Insect repellents are not insecticides since the chemical action is to repel, not kill. However, some produce lethal effects if the concentration is high enough and insects are unable to move away rapidly. The vast majority of insecticide use on horses is in the form of fly repellents. Many, also, are labeled for use against ticks, lice and other equine ectoparasites. Although often called repellents, labeled products may kill the pest as well. Horse owners expect on-animal pesticides to repel flies. Usually house and biting fly populations around horses are high enough that killed flies are quickly replaced. So, the apparent efficacy (reduction of flies on the horse) is nil with products that just kill flies.
The active ingredients for equine fly repellents consist primarily of 1) natural pyrethrins (the single best muscoid fly repellent) synergised with PBO and/or MGK 264, 2) pyrethrins in combination with other pyrethroids such as permethrin or cypermethrin to extend the efficacy, and 3) either of the first two combinations with additional non-insecticidal repellents such as MGK 326, Stabilene, or citronella.