I would like to detect ant and termite using either one of the following sensor. PIR or infrared object detection sensor. Is it possible to detect insects with them or it can detect only larger sized animals ? I came to know that these things will detect these spiders and ants by this link. Does this work?
This might be tough unless you get line-of-sight.
Perhaps a chemical sensor would be more beneficial in your case? You might be able to detect chemical markers for specific insect types.
May be you should use microwaves. Ants are really small, any thing you would use, it should be precisely setup - microwaves with some millimeter wave length could does the job. May be in a thin stripe where the wall stick with floor. Possible you could use commercial microwave motion detector, with some fine tunning or remodeling.