Crawling insect surface sprays work on the principle that after spraying an area, a thin layer of insecticide remains as a residue on the surface of the material. When an insect walks across this surface, they pick up the insecticide on their legs and body. The insecticide either enters their body directly or as a result of grooming and the insect dies. The speed of kill depends on the amount of insecticide they pick up and their susceptibility to the insecticide. However, even on a freshly sprayed surface, the insects don’t pick up much insecticide and it will take minutes or even hours for the insect to die. It is therefore quite possible for homeowners to see insects apparently walk across treated surfaces with no apparent effect. However, assuming the insect has picked up enough insecticide they will eventually die.

Most surface sprays include a type of insecticide called a pyrethroid. These insecticides tend to be repellent to insects. However, the level of repellency varies depending on the concentration of insecticide on the surface and the type of insect. Typically the level of repellency is at its highest when freshly sprayed. It is probably this feature that has given rise to the “force field” adverts and the perception that these products create a magic force field to keep insects out of the home. However, the repellency quickly reduces and the main mode of action is the kill properties provided when the insect crawls across a surface.

Some pesticides, like  sprays, are fast acting. They knock down the pest, then break down quickly. Other pesticides, like surface sprays and termite treatments, linger in the environment for days, weeks or even longer after application. While most pesticides are designed to only affect the target pest, other insects or animals may sometimes be harmed.