Anyone who has walked through a barn or cattle pasture in the summer knows that flies are a nuisance and even a health hazard. Face flies can spread diseases like pink eye to cattle, and horn flies—biting flies that live on cows and take up to 20 blood meals per day—in large enough numbers can impact animals' health and growth. But insecticides frequently used to combat these pests may actually be reinforcing the problem by killing dung beetles, which naturally control flies, and potentially harming other beneficial insects.