Description & Statistics

Trigonalidae. -- Trigonalids are a small group of rare hymenopterans that are of average size and quite brightly colored. Their bodies are stout and they resemble wasps, but have long and multisegmented antennae.

Trigonalids are parasitoids of Vespidae or hyperparasitoids of caterpillars. The very small eggs are laid in large numbers on plant foliage. In the case of the species attacking caterpillar parasites, the eggs hatch when eaten by a caterpillar, and the wasp larvae attack the ichneumon, tachinid, or other parasitoid larva that may be present. In the species that attack vespid larvae, the eggs are eaten by a caterpillar, which is in turn eaten by a vespid wasp, which in regurgitating the caterpillar and feeding it to its young, transfers the trigonalid larvae from the caterpillar to the wasp larvae (Borror & DeLong, 1954).

This is a little known family represented by relatively few genera and species, but is nevertheless distributed worldwide. Wheeler (1923) considered it to be one of the most archaic groups of Hymenoptera. Some researchers consider the family to be most closely allied to Vespoidea, although its habits and parasitic life link it closely with Ichneumonoidea, in which superfamily is usually placed. Host preferences are obscure. Various species have been reared from the nests of Vespoidea and several from cocoons of ichneumonoid Hymenoptera and from puparia of Tachinidae. In the case of Vespidae, the species reared from the nests appear to be primary parasitoids, while those from ichneumonid cocoons and dipterous puparia are hyperparasitoids, through these hosts, of caterpillars and sawfly larvae. Australian Trigonalys maculatus Smith, however, is a primary parasitoid of sawfly larvae in the genus Perga (Clausen 1940/1962).

Mason (1993) noted that in trigonalyids the antennae are inserted on the frons under a small lobe, and there are >20 flagellar segments. The forewing has 10 closed cells, the costal cell being wide. The hind wing has 2 closed cells. Tarsomeres 1-4 has apicoventral plantar lobes. The metasoma is pedunculate, and the ovipositor is reduced.

Life histories are complex, with many details being unknown. These parasitoids lay thousands of minute (0.1 mm) but thick-shelled eggs on the underside of leaves near the margin. The eggs remain unhatched until they are eaten by a caterpillar (either Symphyta or Lepidoptera). Cracks are made by the jaws of the caterpillar and its digestive juices cause hatching. The newly hatched larva makes its way into the body cavity of the caterpillar. If the host already has a parasitoid larva, that parasitoid is attacked. Otherwise the trigonalyid larva seems to wait until the caterpillar is parasitized when it attacks only the parasitoid, not the primary host. Some trigonalyids have been roared from larvae of Vespula. They are thought to reach their host by way of infested caterpillars that are fed to the wasp larvae by worker wasps. There are ca. 75 rare species in 22 genera worldwide. In North America only 4 rare species are found in Canada (Mason 1993).

Schultz (1907) revised the world genera and Bischoff (1938) and Weinstein & Austin (1991) did a catalogue of the species of the world. Clausen (1940) and Weinstein & Austin (1991) reviewed their biology. Other Key references are Townes (1956), Oehlke (1984) ,Tsuneki (1991) and (Mason 1993).

Biology & Behavior

Detailed biological studies were made by Clausen (1929, 1931a) on Poecilogonalos thwaitesii Westw. and species of several other genera, van der Vecht (1933) on Nippogonalos jezoensis Uch., parasitic in larvae of Vespa spp., and Raff (1934) on T. maculatus. All species seem solitary in habit and develop internally in the mature larvae and prepupae of the various hosts. A complete life history, with descriptions of immature stages, was not available for any species when Clausen (1940/1962) wrote his monumental book, Entomophagous Insects. Thus, it is not known how the young larva gains access to its primary host.

Oviposition habits of the family are quite uniform. Bugnion's (1910) study of the anatomy of Pseudogonalos hahni Spin, revealed 3-4,000 minute eggs in the ovaries of each female. Wheeler surmised that oviposition would be found to take place upon foliage and that the first instar larva would be an active form of the planidium type. The first part of this conjecture was proven correct. Leaf oviposition trigonalids was first observed by Teranishi (1929) in Poecilogonalos maga Tera, and a similar habit has since been noted by other researchers in P. thwaitesii, P. henicospili Roh, Orthogonalos debilis Tera, Nippogonalos jezoensis Uch, and Pseudogonalos sp. (Clausen 1940/1962). The female stands on the upper surface of the leaf, curves the tip of the abdomen beneath the margin, and deposits the egg on the lower surface at a distance of 0.5 to 1.0 mm from the edge. In N. jezoensis, a modification of this habit was noted by van der Vecht, where the eggs were placed singly in minute incisions in the leaf tissue but near the margin, and the leaf tissue was damaged on both sides. They are placed in the foliage of a wide variety of plants and, in some cases, in the petals of the blossoms.

The eggs of a particular species under field conditions in a given locality are usually deposited on a single species of plant, although in another locality the plant chosen may be entirely different (Clausen 1940/1962). Physical qualities of the leaves have a direct bearing on the readiness with which females oviposit on different plants, but the principal influence may be the occurrence of the caterpillar or sawfly host on the foliage (Clausen 1940/1962). There is no direct evidence to indicate that the latter is true, and caged females oviposit quite as readily on clean foliage as on that on which caterpillars are present or on which they had previously fed.

Oviposition capacity is exceedingly high, as found by Bugnion. Actual oviposition records show the deposition of 3,559 eggs in four days by a female P. maga, while 10,641 were secured from a single P. thwaitesii female in 14 days and 5,782 in 6 days from P. henicospili. The P. thwaitesii individual referred to by Clausen (1940/1962) deposited 4,376 eggs in a single day. This high reproductive capacity was considered essential in view of the mortality factors operating prior to the time the primary host is found.

Clausen (1940/1962) noted that the microtype eggs of trigonalids consistently fail to hatch when left on foliage, although eggs several months old were determined to contain viable larvae. Experiments with chemicals, such as a weak solution of K(OH), induced emergence provided that the chorion was first ruptured. This led to the belief that the eggs must be eaten by the host in order to secure normal hatching, a conditions already known in Tachinidae. Tests with lepidopterous larvae proved this to be true, and hatching resulted from the cracking of the chorion by the mandibles of the caterpillar followed by the stimulating effect of digestive juices. The larvae were found free in the alimentary tract 1-6 hrs after ingestion of the eggs by the caterpillar and shortly thereafter they entered the body cavity. Several 1st instar larvae of P. maga were found within the body of a sawfly larva collected in an area where the species was known to occur, thus verifying the conclusions arrived at experimentally (Clausen 1940/1962).

The larger larvae of Vespa velutina Lep. and V. analis F., which are the hosts of N. jezoensis, are fed mainly, if not exclusively, with fragments of bees, flies, ants, etc., and there is thus no direct clue as to the means by which the Nippogonalos larva reaches its host (van der Vecht 1933).

First to third instar larvae develop internally in the body of the host, and the surplus individuals are eliminated in the 3rd stage. The mandibulate 3rd instar larvae show a strong cannibalistic tendency. Just prior to issuance, the parasite larva assumes a position immediately beneath the derm of the thorax, with the head embedded in an eye of the developing host pupa, and emergence of the 4th instar larva always occurs at this point. Emergence from the host prepupa or pupa occurs immediately after the third molt of the parasitoid, and death of the host follows. Feeding then takes place externally until the 4th molt. Despite its heavy tridentate mandibles, the 5th instar larva feeds very little; only a portion of the fluid contents of the host body is removed, and no solid tissue is eaten.

Mature larvae spin irregular cocoons within that of the host, and partitions off the meconium of the host and its putrefying remains. Raff (1934) found that Trigonalys maculatus does not spin a cocoon. Instead, a transverse partition of host origin in the Perga cocoon separates the pupa from the exuviae of the sawfly host, and the prepupal remains of the latter are within the cell occupied by the trigonalys pupa. Van der Vecht (1933) thought that the larva of Nippogonalos emerged from the body of its Vespa host after the latter had closed its cell in preparation to pupation and that the mature parasitoid larva, after completing its feeding externally, made a cross wall of silk at the middle of the Vespa cell, thereby isolating the host remains in the lower portion. This lower compartment was apparently opened and cleaned by the Vespa workers, and the parasitoid adult later emerged through a hole made in the cross wall.

The life cycle has not been definitely determined for any species, but evidence points to a general one year cycle in temperate regions, while in tropical areas it must be longer than that of the host, for the duration of the egg stage is exceedingly variable and may extend over several months (Clausen 1940/1962). Development of the early stage larva is then delayed until the host approaches the prepupal stage.

Please refer to Clausen (1940/1962) for an account of the developmental cycle of members of this family.

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

Bennett, D.J. & A. S. Lelej. 2003. To the knowledge of trigonalyid wasps (Hymenoptera) of the Sakhalin. Far Eastern Entomologist 130:8

Lelej, A.S. 1995. [Fam. Trigonalidae - Trigonalid wasps]. In: Kupianskaya A.N., Lelej, A.S., Storozheva, N.A. (eds) [Keys to the insects of Russian Far East]. IV(2): 8-14

Lelej, A.S. 2003. A review of the Family Trigonalyidae (Hymenoptera) of the Palaeartctic Region. Far Eastern Entomologist 130:1-7.

Nel A., V. Perrichot & D. Néraudeau 2003. The oldest trigonalid wasp in the Late Albian amber of Charente-Maritime (SW France) (Hymenoptera: Trigonalidae). Eclogae Geologicae Helvetiae 96: 503-508.

Poinar G. 2005. Fossil Trigonalidae and Vespidae (Hymenoptera) in Baltic amber Proc. Ent. Soc. Wash. 107 (1): 55-63

Smith, DR & I. C. Stocks. 2005. A new trigonalid wasp (Hymenoptera : Trigonalidae) from eastern north America. Proc. Entomol. Soc. Wash. 107: 530-535