Adulticiding, or the killing of adult mosquitoes by ground or aerial applications of chemicals, is considered by the Center for Disease Control and Prevention to be the least efficient mosquito control method. There are several reasons adulticiding is not highly effective. For example, mosquito species have different preferences for flight times and habitats, which makes timing spray events difficult. In areas where there are many houses, trees, and other obstructions, the chance of actually hitting a mosquito with the spray cloud is greatly diminished. Although it is very difficult to measure the efficacy of a large spray event, the results of trap experiments conducted in Houston and Florida suggest that a large spray event will typically reduce the adult population of mosquitoes by only about 30%.