Chapter 20
THE PARASITE OF MY PARASITE IS MY FRIEND
Here is the story of how parasitic wasps saved twenty million lives.
But to tell the tale, first you get to hear about mealworms, a kind of insect that's just as unpleasant as its name. Mealworms aren't very big - a cluster of thousands looks like a tiny white speck. But this single speck can devastate whole continents. Here's how:
The average mealworm has eight hundred kids, almost all of which are female. Each of these offspring can have eight hundred more kids. Do the math: One mealworm can produce five hundred million great-grandchildren. And they aren't really worms at all; the young ones can fly, carried from plant to plant on the wind, spreading infection as they go.
Thirty years ago, a species of mealworm rampaged through Africa, attacking a staple crop called cassava and almost starving twenty million people. That's a pretty big death toll for a microscopic parasite. Fortunately, however, cassava mealworms have their own parasite, a species of wasp from South America.
A word about parasitic wasps: nasty. Instead of a stinger, they kill with something called an ovipositor, which injects eggs instead of venom. And, believe me, these eggs are much worse than poison. At least with poison, you die fast.
Here's what wasp eggs do to their unlucky hosts: Some hatch into "soldiers," which have big teeth and hooked tails. They roam around in the victim's bloodstream, sucking out the guts of any children left by other wasps. (Parasitic wasps are very territorial.) Other eggs hatch into wasp larvae, which are basically big bloated stomachs with mouths. Protected by their soldier siblings, they ravenously consume the host from within, sucking away its juices as they grow into wasps themselves. Once they're big enough to grow wings, the larvae eat their way out into the world and fly off to lay more eggs. The soldiers don't leave, they just stay behind with the dried-up, dying host, having done their duty for their waspy brothers and sisters. (Isn't that sweet?)
So what happened in Africa? Long story short: The crops were saved.
Once the right species of wasp was let loose, the mealworms were dead meat. Mealworms may spread as fast as the wind, but wherever they go, the wasps can follow. Wasps can fly too, after all, and they're pretty much psychic when it comes to finding mealworms. If a single plant in a huge field is infected, the wasps will find the mealworms and inject them with their eggs. No one really knows how wasps track down microscopic mealworms, but some scientists have an intriguing theory:
The infected plant asks for help.
That's right: When a cassava plant is attacked by mealworms, it begins to send out signals to any wasps in the area. Some unknown chemical rises up and draws the wasps toward it, like a big red highway flare saying, Help me! Help me!
Of course, another way to translate the message is: Mealworms! Get your hot delicious mealworms!
You could say that the cassava and the parasitic wasp have an evolutionary deal: "I'll tell you when I'm infected with mealworms, and you come and deposit your deadly eggs in them."
It's a great relationship, because the parasite of your parasite is your friend.