Why do people still use pesticides when they know it could be doing harm to the wildlife and pets around them?

This is purely for economic reasons. Without the application of pesticides, the crop would be destroyed by pests thus incurring heavy loss to the farmer.

Moreover, the farmer does not have choice other than harmful pesticides to save his crops from devouring pests.

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Pesticides do not have to harm wildlife and pets. The flea medicine you use on your dog or cat is a pesticide, as are substances used to control mosquitoes and ticks- which carry disease. Without pesticides, thousands of people would die from malaria.