In 2017, researchers discovered that the oriental fruit fly (Bactrocera dorsalis) had developed resistance to the insecticide trichlorphon thanks to a species of symbiotic bacteria, Citrobacter freundii, in its gut that helps the fly degrade the chemical. A new review paper published in the Annals of the Entomological Society of America compiles this case and other existing research on the links between insect symbionts and insecticide resistance. (Merle Shepard, Gerald R.Carner, and P.A.C Ooi, Insects and their Natural Enemies Associated with Vegetables and Soybean in Southeast Asia, Bugwood.org)

By Leslie Mertz, Ph.D.

Probiotic products now line store shelves, promising to improve human health by replenishing the gut microbiome, or the collection of bacteria and other microbes that live in the digestive system. Insects have a gut microbiome too, and it not only benefits their general well-being but may also help them adapt to and overcome pesticides, according to a new review paper published in the Annals of the Entomological Society of America.

Leslie Mertz, Ph.D.

“Whether you’re looking at agricultural pests, household pests like cockroaches, or medical pests like disease-carrying mosquitoes, insects are great at adapting to whatever we throw at them, especially when it comes to different chemicals,” says lead author Jose Pietri, Ph.D. “I’ve always been interested in both microbes and insects, and I’ve been hearing my whole career about the importance of microbes to human and animal health, so it seemed very obvious to ask about the connection between the diverse community of microbes that live inside an insect and the insect’s ability to evolve resistance to pesticides.” Pietri is principal scientist at Apex Bait Technologies Inc. of Santa Clara, California, and a former postdoctoral researcher at the University of California, Santa Cruz.

Other researchers have recently begun to look into the connection between gut microbes and insecticide resistance, but, as with any new area of study, publications are limited in number and scattered over a wide range of journals. That’s why it was a good time for a review paper, Pietri says. “We wanted to compile everything that we could find and everything we currently know about this association between insects, microbes, and insecticide resistance and put it all in one place to start to identify patterns and to facilitate people learning more about it.”

For the review paper, the researchers pored through scores of publications that at least superficially broached the topic, and they found several dozen studies that involved more thorough analyses of microbes’ roles in resistance. “From that, we figured out a few interesting things,” Pietri says. First, just because a certain microbial community is present in resistant insects does not mean that the microbial community is responsible for the resistance. “What happens is that the insect evolves to resist insecticides, and those evolutionary processes also change what happens in their bodies, which includes the gut microbes,” he explains. In other words, the microbes are often just passengers on the evolutionary ride.

Researchers Jose Pietri, Ph.D. (shown examining a cockroach under a microscope), and Dangsheng Liang, Ph.D., are co-authors of a review paper that that looks into possible connections between the microbes that live inside the insect gut (the gut microbiome) and insects’ ability to develop pesticide resistance. Both Pietri and Liang are scientists at Apex Bait Technologies Inc., a company that focuses on developing insect baits to control urban pests. Part of the company’s work includes investigating whether insecticide exposure might alter the insect’s gut microbiome. (Photo credit: Apex Bait Technologies Inc.)

That said, numerous studies did show that certain microbes do sometimes take the driver’s seat in conferring resistance. “There are several possible ways they can directly help insects deal with the toxins in insecticides, and one is a very direct mechanism whereby a toxin comes into the gut of an insect and, through enzymatic means, the microbes break it down and render it ineffective,” he says. Other microbes work by binding to the toxin, which effectively takes the toxin out of play. Still others appear to boost the insect’s own defense mechanisms.

“Insects have evolved genes, proteins, and other means to help flush away or metabolize insecticides, and some studies have shown that the presence of certain microbes can change the way that insects express these genes and can heighten the function of the insect’s own enzymatic detoxification mechanism,” Pietri says.

The review paper has spurred Pietri’s Apex research group to pursue a study on German cockroaches (Blattella germanica) to figure out how exposure to insecticides might alter microbial growth. “We’re anticipating the results of that study to be out later this year,” he notes, adding that the knowledge may be useful in developing new control measures against this nuisance pest.

Outside of his lab, Pietri hopes the review paper will also promote additional “thorough, controlled, detailed studies” and encourage interdisciplinary efforts that involve microbiologists, entomologists, and other scientists who can bring different perspectives and new technologies to bear. In addition, he hopes research groups will begin to move beyond the lab and to real-world settings.

“From the review paper, it’s clear that you can affect resistance across diverse insect species by targeting microbes in a lab setting. Now, I would like to see people take their work and translate it to a broader environment to see if we can apply it to effective control in the field, because that’s ultimately what we want to be able to do,” he says.

Beyond combating insecticide resistance and improving insect control, he envisions a slew of untapped research areas relating to the insect microbiome. For instance, a study might consider the detoxifying capabilities of insect microbes in bioremediation, such as cleaning up pollutants in a water supply, and another might use the insect microbiome as a model system for learning how the human gut bacteria might influence a patient’s response to a medication.

He adds, “This is an interesting field to be sure, because there are a lot of ideas that can be tested and a lot of room for researchers to be innovative and creative. I think that’s exciting.”

Leslie Mertz, Ph.D., teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.