It's bad enough that bacteria are quickly developing resistance to our best antibiotics – now it looks like another kind of bug will soon earn the "super" moniker. Cockroaches have been known to develop resistances to individual insecticides for decades, but new research has shown that they're also able to rapidly develop cross-resistance, even to chemicals that they've never been directly exposed to.
Unfortunately, there's no one insecticide that's able to wipe out all cockroaches. Some of the bugs have natural immunities to certain chemicals, letting them survive the exterminator's wrath that may take out the rest of a colony. And as the principal of natural selection goes, those survivors go on to be the only ones to breed, passing those genes on and ultimately immunizing the whole population against that poison. It's the same thing that's happening with bacteria and antibiotics.
To get around that, exterminators have traditionally used a mix of different classes of insecticides. That way, even if some roaches are immune to one of the chemicals, the other poisons should wipe them out. At least, that's the idea behind how it should work.
For the new study, researchers from Purdue University tested what was the most effective method. Over six months, they subjected cockroach populations in several buildings to three different insecticide treatments, and compared the results. Before the study, roaches were caught from each location and studied to determine which classes they were resistant to, informing the scientists' choices.
One treatment cycled through three different classes of insecticides, alternating each month in an ABCABC pattern. In the second treatment plan, the team mixed insecticides from two different classes, and used that mix for the entire six month period. And in the third, the team chose one insecticide which the local roaches had the least resistance to and used that the whole time.
The results were mixed, but fascinating. The rotating insecticide treatment was found to keep roach numbers flat, but wasn't able to reduce them. The two-insecticide mix wasn't effective at all, with populations flourishing in spite of the treatment.
Experiments with the single, targeted insecticide were varied. In one, the bugs were basically wiped out, but in another test populations actually grew.
At the end of the experiment period, the researchers caught some of the remaining cockroaches and studied them in the lab. As expected, they found that many had resistance to the insecticides used. Worryingly though, they also seemed to have gained resistances to other classes of the poisons, even if they hadn't been used. This kind of cross-resistance could make cockroaches even harder to kill in future.
"This is a previously unrealized challenge in cockroaches," says Michael Scharf, lead author of the study. "We would see resistance increase four- or six-fold in just one generation. We didn't have a clue that something like that could happen this fast. Cockroaches developing resistance to multiple classes of insecticides at once will make controlling these pests almost impossible with chemicals alone."
The team says that the most effective method of would be to combine these insecticides with other forms of pest control, such as bait traps and good old-fashioned cleaning up after yourself.
The research was published in the journal Scientific Reports.
Source: Purdue University
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