PESTS
There are plenty of biting insects around, and the mosquitoes can be downright obnoxious at times (especially if you are sleeping outside). Carry some kind of insect repellent (except in winter). Liquid repellents seem to last longer than sprays.
Watch for ticks in brushy areas. No matter how hard you try you will probably get some anyway. Never pull one off. The proboscis usually breaks, whereupon infection may occur. They are best removed by applying rubbing alcohol (Some of the more obnoxious ticks seem to get “bombed” and then hang on for dear life.) or a hot instrument such as a recently burnt match.
Rattlesnakes do occur in the western part (particularly in the badlands), although you would be doing well to see one.