Study reveals why cockroaches are growing immune to insecticides

Monday July 15 2019


A study on the ‘German cockroach’ has found that they have developed a cross-resistance to a range of powerful insecticides and may soon become nearly impossible to kill with chemicals. PHOTO | FILE | NATION MEDIA GROUP 

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Have you had cockroaches in your kitchen, or building, that won’t just be exterminated no matter what?

Well, here’s what’s going on, apparently the little pesky crawlers have become immune to most available insecticides. In fact, spray mixture treatment will not only fail to lower cockroach numbers, according to recent research, but might have a repellent effect that distributes cockroaches to previously uninfested neighbours.

A study on the ‘German cockroach’ (Blattella germanica) has found evidence that they had developed a cross-resistance to a range of powerful insecticides and may soon become nearly impossible to kill with chemicals of whatever kind. This is the most common species here in Kenya, according to the International Centre of Insect Physiology and Ecology (Icipe) — Kenya.


The German roach is the single-most cockroach of concern around the world.

The other species of cockroaches found in homes in Kenya include Oriental cockroach (Blatta orientalis), American cockroach (Periplaneta americana) and brown-banded cockroach, Supella longipalpa (Fabricius), said Icipe.


Study co-author of the research paper, Michael Scharf, a professor and chair with the Department of Entomology at Purdue University in Indiana, and Rollins-a global leader in pest control, said the roaches' offspring are also being born already resistant to the chemicals in the insecticides.

The scientists said insecticide resistance has occurred to every insecticide class introduced for cockroach control since the early 1950s.

“Surprisingly poor performance of a majority of treatments in the field study suggested significant levels of resistance,” said the study. “Insecticide resistance has occurred to every insecticide class introduced for cockroach control. And resistance to multiple classes of insecticides was found to exist among entire cockroach populations. This is because German cockroaches live in relatively closed populations which facilitates rapid selection for high-level resistance.”

In Kenya, Dr Chrysantus Mbi Tanga, an insects’ research scientist, at Icipe told the Nation that this was a problem they had seen Kenyans grapple with for years now.


“It is a very big problem that has concerned us scientists in Kenya. A lot of Kenyans have been complaining and looking for effective control measures because commercial products seem not to provide adequate results as intended,” he said.

Cockroaches are a serious threat to human health. They are a vector of human-associated pathogens like bacteria, Salmonella, Enterococcus and E. coli, which can cause illness to humans.

According to the authors of the US research, the German cockroach is capable of hosting tens of bacteria in its digestive tract, including antibiotic-resistant strains.

The nocturnal creatures with filthy habits search for food in kitchens, food storage places, rubbish bins, drains and sewers and transmit disease to your food, utensils, and kitchen surfaces and can easily contaminate food by leaving saliva, body parts and faecal droppings which may contain these bacteria that can cause food poisoning, contributing to unhealthy indoor environments.

Dr Tanga warns that they play a major role as carriers of intestinal diseases, such as diarrhoea, dysentery, typhoid fever and cholera.

Also the report said sensitisation to cockroach allergens is one of the strongest risk factors for the development of asthma, as their droppings leave behind allergens that trigger allergies and asthma.

“Recent evidence has suggested links between cockroach-vectored bacteria and asthma. Recent evidence further suggests links between cockroach-vectored bacteria and asthma. Cockroaches contribute to house dust microbiomes, which in turn intensify cockroach-induced asthma,” said the scientists in their report, “Effective cockroach control can reduce allergen loads.”

, asthma morbidity and associated economic costs.”


They said this resistance had exacerbate impacts of cockroaches on public health.

Yet according to Prof Scharf, while insecticides are essential for reducing cockroach populations and improving health outcomes, “insecticide resistance has been a consistent barrier to cockroach control, as cockroaches have developed resistance to multiple classes of insecticides all at once”, a thing the researchers reckon will make their control almost impossible with chemicals alone.

For the study, the researchers tested three different courses that included single Active Ingredient (AI) treatments, mixture and rotation of professional-grade insecticides with active ingredients thought to be the most active on roach populations in housing facilities in Danville, Illinois and Indianapolis, in the Midwest USA, over six months.

One group of roaches was exposed to a single insecticide. A second population received two insecticides from different classes. And a third was dosed with rotations of three insecticides — one per month, for two three-month cycles.

Beyond the multiple resistance that appears ubiquitous among cockroach populations, the scientists also tracked roaches' resistance to insecticides across multiple generations.

Apparently, even after dosing the fast-breeding roach populations with the different classes of insecticides, the study authors reported they either remained stable or increased. Rotating the pesticides was found to be "mostly ineffective" at reducing their numbers, "due to cross-resistance,”.

Regarding the spray mixture treatment, cockroach numbers increased at both locations despite four months of treatment.

Mixture products containing pyrethroids as active ingredients were found to be universally ineffective and highly repellent. But worse, that evolution of cross-resistance among active ingredients is a significant, previously unrealised challenge.


According to Dr Tanga, in Kenya Total Release Foggers (TRFs) or “bombs”-spray cans that usually contain pyrethroid, pyrethrin or both as the active ingredients- are the commonest AIs used to manufacture insecticides for cockroaches. Boric acid, Fipronil, carbamates and others with a synthetic form of nicotine are also listed by the Pest Control Products Board as ingredients used by manufactures of cockroach insecticides in Kenya. Diazinon, Carbaryl, Chorpyrifos-methyl, Pirimiphos-methyl are others.

The research in US included these AIs used in Kenya and more, in the study that established that offspring were not only resistant to the pesticide that their parents encountered but also showed signs of resistance to other classes of insecticides as well.

“Resistance levels increased to almost all AIs tested in this study,” the researchers said. In one of the experiments the researchers tested a group of roaches that had slightly more resistance, whose number of roaches actually increased, with generations born to resistant survivors.

A single female German roach can produce up to 40 offspring every few months, which quickly replenishes depleted communities.

Ridding homes of these pests will require strategies more complex than chemical treatments alone, Scharf said in the report in the journal Scientific Reports published last month. He suggested a combination of approaches such as improved sanitation, traps and even vacuums to suck them up, instead of relying on pesticides to do the job.

“Some of these methods are more expensive than using only insecticides, but if those insecticides aren’t going to control or eliminate a population, you’re just throwing money away,” said Scharf.


Another proposed strategy for managing resistance, according to the scientists include rotating between different products or using mixture products with multiple modes of action, rather than using single AI products with single modes of action. But then the researchers also expressed concern of overuse of the chemicals that are also environmental contaminants.

A combination of insecticide resistance mechanisms documented in this type of cockroach included bodily mechanisms in which the insects were able to detoxify their own bodies of the chemicals contained in the insecticides. Also, the cockroaches’ outer shell mutated to reduced ability of insecticides to penetrate. And interestingly the bugs had also become smart as to make social changes including behavioural avoidance of the baits.

Dr Tanga said the research paper from the US was a pointer of the new opportunity in research especially in the evaluation of alternative options to deter resistant. He suggested that use of integrated pest management strategies might help minimise resistance for now.

“Unfortunately, very little attention or research has been directed at this problem, yet it has significant public health ramifications,” he observed.

He said scientists at Icipe were investigating low-cost and eco-friendly alternatives like Bio-pesticides in an integrated fashion for management of the nuisance insect.

“If this is successful, it will greatly reduce the dependence on insecticides which are not effective and have health and environmental related negative implications,” he added.