Many fly control products also affect other external parasites and some, most notably those containing the macrocyclic lactone compounds, also affect internal parasites. Use of these chemicals for fly control can also select for resistance in the other parasites. It is important to read the label to determine which parasites will be controlled. Resistance is a significant issue in buffalo flies, ticks and cattle worms and when choosing a chemical for fly control consider the possible effects of increasing selection for resistance in non-targeted parasites.
Figure 1. Consider if the product you are using to target one parasite might also be affecting other parasites such as worms, ticks, flies or lice. Images courtesy of AR Walker Wikimedia CC (worm), Lex Turner (ticks) and Jess Morgan (flies and lice)