Some of the largest holes in your home's exterior are more difficult to cover. After all, you can't exactly fill your chimney or roof vents with caulk. To fill larger openings and keep bugs out, use very fine wire mesh, often called hardwire cloth. This material comes in rolls that can be stapled over holes to keep out pests. It not only keeps bugs out, but can prevent squirrel and raccoon infestations, which often bring fleas, ticks and other insects into the home [source: Potter].
Look for large holes on the roof, which are often found at the chimney and roof vents. A pre-fabricated chimney cap can be used in lieu of wire mesh, and may be more successful at keeping a variety of pests out of the chimney. Wire mesh should also be installed over holes in crawl spaces and basements, as well as over grilles, vents and registers [source: Gouge et al].
Some vents have pre-installed dampers, which are designed to keep bugs out. Check to see if yours are operating properly, and repair or replace the dampers as necessary. You can also replace existing grilles or vents with screened models that are designed to keep bugs out.