What do you do when you’re chasing a spider or a roach only to have it escape into an air vent? If you douse the opening with Raid or poof some boric acid dust into it guess what will be coming out in about 10 minutes when the air kicks on. That’s right, you’ll either have the smell of your aerosol and or any airborne pesticide particles floating around your living area and this sometimes can go on for days. This is not good and for obvious reasons you should ALWAYS avoid spraying anything into air vents or air returns.
I’m often asked to spray in air vents but my usual reply is; “Sure, I can do that and when I’m done can I spray in your fish tank?” Of course the answer is always NO and a look of puzzled concern always follows. I explain that both actions are about the same because I’m putting pesticide in a place where you cannot avoid breathing it and I have no control of where it goes. Just as your fish will surely take ill it would be no less irresponsible for me to treat your vents in this way.
Can vents be treated?
The short answer is yes but your options are sort of limited. The best way is to simply remove the vent and place a fold up glue trap inside the vent. Since roaches and spiders can easily walk around the trap there is no guarantee you’ll catch them and while baiting with a smear of peanut butter might entice the roach it will give your home that nice aroma of a 3 day old school lunch sack. Maybe you want to think about that. You could spray around the vent but drywall sucks up liquids and it’s doubtful any residual would be on the surface in enough of an amount to provide a lethal dose. Plus you still may get airborne chemicals when the heat or or a/c turns on. Some vents are vertical and it is difficult place a trap inside, for these types what I sometimes do is place wide strips of clear packing tape over the vent leaving thin slits (1/4 to 1/2 inch wide) so that the air can still come through but when the insect tries to crawl out to freedom he will get stuck and die. Packing tape is great because you can still see into the vent and it usually comes off cleanly whereas masking or duct tape won’t.
Can bugs live in vents & do they come in from them?
Almost all bugs are sensitive to air currents in one way or another and roaches will actually avoid the air from a fan. To much direct forced air will begin to dehydrate the bug and if they do spend some time in a vent it won’t be long before they start looking to get out. As far as spiders or bugs entering your living areas from the vents, it’s not likely unless you have a break or tear in one of the ducts. In one case I treated a home that had fleas only in a tub and not all the time. After being 
puzzled for awhile the a/c kicked in and I suddenly had fleas on my shoulders and head. The air vent above the tub was kicking them out every time the air went on and upon further inspection we found a birds nest in a large tear of the flexible air tubing in the attic. The nest was removed, attic treated, the duct fixed and the clear tape trick got the rest. What happens most concerning vents and pest entry is that the vent itself is not sealed tightly up to the ceiling. Critters in the attic follow the air current, see the light or detect food odors through the opening and the slip through right next to the vent. This gives the illusion that they are in the vent which is almost never the case. While I may dust into the void to block them I most often suggest sealing up the opening.
These same rules apply anytime you are using pesticides and would include car vents, box fans, window a/c units or anything that will return your spray back to you in an uncontrolled way. If you think creatively you can get around the use of residual sprays in these areas and still get rid of your pest. You just need to think outside the vent.
