. However, these ratios may be upset by abnormal temperature and RH. Schread & Garman (1934) found that refrigeration of mature larvae at 8.3°C or lower resulted in a preponderance of males in the following generation. Flanders (1935a) found that T. evanescens is always of female sex when developing from a solitary egg in an Estigmene egg, while if three mature in each host, which is normal, the brood usually consists of two females and one male. Jones (1937) determined that the ratio in field-collected material of T. lutea Gir. was circa 2:1 and that, as in T. evanescens, a solitary individual is of the female sex, while if two are produced in the host egg they are usually of opposite sexes. With greater number there is usually only a single male in the group. Salt found that in using host eggs of smaller size, the proportion of males increased directly with the extent of superparasitization, for males dominate when in competition with females (presumably from a lower food requirement). Taylor (1937) observed that Oligosita utilis usually produced 3-4 individuals in each egg of Promecotheca, and that a single male was usually present in each group, the normal sex ratio being circa 2.5:1.

Unisexual reproduction is normal in several geographic races or strains of the more widely distributed species, such as T. evanescens and T. embryophagum. It occurs in T. cacoeciae and in one race of T. fasciatum in Europe; but in T. flavum Ashm., discussed by Marchal as a form of the first-named species that develops in Mamestra eggs, an occasional male was produced. In laboratory cultures of T. cacoeciae, the first male appeared in the 13th generation and several additional in the 26th and 27th generations. The latter mated normally and all progeny were females. Hase (1925) working on an undetermined species of the genus, found that virgin females produced about equal numbers of male and female progeny. The true taxonomic position of these various supposed species, races, etc. is confused, however (Clausen 1940/1962). Marchal reported on the occurrence of both bisexual and unisexual strains of a T. evanescens reared from Sialis eggs in France; but this is in contradiction to results of Salt (1938) with a species from eggs of the same host in England, which he considered to be T. semblidis. All progeny from virgin females were males. The work of Stouthamer on Trichogramma species has revealed the presence of bacteria in the reproductive tract of males and females which cause the thelytokous state. Work by Legner on other parasitic Hymenoptera has also pointed to the possible involvement of microorganisms in thelytokous reproduction.