Russian Doll Warfare: Plant, Virus, Bacteria, Aphid, Wasp

There are times when I want to retitled this blog The Continuing Adventures of Parasitic Wasps and Their Unfortunate Hosts. Because there are just so many stories of these sinister insects and how they lay their eggs inside other animals. That’s no surprise, really, because there are hundreds of thousands of species of parasitic wasps on Earth, all evolving in different directions as they adapt to their host’s defenses.

Last week, for example, I reported in the New York Times about a newly discovered defense that flies use against certain wasps: when the wasps inject their eggs into the flies, the flies drink alcohol to literally turn the parasites inside out.

Since then, I’ve become obsessed with another species of wasp that attacks aphids. The battle between these two species–and their many allies–makes the story of the boozy flies seem positively pedestrian.

The wasp is known as Aphidius ervi, and its hosts are aphids. Because aphids are major pests on farms and in gardens, researchers have turned A. ervi into a biological weapon against them. If you so desire, you can order 250 mummified aphids with wasps ready to emerge through the mail for $69.95.

To find an aphid host, A. ervi wasps take advantage of the struggle between the aphids and the plants they eat. When a plant gets nibbled by an aphid, it releases a cocktail of molecules into the air. The wasp detects those chemicals, sniffing its way to the plant–and to the aphid.

The wasp may lay only a single egg inside an aphid, or it may choose to lay additional ones. Along with her eggs, the wasp will also inject a venom that stunts the growth of the aphid’s own ovaries–thereby stopping the host from wasting energy on its own reproduction so that there’s more food for the wasps. The wasp eggs have no yolk, so they depend entirely on their host from the start. When the wasp egg hatches, the larva develops a thick, tentacled blanket of cells that extends out into the aphid’s body and draw in nutrients–in other words, a placenta. The placenta also buds off a special class of cells that swim through the aphid’s body, releasing enzymes that degrade the aphid’s cells and bind fatty acids, making it easier for the wasp to feed on its host.

But there’s a big puzzle about A. ervi’s strategy. No matter how many eggs it lays in an aphid, only a single adult wasp at most emerges from a host. Why the extra eggs?

In many cases, no wasp emerges at all. That’s because the aphids have defenses of their own. Their immune cells go after the wasp larvae. And in addition to their own defenses, many aphids are home to allies–namely a species of bacteria called Hamiltonella defensa. Some aphids are infected with the bacteria and pass it down from parents to offspring (or spread it by sex). The bacteria produce a toxin that sickens the wasps and keeps them from developing. Weirdly, Hamiltonella defensa only protects the aphids if they are, in turn, infected by a virus of their own, which carries the wasp-attacking toxin gene. (See Ed Yong’s post for details.)

That would be remarkable enough. But now scientists have discovered another dimension in this multi-species battle. A team of researchers led by Kerry Oliver at the University of Georgia has found that the wasps can tell the difference between aphids that are protected by bacteria, and the ones that are defenseless. Into the defenseless aphids, they lay only a single egg. And into the protected aphids, they are more likely to lay two or more. Oliver found that the wasps seem to boost their odds of surviving against the bacteria by boosting their numbers. It’s possible that the extra venom and enzymes let the wasps grow despite the poison supplied by the bacteria and their viruses. No matter how exactly the extra eggs help, Oliver’s finding hints at an answer to the puzzle of the wasp’s egg laying patterns.

How the wasps can figure out that the aphids have friends inside, the scientists cannot say. It’s possible that when the wasps lay the first egg, they can probe the chemistry of the aphid and figure out if it’s infected with H. defensa. Oliver and co. raise an even more intriguing possibility. The aphids infected with H. defensa seem to release fewer alarm pheromones in times of trouble. Perhaps they are blase about parasites because of their protection. It’s possible that the wasps have evolved to use that clue to tell whether they need to lay extra eggs.

Regardless of which trick they actually use, Aphidius ervi is my favorite parasitic wasp. But it will probably not keep the trophy for long.

Reference: Parasitic wasp responses to symbiont-based defense in aphids. Kerry M Oliver, Koji Noge, Emma M Huang, Jamie M Campos, Judith X Becerra and Martha S Hunter. BMC Biology (in press)

(For much more on the wasps, see my book Parasite Rex)

[Image courtesy of Alex Wild]