Your drilled well must be capped with a commercially manufactured vermin-proof well cap, as regulated by the Department of Environment and Conservation. Vermin-proof caps prevent the entry of "foreign material" such as vermin, insects, and decaying plant material into the well. The cap should also contain a screen vent to allow the circulation of air. The standard vermin-proof cap for a drilled well has rubber gaskets and screened vents inside to ensure vermin stays out and air flows through. Loose fitting caps found on older wells make these wells a comfortable home for insects and vermin.
Finding a vermin-proof, watertight cap for a dug well is more of a challenge. Because dug wells vary in size, they must be custom made. Suggested construction materials include fiberglass, food grade plastic, and concrete. Avoid wood because when it rots it can no longer support the weight of animals and snow, is not watertight, and can provide a home for vermin and insects. In some cases a fence will be constructed around the well to keep wild animals off the cover.