Crawl spaces, enclosed porches and garages are all spaces that tend to attract pests. This is so because they provide some shelter from the elements while not being regularly occupied by people.
Crawl spaces are especially problematic in part because we so rarely enter them. They exist to help protect a house from flood and give relatively easy access to plumbing, ductwork and electrical work. The vast majority of the time, they are very much a case of "out of sight, out of mind."
But you really don't want pests to get into the crawl space. It is the foundation of the house and damage there is a serious issue. Plus, you don't want it to be a path for pests to get into the rest of the house.
Crawl spaces can become home to a variety of different pests, from rodents to insects to wasps. Some can carry diseases. Others, like termites, can be a danger to the structural integrity of the house.
So just what can you do to protect the crawl space from being invaded by pests?
Start by blocking all holes and sealing the area shut. Ideally, you want to make it impossible or at least extremely difficult to get into the space at all.
If there are large holes, you may need to use wood or similar to repair them. These larger holes are a risk for allowing in rodents and other animals.
Small holes can let in smaller vermin, like insects. For small holes, caulk will help. Caulk is inexpensive and very do-it-yourself friendly.
You also want to control moisture. Excess moisture can not only attract pests, it can promote wood rot and mold growth. No good will come of allowing the space to be moist.
A vapor barrier can help keep out both moisture and pests. A vapor barrier goes directly on the floor and prevents moisture from infiltrating from the ground.
This may seem silly, but crawl spaces are often favored in places where the water table is too high to allow for a basement. The ground can contain quite a lot of moisture and it will tend to rise, especially during warm weather.
You should also use a dehumidifier. While sealing the space helps keep out moisture, it also makes it impossible for moisture that gets inside to readily escape. A dehumidifier can remedy this issue so that your prevention measures don't become counterproductive.
You should perform regular exterior maintenance on the house. For example, don't let weeds grow too close to the base of wooden walls.
Inspection of your Crawl Space
Last, you can inspect the crawl space on a regular basis. At a minimum, you should do this annually.
If you aren't confident that you know enough to inspect it yourself, you can hire a professional to inspect it for you. This is a relatively small expense compared to the value of your home and can prevent larger costs down the line. An ounce of prevention is worth a pound of cure.