What could be a good way to remove critters from a car (cockroaches, ants, etc.)? I want to avoid using insecticides, since this will leave an unpleasant odor on the car.
I believe that first of all you should have a thorough carwash. Then let it be in the sun to dry. There are some insecticides in solid form that can be placed at different locations inside car which have less odour. You can also place camphor at different locations as it also repels ants and many insects. The only thing is it is imflammable but I don't think it will mean a lot when properly placed. Moreover camphor has sweet smell and is also good for health. Camphor also acts as biopesticide.
*Leave a few tea bags of mint tea near area where the ants seem most active.Dry crushed mint leaves or cloves also work as ant deterrents.
*Usage of diatomaceous earth could be very effective method and you would find coackroaches dying within 48 hours.Keep this in car and also sprinkle it in appropriate locations.
*Mix 1 litre water,one teaspoon of borax and a cup of sugar.Soak cotton balls in the solution and place them in containers containing hole in lid.Ants will carry it back and eventually kill their colony.
You need to find out how they are getting in. Cars should be able to keep bugs out unless the windows/doors are open.
If those are closed and you still get bugs you need to figure out where they are getting in or they'll keep appearing. If you've got a rust problem creating holes, you can get expanding foam insulation to fill the holes and keep them out.
First, you have to remove whatever is attracting the bugs. Insects don't colonize your car because they like to annoy you; they do so because there's something inside which they like (probably that they consume). So, if your kid's been spilling Jolt on the upholstery, you'll need to clean out all that sugar (I'm guessing the critters don't go for the caffeine). And, while you're at it, clean the whole car.
Second, there are passive bug traps that may do what you need (e.g. Roach Motels). Tape a few down under the seats, and check them in a week. If there's nobody home, throw them out and declare victory; if there is, then replace them.
Third, if you still have problems, you may have to bend your "no pesticide" rule. Camphor explicitly works by smell, so you may want to avoid that. But, if you selectively spray in critical areas (e.g. edges of carpet) then you may get what you want without too much odor. And then, leave the car windows open to get the smell out.