Why can insecticide keep killing bugs for 3 months even after it has dried up?
The residue remains even after the insecticide has dried. Just as if you spilled some pepsi on the table and waited for it to evaporate; the table would still be sticky in that spot.
This is why pest control technicians will ask if you have any young children or pets, as the insecticide is still there, and is still very toxic. Kids can crawl on the ground and put their hands in their mouth after touching the area which has been treated.
It’s recommended that a professional apply insectice for several reasons, but a main one is that they’ll know where to spray it so that it remains effective in an area where it will not likely be disturbed. This is because, just like in our pepsi example, insecticide remains effective only if left alone. If you wash the area where the insecticide is, it will no longer remain and you’ll need to have it reapplied if you want to keep killing bugs. Or if you apply it in an area with too high of traffic, it will get wiped away over time.
Because the insecticide is still there sticking to the surface, it’s just the water that had been carrying it has evaporated.