There are two reasons that sealing your foundation prevents basement bugs and rodents.
One obvious reason is that when there are no open vents, foundation cracks or exposed dirt, there is no entrance for insects and rodents to get into your basement and crawlspace. Open vents and cracks are an easy way for insects and other pests to get inside. Exposed dirt crawlspaces practically invite rodents to live under your house. There is nothing stopping them. When you close and seal the crawlspace you make it difficult for the pests to get inside. By preventing the pests, you control the waste that actually is a serious allergen and can impact your indoor air quality.
The second way sealing the foundation controls pests is by controlling the water. When Advanced Energy conducted their ground breaking crawlspace studies where they closed, sealed and waterproofed previously vented crawlspaces they were surprised to find when they controlled the moisture they saw a decrease in pests and insects by 70%. Pest control professionals will tell you that controlling water is one sure way to control bugs. Bugs need water to live. They come to a moist environment like a dark, damp crawlspace because it has all the elements they need to survive. Control the water and you automatically control the pests.
By sealing your foundation, closing the vents, filling the cracks and making the basement or crawlspace a clean, dry and insulated part of the house it is like putting up a “No Vacancy” sign for the pests. They will get the hint!