Various droplet sizes of formulated chlorpyrifos were evaluated by ground and aerial application methods to determine optimal aphid control with the least drift potential. Droplets in the small (200 m) to medium range (400 m) generated by ground equipment provided significant control compared with an untreated check, whereas the largest size (600 m) did not. Increased efficacy of small and medium size droplets was also found with aerial application. Because control of some insects may decrease as droplet size increases, medium size droplets may provide acceptable control while minimizing drift.