Pesticides properly applied to water kill 95% to 100% of the mosquito larvae and, thus, are very effective. It is more difficult to tell how effective sprays to kill adult mosquitoes are, because it is not known exactly how many mosquitoes are present before spraying. Adult sprays applied by aircraft are very effective because the spray covers a very large area, usually several square miles. Reductions of 90% are common. Adult sprays applied by trucks can be just as effective where parallel roads are close together, where there is a light wind and the vegetation around houses is not thick. It is less effective if there is no wind, high humidity, too few roads and heavy vegetation. Traps are often used to measure the population levels before and after spraying.