I have filled up automatic fish feeders thousands of times over the past several years.
On occasion, I will get one that will have tiny bugs crawling around inside and out. I have discovered that these bugs are called grain mites and their eggs can be found in fish food, livestock grain, etc. They will hatch and reproduce when the food gets moist from water or high humidity. I normally will get them when the fish feeder is inside of a canopy over an aquarium.
The easiest way I have found to get rid of and prevent them is to attach an air pump to a fish feeder that has an air attachment port. The Hydor fish feeder is a good example of this. Place the air pump away from humid air such as underneath the tank in or behind the aquarium stand. The will keep continuous, dry air circulating through the food and will prevent the eggs from hatching.