A common kitchen scrap can help you keep wasps and spiders off your patio or even deter smells in your car and refrigerator. Spiders, mosquitoes, wasps and bees are all repelled by coffee grounds. The only catch is that the coffee grounds must be completely dry before use. So, while a kitchen scrap, used coffee grounds need to be dried out first in a plate or on a baking sheet. These areas have a tendency to aggregate strong smells and keep them lingering.