I think that foam is a good immediate way to close off entry points or to fill large holes, but once it dries rats and mice will easily gnaw through it again. Better would be to permanently close that opening as soon as possible with some more impermeable material like metal flashing or caulking or concrete, depending on where the hole is. You might even shove a wad of copper or stainless steel mesh (Univar sells both) into the hole before then filling it with the expanding foam, as mice and rats appear to be deterred from gnawing by the wire materials.
Odors definitely are an attraction to rodents, including even the odors of foods that may seep through gaps or holes from the interior of a home or other structure. Even warm air flowing out a gap could be an attraction to the rodent looking for a warmer, cozier place to be, and if the gap is wide enough the rat or mouse easily will move through to investigate. In his excellent book on Rodent Control Dr. Robert Corrigan stresses once more that the House Mouse can move through any gap 1/4 inch high and 3/8 inch wide (about the width of a wood pencil or diameter of a dime) and a rat can squeeze through a gap 1/2 inch high or 3/4 inch wide (about the diameter of a quarter). Of course, these kinds of gaps also could admit many other unwanted critters, such as most bugs, snakes, or bats if those gaps are higher up on the building.
I don't know if a rat or mouse would necessarily completely miss a hole or gap if no odors or other air currents were coming out of it. They don't have great eyesight, but they still can see and they have whiskers (vibrissae) that are very sensitive that might detect that hole as they pass by. Since house mice in particular are very curious and always looking for places to explore, the discovery of a hole or gap could lead them to move through just to see what's on the other side. Heck, there could be food or just a nice place to hang out, so checking out openings is normal for them. It is suggested that this is one reason they enter bait stations, to see what may be inside. The hole also could represent an opening leading to a secure cavity that hides them from the dangers they face from predators when they are exposed and moving about in the open.
Pheromones are not restricted to insects, and mice and rats use them effectively too as chemical messages. Mice are well known for urinating all over the place, leaving behind pheromones that communicate to other mice that this one has been there. Mice may leave more than 3,000 "micro" droplets each day in their territory, and this is not just due to poor bladder control. They leave all these droplets as messages and as signals for them to use as they move around. As Dr. Corrigan stated it, mouse urine is their main source of social odors. It may enhance recognition of new objects or of hazards and it provides social orientation, determining dominance by some mice. These urine and fecal paths are vital for the PMP to locate in mouse management.
Now, whether or not you can successfully eliminate ALL odors from moving past a point is debatable, but by closing off gaps completely you probably do a pretty good job of reducing the air flow and associated odors to a point that at least reduces a rodents interest in a former opening.
Mr. Pest Control
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Please note, Mr. Pest Control is answering questions supplied by PMP customers across North America. His answers are generated from industry and manufacturer-provided information. The answer may not be specific to the laws and regulations for your State, Province, Territory or Country. In addition, products mentioned may not be registered and or available in all areas. Always check with your local Univar office for specific information to your area. Always read and follow label directions.