The single most effective bug repellant is one containing DEET. It is not harmful, as long as it’s not overdone. It is usually the propellant that creates the problem (aerosol cans), so just get either a pump-spray or lotion.Lemongrass, eucalyptus oils, etc. have very limited results.
DEET is the best choice for ticks, and still a good choice for mosquitoes. The best option for black flies is picaridin