Warm weather (and sometimes hot weather) and more-humid conditions are all part of the coming of spring and summer. This creates ideal conditions for flies and other insect pests. That makes this an excellent time to focus on preventing the problems caused by flies for cows and calves, and optimum production goals. Flies can be costly to a dairy herd (especially calves and replacement heifers) by reducing feed intake and helping to carry or transmit diseases and other pathogens. Proper control of flies and their effects can be a challenge for many farmers. Prevention and appropriate management depends upon the fly species in question.