Biological control is part of the integrated pest management strategy. Biological control considers the active human role in controlling and reducing pest populations, by attracting natural enemies (i.e. predatory insects) to reduce unwanted pests in your garden or on a farm. By introducing natural enemies, you can achieve a reduction in the pests that are affecting the crops without bringing in harmful chemicals or using other harmful methods. Cornell gives a good overview of how to achieve biological control. For instance, have a harmful amount of aphids taking over your corn stalks? Bring in an army of lady bugs, an aphid's natural predator. Serve the ladybugs and your corn, at the same time.