Humboldtians in Focus

The Secret Number One

By Lars Krogmann

Hardly anyone knows them, but if there were a contest for beauty, versatility and evolutionary success in the insect kingdom, chalcid wasps would be up on the winners’ rostrum. And quite apart from this, they make an important contribution to conservation, too.

The caterpillar is happily munching on an oak leaf when, suddenly, a tiny wasp lands on its back. Before the caterpillar can throw off the unwelcome guest by wiggling violently from side to side, it feels a sting. The caterpillar has no idea that this heralds its death knell. The parasitic wasp has already injected several eggs into the caterpillar; in a few days, tiny larvae will hatch. They will continuously feed on the host tissue and gradually hollow out their host from inside, whereby they initially spare the vital organs. The caterpillar will gradually get weaker and weaker and, after a final few agonising days, perish pitifully.

Two weeks later, all that is left of the caterpillar is a dried out shell with tiny yellow larvae crawling out of it. They pupate very near their host and after a further three weeks, numerous shiny green wasps emerge from the pupae.

A frequent inhabitant of gardens and parks: the chalcid wasp.
A frequent inhabitant of gardens and parks.
This chalcid wasp (Cecidostiba semifascia)
lays its eggs in the larvae of other
wasp species.
Foto: private

What sounds like a horror film scenario does actually take place billions of times a day all over the planet. Chalcid wasps dominate every habitat on land, and your average city-dweller would be amazed at the huge number of species of chalcid to be found in the garden.

Jewels of the insect kingdom

If the animal kingdom were awarding prizes for evolutionary success, beauty, number of species, or versatility chalcid wasps would have a good chance of winning every category. Chalcids have a shiny metallic surface which has earned them their English name, jewel wasps. These “jewels of the insect kingdom” are one of the greatest natural treasures on our planet. Their way of life is their capital. While they are larvae, nearly all chalcids live parasitically in the eggs, larvae or pupae of other insects, killing their host in the course of their development. Thiis sounds cruel, but actually it is a decisive regulating mechanism in the ecosystem. In the absence of parasitic wasps there would be an explosive proliferation of all the other types of insect, most of which are vegetarian. As a result, nearly all land plants would be stripped bare and destroyed in just one growing season.

So far, science has identified about 22,000 species of chalcid wasp which is probably only a fraction of the actual number of species. Roughly 800 of these species have already been used more or less successfully by humans for biologically protecting crops and stored produce from insect pests. The advantages of this are obvious: reduced crop failure and savings on insecticide and its damaging consequences. But to use this method successfully it is essential to identify the right species of chalcid wasp. Some types are so similar that you can barely tell them apart from their useful close relation, but they have specialised in different species of insect so that the real pest gets away and is not parasitised. The environment in which they are introduced can also be decisive. There are two species of chalcid, for example, which attack the larvae of the corn weevil, a dreaded stored produce pest. But, as current research shows, only one of the two is ideally suited for use in grain stores.

“In the absence of parasitic wasps there would be an explosive proliferation of all the other types of insect, most of which are vegetarian.”

Using useful insects to fight pests that have been introduced in other countries is not unproblematic either. The newly-introduced useful insects may not only attack the pest itself but closely related indigenous species, too, with unforeseeable consequences for the ecosystem. An instance of this has been well documented in the case of a braconid wasp which was only actually introduced into New Zealand for the biological pest control of two species of alfalfa weevil; the braconid did not only attack these pests but a total of 13 other indigenous species of weevil as well.

Chalcid wasp under a raster electron microscope.
Chalcid wasp (Gonatocerus morrilli)
under a raster electron microscope

Foto: private

So in order to apply parasitic pest controllers successfully it is essential to know all about the biology and taxonomy, i.e. the classification of species based on relationships. This is not as easy as it sounds, however: the average chalcid wasp is only about one to two millimetres long and can only be differentiated by tiny details, such as the form and length of the pronotum or the surface structuring of the cuticula.

Seeking junior researchers!

The difficulties involved in identification are not the only problem. In the entire world there are only a handful of taxonomists investigating chalcid wasps, most of whom work at natural history museums. And as the number of jobs for taxonomists is decreasing, they are likely to die out without successors at some stage. At the same time, there are many new species to be discovered and work to be done on investigating why so many different ways of life have emerged amongst chalcid wasps in the course of evolution. The author and an American colleague, for example, are in the process of describing a particularly spectacular new species in the Thai rain forests. Measuring almost four centimetres it is the largest species of chalcid wasp ever discovered. Its victims are probably the larvae of xylobiontic beetles.

Interrelations between the innumerable parasites, hosts and plants have existed for millennia – a complex symbiosis that is only disrupted when humans start cultivating monocultures, for example. Thus, to some extent, the useful insects and pests of the modern agrarian landscape have been created by humans themselves. Controlling their use and controlling them are two of the biggest challenges facing biological research.

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Lars Krogmann Lars Krogmann

Dr. Lars Krogmann is an entomologist and curator of hymenoptera in the Entomology Department of the State Museum of Natural History in Stuttgart. From January 2007 to March 2008, he was a Feodor Lynen Fellow, working at Adelaide University in Australia.

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