The Most likely reason the writer uses a problem-solution structure in this excerpt is because the problem is related to public health, and it needs an awareness of the risks in the minds of people.
It is a format of writing, where the writer first explain the issue or the problem, to create emphasis on it's seriousness. And than explains, in the second part, the solution of the problem.
In the given excerpt, the writer in the first part, explained the health hazards of insects like mosquitoes to make people aware of it. And in the second part, he explained the solution of the problem.
Therefore, we can suggest that the writer uses problem-solution structure of writing because he wanted to make people aware of the seriousness of the issue.
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