听力原文： The volcanic ash from the eruption of Mount Saint Helens certainly caused a lot of damages, didn't it? It did, but not as much as experts had predicted. It seems to have a beneficial effect as well. Really? But were crop losses great? Yes. However, since the weather following the eruptions was unexpectedly good, some crop yields remained steady or even increased beyond normal expectancy levels. I'm glad to hear that. But what was the beneficial effect you mentioned? A mass destruction of various insect populations. How could the ash kill insects without affecting people too? 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 fatal. How about that? Sounds like an effective yet natural insecticide. (30)