Most common household cockroach able to develop ‘cross resistance’ to multiple types of chemicals, US study finds

This article is more than 1 year old

This article is more than 1 year old

Cockroaches have become harder to kill and could soon be “almost impossible” to control using pesticides alone, according to a study funded by the United States housing department.

Researchers from Purdue University in Indiana spent six months trying to eradicate German cockroaches (Blattella germanica L.), one of the most common species of household cockroach in the US, Australia and Europe, from three low-rise apartment buildings in Illinois and Indiana.

The results were published in Scientific Reports last month.

The study used three different professional-grade pesticide mixes: a rotation of three different insecticides, changing every month for six months; a mix of two insecticides sprayed monthly; and abamectin gel baits applied once a month in an area where the cockroaches had been tested and showed a low resistance to abamectin.

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Of the three, only the abamectin gel baits succeeded in reducing the cockroach population, and then only in populations which had a low-resistance. In another area where 10% of the cockroach population was resistant to the chemical, populations grew.

Cockroach populations in the building treated with three rotating pesticides remained flat, and populations in the building treated with the mixed spray “flourished”.

Populations were surveyed using glue traps before testing began and once a month thereafter, just before the next round of pesticide treatment.

“If you have the ability to test the roaches first and pick an insecticide that has low resistance, that ups the odds,” the study’s lead author Michael Scharf said in a press statement from the university. “But even then, we had trouble controlling populations.”

Subsequent laboratory tests showed that cockroaches were able to develop a “cross resistance” to multiple types of pesticide, meaning that those which survived the spraying would go on to survive other eradication attempts even if a different class of pesticide was used.

Previously, it was believed that cockroaches only developed a resistance to one class of pesticide following exposure.

“We would see resistance increase four- or six-fold in just one generation,” Scharf said. “We didn’t have a clue that something like that could happen this fast.”

Rapid breeding rates – a female German cockroach can have 50 offspring every three months, expanding out to up to 10 million descendants over four generations within 12 months – mean that chemical pesticides can swiftly become ineffective.

“Cockroaches developing resistance to multiple classes of insecticides at once will make controlling these pests almost impossible with chemicals alone,” Scharf said.

He said pest controllers would have to use a mix of traps, improved sanitation in housing developments, and bug vacuums to reduce cockroach numbers.