Consumer insecticides are useless for fighting cockroach infestations

People buying insecticide sprays to battle cockroaches are wasting their money as the insects have grown resistant to a key ingredient, sparking a call for US regulators to toughen up product testing rules.

About 30 different cockroach species live among humans, but the German cockroach (Blattella germanica), which is common throughout the world, is the one most likely to infest buildings. Previous studies have demonstrated that it has developed widespread resistance to pyrethroid insecticides, which are commonly found in consumer products.

Now, Johnalyn Gordon at the University of Florida and her colleagues have shown that while consumer sprays continue to work effectively on laboratory-sourced German cockroaches, bred from a line with no exposure to insecticides, they are ineffective against insects collected from real-world infestations.

The products are designed to be sprayed on to surfaces, killing the insects when they walk over the coating. But in the team’s tests, coated surfaces killed fewer than 20 per cent of the collected cockroaches after exposure for 20 minutes. “Even directly spraying them in an enclosed container with these products, we weren’t getting 100 per cent mortality,” says Gordon.

When the cockroaches were forced to remain on the treated surfaces, most of the products took between 8 and 24 hours to kill. Previous research has shown that these insects avoid lingering on surfaces treated with pyrethroids, making it unlikely this would happen in reality.

Pest-control products sold in the US are regulated by the Environmental Protection Agency and manufacturers must prove a 90 per cent efficacy rate, but there is no general requirement for testing on field-collected insects.

This should change, says Gordon, who points out that products sold in the US for treating bedbugs must be tested on recently collected specimens. “Something like that on the cockroach side… we believe that would raise the bar for cockroach control products and help to ensure that the products that are on the shelves are serving to provide the control that people would rightfully expect them to,” she says.

In the meantime, cockroach baits are probably the most effective consumer product for controlling infestations, says Gordon, and come with the added benefit of minimising human exposure to pesticides. Other in-home strategies, such as removing food and water sources used by the insects and clearing clutter, can also help. Professional pest control, which relies on pesticides beyond pyrethroids, is also recommended.