Some of you guys are saying deet does not repel black flies????

You likely were using a bad batch or expired deet because in all my years working in the Great Northwoods of northern NH and Maine deet has always worked very well at repelling insects of every species. They may continue to buzz around, but if they landed and stayed on any part of my body that meant there was no deet on that spot or it had worn off on bushes, or due to sweat, etc.

I know some sprays are pretty weak in percentage of deet and that could be a problem for longer term protection.

If an insect were to become covered in viable deet it would live about 1/10 of a second.