Asobara japonica

Drosophila suzukii is a fruit fly like no other. Native to Asia, it is related to that common lab workhorse(fly) Drosophila melanogaster, but unlike most Drosophila, which lay their eggs on overripe and rotting fruit, D. suzukii has a saw-like ovipositor that allows it to lay its eggs in fruits that are still ripening. Recently D. suzukii has been spreading its wings over the American and European continents, earning the title of being a pest species as it attacks a wide range of soft-skinned fruits including strawberries, cherries, grapes, nectarines, pears, and peaches.

The usual adversary of fruit flies is Leptopilina heterotoma, a parasitoid wasp that can devaste Drosophila maggots. It is such a threat that some maggots resort to imbibing alcohol to stave off this parasitoid. But while L. heterotoma is a menace to most fruit fly maggots, the maggots of D. suzukii is the first Drosophila found to stop that wasp in its tracks. The secret lies in the fly's blood. Insects and other invertebrates have blood cells called hemocytes that patrol their bodies, clotting wounds and entombing foreign invaders in hardened capsules. Leptopilina heterotoma disables those defensive cells by unleashing a virus that destroys them.

Asobara japonica photo from here 

However, this feat of biological warfare doesn't seem to work on D. suzukii. In the study we are featuring today, researchers exposed a group of D. suzukii to some L. heterotoma that were eager to lay their eggs in some suitable victims. But while the wasps readily injected their eggs into D. suzukii as they would with any other fruit flies, most of the eggs ended up being trapped in hemocyte coffins and none of the parasitic larvae ever made it out of a D. suzukii maggot alive. When they looked at the blood of D. suzukii, they found that it has five to ten-fold more hemocytes than D. melanogaster, making it a tough adversary for any would be parasite. Furthermore, not only were the hemocytes of D. suzukii not destroyed by L. heterotoma's "pet" virus, their numbers actually increased in response.

But D. suzukii is by no mean invincible; it has its own parasitoid to watch out for and it is also the species we are featuring today - the parasitic wasp Asobara japonica. This wasp is one nasty customer; it would have to be seeing as it has coevolved with D. suzukii. When the researchers unleashed egg-bearing A. japonica upon both D. suzukii and D. melanogaster, the exposed D. suzukii were able to entomb very few of the A. japonica eggs - a quarter of them at most. In comparison, D. melanogaster did not stand a chance - none them were able to entomb the A. japonica eggs that had been laid inside them.

While both L. heterotoma and A. japonica are both parasitic wasps of fruit flies, they have very different methods for subduing their host's immune system. Whereas Leptopilina heterotoma wages biological warfare on its host, A. japonica is a chemical warfare specialist. It injects at first glance what appears to be a peculiar cocktail into its host; a deadly venom and its antidote. Yet this mixture allows A. japonica to manipulate the host's physiology, but only when both serums are injected in combination. The venom alone will disrupt the host's immune system, and then induce paralysis, which is followed by death. But A. japonica also injects the antidote along with it which mitigates some of the venom's effects - it keeps the host alive, but at the same time allows the immune system to be ravaged. So in effect this wasp brings its host to the edge of death, enough to disable its defences, then cures it - but only so that it can then act as a living incubator for its babies.

Reference:
Poyet, M. et al. (2013). Resistance of Drosophila suzukii to the larval parasitoids Leptopilina heterotoma and Asobara japonica is related to haemocyte load. Physiological Entomology. 38: 45-53.