听力原文：M: The volcanic ash from the eruption of Mount Saint Helens certainly caused a lot of damage, didn't it? W: It did, but not as much as experts have predicted. It seems to have had a beneficial effect as well. M: Really? But were crop losses great? W: Yes. However, since the weather following the eruption was unexpectedly good, some crop yields remain steady or even increase beyond normal expectancy level. M. I'm glad to hear that. But what was the beneficial effect you mentioned? W: A mass destruction of various insect population. M: How could the ash kill insects without effecting people too? W: It's highly abrasive and quickly wears away the outer wax layer protecting insect bodies. Removal of that layer causes the insects to lose body moisture by evaporation, the result is usually lethal. (23)