单项选择题

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

What is the main topic of the conversation().

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