Insecticides futile against cockroaches

Commercial insecticides “add little to no value to cockroach control”, according to a new study.

The study, published in the Journal of Economic Entomology, found that pyrethoids – a common insecticide agent – couldn’t effectively kill German cockroaches when sprayed on surfaces near them.

“Due to the frequent use of pyrethroid-based residual products, it is very likely that German cockroaches inside of homes will have some degree of pyrethroid resistance,” says Dr Johnalyn Gordon, a postdoctoral researcher at the University of Florida, USA.

“To the best of our knowledge, a pyrethroid-susceptible German cockroach population has not been documented from the field in decades.”

German cockroaches are one of the world’s most widespread pests.

Pyrethoids, which are synthetically made substances based on compounds produced by a type of flower, are commonly used in household insecticides. They’re generally not tested on cockroaches specifically – just other insects.

The US team of researchers studied the resistance of cockroaches born from those caught at a couple of US locations, by testing their survival both after direct spraying, and inhabiting a range of different surfaces where insecticides had been sprayed.

They compared this resistance with a population of cockroaches called Orlando Normal, which has been bred in a lab for more than 70 years without any insecticides.

The Orlando Normal cockroaches were very vulnerable to pyrethoids, but the “field” cockroaches were much more resistant.

While direct spraying killed some of the field cockroaches, fewer than 20% died when exposed to surfaces with residual insecticide for any length of time.

Since direct spraying usually can’t catch every roach, this means that pyrethoids are insufficient to stop infestations.

The researchers also found that different types of surface had different efficacy, with drywall performing worse than ceramics or stainless steel at killing cockroaches residually.

Even the vulnerable Orlando Normal cockroaches mostly survived the drywall encounters.

“This suggests that how porous a surface is may have a significant impact on product efficacy,” says Gordon.

“Given common applications of residual insecticides along baseboards, reduced efficacy on painted drywall was a particularly striking finding.”

The researchers say that gel or baits are more promising options for cockroach control. Professional pest control services use multiple different methods of dealing with cockroaches, which is better for eradication but expensive.

“There is a strong base of knowledge and research on how we can control cockroaches, but there are numerous economic and procedural barriers that mean that this control is not occurring in these areas, arguably where it is the most needed,” says Gordon.

“If residents do not have access to effective professional pest control or consumer solutions, they continue to be impacted by the effects of an infestation, including the health risks associated with cockroach allergens.”