Some effective black fly repellents are those that contain DEET, picaridin, IR3535, permethrin and metofluthrin. The oil from lemon eucalyptus trees is also used as a natural black fly repellent.
DEET is generally accepted as the most effective black fly repellent in the market. DEET is found in various levels of concentration from 5 to 100 percent. Studies have shown that 30 percent is the threshold beyond which DEET's effectiveness becomes marginal. Also, concentrations above 30 percent can cause skin irritation and rashes.
Picaridin is less harmful and less likely to cause allergic reactions than DEET. Because picaridin is derived from pepper, it is substantially less toxic and does not have the greasy consistency commonly associated with DEET.
IR3535 is non-toxic, however it is 10 to 100 times less effective in repelling black flies than DEET.
Permethrin and metofluthrin are both contact insecticides that double as repellent. Because both are toxic, they are only applied to clothing or other fabrics and not on the skin.
Lemon eucalyptus oil is a more natural repellent. It is as effective as DEET concentrations of 20 to 30 percent, but it only repels flies for a short period of time before it needs to be reapplied.