Researchers in the US have made a surprising discovery about the number of parasites typically found within the bodies of insecticide-resistant cockroaches.

German cockroaches significantly impact human health on a global scale through their transmission of food-borne pathogens. In addition to this, they host a number of parasitic microbes within their bodies, which are left behind in their droppings. Many of these microbes cause illness in their own right, but other microbes can intensify the allergic reactions and asthma caused by the allergens in cockroach cuticles and droppings. The health impacts of cockroach infestations are particularly prevalent in low-income housing areas.

During the course of a research project looking at insecticide resistance in cockroaches and its impact on gene activation, scientists from Purdue University in Indiana, USA, discovered that cockroaches with high levels of insecticide resistance had a surprisingly low number of parasites within their bodies – much lower than the bodies of non-resistant cockroaches.1

Michael Scharf, endowed professor in urban entomology and the first author of the study, explained the unusual findings.

“To process a roach for genetic analysis, you end up processing both the roach and the parasites living inside of it. This is when we found that insecticide-resistant roaches contained less parasite DNA than their nonresistant counterparts, evidence that the former carry lower amounts of parasites.”

Prof Scharf also noted that the insecticide-resistant cockroaches were also visibly healthier, with long, straight antennae and shiny wings, all indications that the cockroaches were not hosting parasites.

The study’s surprising findings suggest that commensal living with insecticide-resistant cockroaches may not be as injurious to health as living with non-resistant cockroaches – although the allergen risk would still be present, the risk of pathogen transfer would be reduced.

As of 2016, the German cockroach was reported as having developed resistance worldwide to 42 distinct insecticide active ingredients in at least 219 documented cases (Zhu et al., 2016). The aim of this particular study was to gain a greater understanding of the genetic processes behind insecticide resistance. The researchers created their own insecticide-resistant cockroach strain in the laboratory as part of the study.

To begin with, the cockroaches were separated into two groups: the control group, which was allowed to live and reproduce undisturbed; and the experimental group, which was exposed to indoxacarb, a commonly used active ingredient in cockroach baits.

Most of the cockroaches in the experimental group died, but not all of them – the survivors went on to reproduce with each other. This second generation was also exposed to the insecticide, and the survivors also went on to reproduce with each other. The process repeated until the researchers reach the sixth generation of cockroaches.

Although this was conducted in a laboratory, the conditions accurately mimic what happens in a real-life infestation in a residential or commercial building. When cockroaches infest a house or apartment, they are a self-contained population, meaning the cockroaches only reproduce with other cockroaches in the building. When insecticide is used, it wipes out the portion of the population that is vulnerable to the insecticide. Those that do survive, however, reproduce with each other and pass on the genetic traits that allowed them to survive. German cockroaches reproduce about every three months, so insecticide resistance can emerge quickly.

“By the sixth generation, we compared each group’s response to the insecticide. The experimental group was able to survive a dose that was 25 times higher than what the control group could survive,” commented Prof Scharf.

When analysing the difference between the two groups of cockroaches, they found that the indoxacarb-resistant group exhibited a range of upregulated genes involved in a range of insecticide detoxification resistance mechanisms, and also a decrease in internal virus, parasite and pathogen levels.

However, it is unclear as to whether this lower microbe loading was due to the direct effects of indoxacarb, the indirect effects of antimicrobial preservatives included in the bait matrix or the selection of general stress response mechanisms that confer both insecticide resistance and microbial immunity. This will be investigated further in future research, not the least to investigate whether these effects are specific to indoxacarb or occur with all insecticide resistance.


Further reading: ‘Study finds insecticide-resistant cockroaches healthier than non-resistant ones’ by Samantha Murray. The University of Florida’s Institute of Food and Agricultural Sciences (UF/IFAS) Blog. March 15, 2022.

1 Scharf, M. E., Wolfe, Z. M., Raje, K. R., Fardisi, M., Thimmapuram, J., Bhide, K., & Gondhalekar, A. D. (2022). Transcriptome Responses to Defined Insecticide Selection Pressures in the German Cockroach (Blattella germanica L.). Frontiers in physiology, 12, 816675. fphys.2021.816675