How pheromones could be used to control insects on a crop?
Many organisms respond to signals other than sight or sound waves. There are chemicals called pheromones that affect organisms within that species in such activities as mating, finding food, etc.
Pheromones usually can be detected in the air and last a long time and persist over a long range. Male insects can locate a female ready to mate by the pheromones in the air that act like a beacon directing the male to the female.
A pheromone that has been identified in a species of insect pests could then be manufactured and subsequently released to lure males to follow the trail. At the end of the trail a trap is set to capture and kill the males or to "scout" them to identify their active times in order to optimize prudent pesticide use. Pheromone traps may also be used in conjunction with chemosterilants to which males are drawn by the sex pheromones in the traps. The chemosterilants act upon the males, that then return in a sterilized condition to the fields where mating produces nonviable eggs and results in population reduction (Dr. J. P. Sharma, Comprehensive Biology XII).
Pheromones can be released over a large area which masks a species' pheromones, confusing the males, and this will prevent mating between males and females. This technique causes confusion where males seek and never actually find any mates.
An advantage to using an insect pest's unique pheromone to lure it to a trap or to confuse the mating process is that it only affects the pest and doesn't harm the other members of the community.
In contrast, traditional insecticides which are released onto a field affect many different organisms, not merely the targeted pest. For instance, the frogs that eat the grasshoppers that ate leaves sprayed with chemical insecticides will all have these chemicals in their tissues due to bioaccumulation through eating and being eaten in turn. These chemicals may be long-lasting and persistent in the environment and can run off into nearby lakes and streams, affecting aquatic life.