ANNAPOLIS, Md. -- A common variety of consumer insecticide sprays is mostly ineffective and of "little to no value" in eliminating cockroach infestations, a new study shows.

Residual insecticides are designed to be sprayed on surfaces where cockroaches are likely to appear, exposing them to the toxic ingredient when they move across the surface later. But laboratory testing by researchers at the University of Kentucky and Auburn University shows that the residues have little effect on German cockroaches (Blattella germanica), a primary species infesting homes and buildings around the world.

The study found that liquid and aerosol sprays using pyrethroid insecticides killed less than 20 percent of German cockroaches that were exposed to sprayed surfaces for 30 minutes. Moreover, even when cockroaches were confined to the sprayed surfaces, most products took eight to 24 hours to kill the cockroaches, with some taking up to five days. Published August 14 in the Journal of Economic Entomology, the study tested the sprays on German cockroaches that had been collected from real-world infestations, where the insects have evolved resistance to pyrethroids, previous research shows.