Army of wasps aims at wiping out Thai mealybug plague

The first wave of an army of 250,000 wasps was deployed this weekend in a campaign to eradicate a plague of mealybugs that threatens to devastate Thailand's £1 billion crop of cassava plants.

Bred in Thailand from a cache of 500 pairs of wasps flown in late last year from Africa, the tiny insects, each smaller than a pinhead, crawled out of small plastic vials in a cassava field in the north-eastern province of Khon Kaen at the weekend and began hunting down their prey.

More accurate than cruise missiles, the wasps home in on the mealybugs, laying their eggs inside them. The larvae devour the bugs from within, emerging in a few days from their mummified shells to seek new hosts.

It is the latest battle in a seemingly unending competition between farmers and predators for the crops that sustain them both, with this species of mealybugs feeding exclusively on cassava and the wasps feeding exclusively on the cassava-eating mealybugs.

"In that sense, it's the perfect biological control," said Rod Lefroy, regional research co- ordinator in Asia for the International Centre for Tropical Agriculture, a non-profit group that has organised the release.

The use of wasps, which has been effective in Africa, is expected to also succeed in Thailand, the world's leading exporter of cassava, which is also known as manioc, tapioca and yucca.

"We've got the start of the first victory, but the greater war continues," Mr Lefroy said.

"It's going to be an international game of cat and mouse," said Tony Bellotti, an entomologist at the International Centre for Tropical Agriculture in Colombia who is a specialist in wasps and mealybugs. "As the cassava mealybug finds its way to new countries, we can send in the wasps."

Early signs of infestation have been reported in Cambodia, Laos and Burma, Thailand's neighbours. "Cassava production in south-east Asia has enjoyed an extended honeymoon, relatively free of major pest and disease outbreaks," Mr Bellotti said.

"But now it's over. And the mealybug isn't the only cassava pest out there. Mites and whiteflies, for example, are also extremely damaging."

Thailand, the third largest producer of cassava after Nigeria and Brazil, accounts for 60 per cent of worldwide exports of the root, which is used in foods such as noodles, the flavour-enhancer monosodium glutamate and products including toothpaste. Vietnam accounts for an additional 30 per cent of global trade in cassava.

Mealybugs, with a life cycle of about a month, can spread fast, with each insect laying an average of 440 eggs and producing ten generations in a year. The bugs feed on the tips of cassava plants, stunting their growth with a toxic saliva.

The wasps breed alongside them, and in an ecological balance, an infestation will stabilise at about 10 per cent of the crop."As an ideal biological control, they kill the majority," Mr Lefroy said. "So as the populations of mealybugs go up, the wasp population goes up and brings it down again. Almost certainly it will never disappear but will be kept under acceptable control."

Hundreds of farmers attended the ceremonial first release on Saturday in Khon Kaen, some of them gathering up handfuls of the tiny wasps for themselves, Mr Lefroy said.