“The Alder Woodwasp and its Insect Enemies”
The film describes in detail the biology of a natural complex of insects and is, in this respect, probably unique. Some of the behaviour patterns seen in the film provide useful clues for systematists, throwing light on the affinities of the parasitoids with other related species.
The sequences on egg emission (Xiphydria, Rhyssella, Pseudorhyssa) have helped to elucidate the method whereby the egg passes down the narrow channel of the ovipositor, which in Pseudorhyssa is only 1/5th the diameter of the egg.
The film is divided into five parts in the following order:
“The Alder Woodwasp” (Xiphydria camelus) Adult 3/4 in. long.
The female woodwasp, with her ovipositor, drills an egg shaft through the bark of recently dead alder and lays a group of eggs between bark and wood. The woodwasp larva at first bores down into the wood and then, after about 10 months, when full grown, it tunnels up to the surface, but not through the bark. The larva pupates, the young adult sheds the pupal skin two weeks later and eats its way out through the cap of bark. Its mandibles are hardened with a zinc-protein complex. Mating is shown on the surface of the bark.
Ist Parasitoid (Aulacus striatus) Adult 1/2 in. long
The female pushes her thin ovipositor down the egg-shaft of Xiphydria and inserts her egg inside a woodwasp egg. She does this to as many eggs as she can reach. When it hatches the woodwasp larva thus has an Aulacus larva inside its body. The Aulacus feeds slowly on the host’s blood and not until a year later, by which time the woodwasp grub has grown quite large and has bored up to the surface of the wood, does the Aulacus finally kill the woodwasp by feeding on its vital organs, and chews its way out of what is left of the woodwasp body. The young Aulacus adult in due course escapes to the outside by gnawing a tunnel through the bark. As with the host the mandibles are hardened with a zinc complex. The delayed development of Aulacus, which allows the host to continue feeding until it reaches a suitable size and location, is called koinobiosis or the koinobiont strategy.
2nd Parasitoid (Rhyssella curvipes) Adult 1 1/4 in. long
The female Rhyssella, by tapping the bark with her antennae, locates a woodwasp grub in the wood below – (sometimes as deep as 3/4 in. in the wood). With her long flexible ovipositor Rhyssella then drills down to the host, (the cutting ridges are hardened with manganese), and, after stinging it, deposits an egg on or near its body. The Rhyssella larva, born in the midst of plenty, feeds rapidly,(the idiobiont strategy), and is full grown in 16 days: it remains motionless in the woodwasp tunnel for ten months before pupating: two weeks later the young adult gnaws its way out through the wood. Mating is shown on the surface of the bark.
3rd Parasitoid (Pseudorhyssa alpestris) Adult 1 1/4 in. long
Pseudorhyssa is also an Ichneumon, similar in size and shape to Rhyssella, but she does not drill her own egg-shaft to reach the woodwasp grub; she makes use of the egg-shaft made by Rhyssella. Pseudorhyssa thus lays her egg on a woodwasp which already has a Rhyssella egg on it. Soon after the two parasitoid larvae hatch the Pseudorhyssa larva, which has very big jaws, seizes the Rhyssella larva and squeezes it to death. It then has the woodwasp to itself and development thereafter is the same as for Rhyssella. Pseudorhyssa is a cleptoparasitoid employing the idiobiont strategy.
4th Parasitoid (Xiphydriophaga Meyerinckii) Adult 1/8 in. long.
Unlike the previous three parasitoids, which are solitary species, Xiphydriophaga is a gregarious idiobiont. After mating on the surface of the bark the female chalcid enters an old woodwasp exit hole and burrows down through the wood-dust until she is near a woodwasp larva in the surrounding wood. To reach her victim she must now burrow through the wood (up to this point the old unoccupied tunnel has provided easy access into the depths of the alder timber). At all times the chalcid keeps her route clear by ejecting to the outside all boring products. Having broken through into the inhabited tunnel the chalcid quietens the woodwasp grub by stinging it and lays up to 30 eggs on its body. The chalcid larvae feed as a group on the surface of the victim and the following year, after pupating, the young adults escape to the outside by the open route prepared by their mother. Mating is shown on the surface of the bark. Having mounted the female the male taps her head with his antennae at the same time vibrating his wings: mating is then attempted. If the female is not receptive the male repeats his routine.