This does not weaken, and so is the correct answer. This claim doesn’t explain why there would be fewer pears lost to insects in the last three years. Overall pear production can change without affecting the proportion of pears that insects destroy, so this just gives us nothing.

This weakens the argument by providing an alternative explanation for the observation. If the pear farmers have been gradually implementing anti-insect programs, then it makes sense that the last three years would see fewer insect losses.

This weakens the argument by providing an alternative explanation for the observation. If insect-eating birds have been spending more time around District 10, they would have eaten more insects, thus protecting the pears.

This weakens the argument by providing an alternative explanation for the observation. If pear-eating insects have had less water in which to breed, then there would be fewer insects, which explains why there have been fewer pears eaten by insects.

This weakens the argument by providing an alternative explanation for the observation. If the effects of the old pesticide get stronger with each use and last for years, then the pears would have been protected by a strong built-up effect during the three years in question.