Cockroaches are becoming increasingly difficult to eliminate because they quickly develop cross-resistance to the best insecticides used by the exterminators, warn US entomologists.
These insects pose a real threat to human health. They carry dozens of types of bacteria, such as E. coli and Salmonella, that can be transmitted to humans. In addition, their excrement and remains of moults can trigger allergies and asthma, but also cause problems in some people, such as gastro.
According to Michael Scharf and his colleagues at Purdue University, the problem is particularly acute in urban areas and in housing for low-income or state-subsidized people, where actions to effectively combat these insect pests are not always taken.
Cockroaches develop resistance to several classes of insecticides at a time, making the fight against these insects almost impossible with only chemicals.Michael Scharf, entomologist at Purdue University
Each class of insecticide acts in a different way to kill cockroaches. Exterminators often prepare a mixture of several classes of insecticides. Thus, if a small percentage of insects resist a class, insecticides of other classes will eliminate them.
US researchers tested these methods in multi-unit buildings in Indiana and Illinois for six months.
In a first treatment, three insecticides of different classes were used alternately each month for three months and again. In a second, they used a mixture of two insecticides of different classes for six months. And in the third, they chose an insecticide to which the cockroaches had low initial resistance and used it all the time.
In each location, cockroaches were captured before the study and tested in the laboratory to determine the most effective insecticides for each treatment, allowing scientists to achieve the best results.
“If you have the opportunity to test cockroaches before and choose an insecticide that has low resistance, it increases the chances,” says Michael Scharf. “But even there, we had trouble controlling the populations. “
Using three insecticides, the researchers managed to contain the cockroach populations for a period of six months, but they failed to reduce them.
In addition, the mixture of two insecticides did not work and populations flourished.
In one experiment with a single insecticide, Scharf and his colleagues found that there was little resistance to the insecticide chosen, and they were able to virtually eliminate the insect population. In another, there was about 10% resistance initially, and populations eventually increased.
Subsequent laboratory tests on the remaining cockroaches showed that cross-resistance probably played an important role. A certain percentage of cockroaches showed resistance to a class of pesticides. In fact, those who survived treatment and their offspring would be essentially immune to this insecticide in the future.
In addition, they also acquired resistance to other classes of insecticides, even if they had not been exposed and had no resistance before.
We have seen the resistance quadruple or sixfold in a single generation. We absolutely did not imagine that such a thing could happen so quickly.Michael Scharf, entomologist at Purdue University
Females have a three-month breeding cycle during which they can have up to 50 pups. However, if a small percentage of cockroaches are resistant to an insecticide and cross resistance develops, a population eliminated by a single treatment could explode again in a few months.
These researchers, whose work is published in Scientific Reports, say that only an integrated approach to pest control can overcome a cockroach problem. They recommend combining chemical treatments with traps, vacuum cleaners, but also improved sanitation facilities.
“These techniques are often more expensive than using insecticides, but if they do not control or eliminate a population, then money is wasted,” says Scharf.
“Combining several methods will soon be the only effective way to eliminate cockroaches,” concludes Michael Scharf.