A precarious existence

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Among the huge diversity of insect life there are some bewildering complex life cycles, but there are few that can compete with the trigonalid wasps for the seemingly haphazard way they ensure their genes are passed to the next generation.

In most cases, a female parasitoid wasp deposits her eggs on or in the host, but this is far too pedestrian and safe for the trigonalids. These mavericks of the wasp world like to make things more difficult for themselves so the female deposits her eggs on plant leaves. If this were the end of the story these wasps would not have lasted very long. No, the egg-in-the-leaf-trick is merely a ruse. The female trigonalid uses her short ovipositor and the unique structure of her abdomen to punch out a small piece of tissue from near the margin of a leaf, replacing it with a flattened egg. She deposits a few eggs in one leaf before moving onto another leaf perhaps on a nearby plant. This goes on until she gets eaten or exhausts her eggs, which in some species of trigonalid can be as many as 10000 – a huge number for a parasitoid, which underlines just how haphazard this strategy is.

The whole purpose of going through the laborious act of laying thousands of eggs in leaves is so that her offspring can get swallowed by a caterpillar or the larva of a sawfly. If the very tough egg is lucky enough to get swallowed it hatches which is thought to be triggered by the physical action of being chewed and/or salivary secretions. For most parasitoids, getting into a plump caterpillar would be mission completed and cause for celebration, but no such luck for the tiny trigonalid larva. The tiny larva winds up in the caterpillar’s gut and wastes no time in breaking out of there to gain entry to the haemocoel, the insect body cavity that is bathed in haemolymph. The trigonalid makes its way through this liquid searching for its real quarry – another parasitoid that has already infected the caterpillar. Lots of parasitoids, such as other wasps and flies, are dependent on the larvae of lepidoptera and sawflies and it is these the trigonalid larva is after. If it’s lucky it will find its prey, attack them and eat them, but in many cases the trigonalid that has defied the odds to get swallowed by a caterpillar will find nothing suitable to predate once inside or the only parasitoid within the caterpillar will be too large for the trigonalid to tackle. In both cases the poor little trigonalid is doomed. However, some species of trigonalid are able to sit tight inside the caterpillar until its gets parasitized by an ichneumon or a tachinid fly. There are a couple of drawings of trigonalid larvae below.

This isn’t the only bizarre life-cycle of the trigonalids. Another strategy used by some species also hinges heavily on coincidence, but the supporting cast is slightly different. These species still depend on a caterpillar or sawfly larva, but this time the caterpillar must be captured by a vespid or eumenid wasp, butchered and fed to one of the wasp’s grubs back at the nest. In the flesh of the dead caterpillar are the eggs of the trigonalid and once swallowed they hatch to feed on the unfortunate wasp larva.

In the world of wasps, trigonalids are something of an enigma. Only around 100 species are known, the adults don’t live very long and their precarious way of life means they are rather rare, so we don’t know a great deal about them. Their geographic distribution, appearance and lifestyle suggest they are very ancient and some entomologists have theorized they may be something of a missing link between the sawflies and the rest of the hymenoptera (the apocrita). Indeed, the oldest trigonalids are known from 100 million year old lumps of Cretaceous amber, which shows this way of life, precarious as it may be, has been going on for some time.

Further reading

Weinsten, P. & Austin, A.D. 1991. The host relationships of trigonalyid wasps (Hymenoptera: Trigonalyidae), with a review of their biology and catalogue to world species. Journal of Natural History 25, 399-433.

Yamane, S, 1973, Descriptions of the second to final instar larvae of Bareogonalos jezoensis with some notes on its biology (Hymenoptera: Trigonalidae), Japanese Journal of Entomology (Kontyu), 41, 194-202.