Parasitic Wasp saves Cassava Crops

27 Jun

In the early 1970’s a farmer in Zaire stood and looked at his crop and despaired. His formerly lush and fertile field of cassava was wilting and dying, the leaves were curling in upon themselves, the thick roots were stunted. Instead of the thick potato-like tubers he was expecting the ground brought up nothing but strings.

Cassava, also known as manioc, is one of the most important crops in the world. After rice and wheat, cassava is the plant by which the most people get their daily carbohydrates. It is hardy and nutritious, growing where many other plants would not, making it especially valuable in sub-saharan Africa. And, unfortunately, it was dying. The culprit was a species of parasitic insect known as the cassava mealybug (Phenacoccus manihoti). They would cluster well into the thousands, forming fuzzy white mat on the plants, and then literally suck them dry. The plague was spreading throughout Africa, threatening to starve an area larger than the continental United States.

Then a Swiss scientist named Hans Herren hit upon what was, at the time, a unique idea. If the mealybugs, like the cassava they preyed upon, were not originally from Africa but instead South America it was likely that there was a South American predator which could easily wipe out the pests. After years of searching Herren hit upon a small parasitic wasp known as Anagyrus lopezi. It would be biological pest control.

The Mealybugs

Herren’s plan was successful, wildly successful. It’s become a textbook case of biological control and a well-used method of pest control. The wasps, barely visible predators, could hone in on the mealybugs. Once found, the wasps would inject their own larvae into the bugs. A parasite of a parasite. Even better, the wasps would only parasitize the South American mealybug, leaving the native insect population alone.

Last year, in early 2010, an explosion of mealybugs cropped up in Thailand, another producer of cassava. As soon as the bugs appeared, the scientists brought in the wasps. But this could have easily gone horribly awry. The cane toads that ate Australia were originally introduced to combat the cane beetle. Today the toads are a ubiquitous and universally loathed pest in and of themselves. The difference between the cane toad and the wasp was careful study and regulated release. Today, Herren’s technique and his wasps have saved countless people from starvation throughout the world.

Sources: New Agriculturalist, The New York Times, The Kebun, Parasite Rex by Carl Zimmer

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Posted by on June 27, 2011 in Natural History


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