Overview

The biology of trigonalids is somewhat unusual compared with most other parasitic Hymenoptera in that oviposition does not directly target a host insect. Female trigonalids lay several very small eggs into incisions in a leaf, and each may lay up to several thousand eggs over a season. The eggs are then eaten by phytophagous lepidopteran (moth or butterfly) or sawfly larvae. The trigonalid eggs hatch inside the larvae where they may then develop as primary parasitoids of the phytophagous larvae, but most develop as secondary parasitoids (hyperparasitoids) if the primary phytophagous larvae is parasitised by an ichneumonid or tachinid fly (Diptera). Outside the Australasian region, trigonalids have also been recorded emerging from larval cells inside vespid nests , presumably due to parasitised caterpillars being fed to the vespid larvae by the adult worker wasps. More than one trigonalid larva may orginally infest a host, but ultimately only one adult trigonalid emerges. Australian species are known to be both primary parasitoids and hyperparasitoids (via tachinids or ichneumonids) of sawfly (Pergidae) a nd lepidopteran larvae.

Description

Adults are of moderate size (4–  12 mm) and superficially resemble some vespid or crabronid wasps. However, they can be recognised easily by their filiform antennae inserted below a frontal lamella, complete wing venation including a costal cell and pterostigma, tarsi with plantar lobes, and females usually having the ventro-posterior metasoma turned under and deeply notched so they can push eggs into the underside of leaves with their ovipositor.

Distribution

Trigonalids are a small family of about 120 species worldwide, with 12 species described from Australia. The family is absent from New Zealand.

Further information about the Trigonalidae can be found in Carmean 1991, Carmean 1988, Naumann 1991, Weinstein & Austin 1991 and Weinstein & Austin 1995.