VOA慢速英语 2008 0219a（在线收听）

This is the VOA Special English Agriculture Report.

Back in the year two thousand, big producers of poplar trees in the American Pacific Northwest needed help. Their hybrid poplars, nearly ten years old, were under threat. Young insects were getting into the heartwood, weakening a tree and making it likely to break and fall. Small, newly planted trees were being killed.

Two professors from Washington State University discovered that the threat was not from traditional poplar pests but from a new one.

Doug Walsh and John Brown found ninety-five western poplar clearwing moths in traps in a four-week period in two thousand one. Then, during a four-week period in two thousand two, they found more than eighteen thousand moths in traps placed in the same locations.

Unlike most moths, this one is active during the day. As a defense, it can make itself look like a yellow jacket.

It was a threat to fourteen thousand hectares of poplar planted in eastern Washington state and Oregon. The producers used twenty thousand kilograms of a pesticide, Lorsban, in two thousand two to try to control the outbreak. But that and other poisons failed to stop the moths.

So the professors asked for help from an expert at the University of California, Riverside. Years earlier, Jocelyn Millar had copied the sex pheromone of the clearwing moth.

Pheromones produce chemical signals that animals and insects use to identify friends and enemies. Pheromones also attract the opposite sex. The Washington State team had used Jocelyn Millar's version of the pheromone in the traps.  

The researchers began treating poplars with the synthetic pheromone in two thousand three. The idea was to confuse male moths. They would sense the presence of females and not be able to find them, and that would interfere with reproduction.

After the success of tests, and improvements to the treatment, it won full approval from the United States Environmental Protection Agency. That was in two thousand six.

Professor Brown says the synthetic pheromone is safe so workers can re-enter a forest after a few hours. And only small amounts are needed -- as little as one gram per two and a half hectares. Professor Walsh says the treatment reduces clearwing moth populations quickly. Today, the population is under control, but preventive treatments continue.

And that's the VOA Special English Agriculture Report, written by Jerilyn Watson.