Pestology Blog
Controlling House Flies, Insecticide Resistant Mosquitos, and Tracking Red Imported Fire Ants
Fairfax, VA – March 5, 2025
In the March 2025 episode of NPMA BugBytes, Laura covers new research on housefly control, Mike talks about new research tracking changes in insecticide resistance in mosquitoes, and Ellie covers a publication focused on tracking the red imported fire ant. We're joined by special guest, Megan Moloney with NPMA!
Featured Article Summaries
Controlling House Flies
Decreased Emergence Rates of Adult House Flies (Musca domestica; Diptera: Muscidae) Due to Exposure to Commercially Available Insecticidal Baits During Larval Development
House flies, or Musca domestica, are a pest that transcends both the agricultural sphere and the structural sphere of pest management. On the agricultural side alone, the estimated annual costs of management of house fly populations approaches one billion US dollars (Geden et al. 2021). Therefore, many methods under the umbrella of integrated pest management have been devised to try and mitigate not only the house flies themselves, but the pathogens that they are known to carry.
Fig. 1 Musca domestica, USDA
That being said, insecticides still reign supreme in terms of house fly management, and are the most commonly reached for management strategy. However, given the historical use of insecticides for these pests, we now currently have documented insecticidal resistance for every single insecticide class that is currently used against house flies. While most of these products are targeted towards the adult life stage, little research has been done to determine if the larval stage is also affected by these products.
Many of these products, such as house fly insecticidal baits, are intended to be scattered on the ground. Yet, many of these areas where these baits are applied may additionally be areas where larvae are developing. The researchers sought to determine if house fly larvae were affected by the application of these products at label rates.
The researchers used three strains of house flies, which included two field-collected populations, and one lab-reared population. They additionally used four different commercial house fly insecticidal baits, each of which featured a different active ingredient with different modes of action. These active ingredients included methomyl, imidacloprid, dinotefuran, and cyantraniliprole. These trials were conducted in a lab, where the baits were applied to larval development medium at label rates in plastic cups. Larvae developed in these cups, and then the number of adults that emerged following development was recorded.
Overall, the researchers found that adult house fly emergence was reduced by nearly 40% due to the application of commercial house fly insecticidal baits. This indicates that although this product is developed solely for the adults, it may have some unintended effects elsewhere in the house fly life cycle.
This sounds like good news, with the product not only affecting the adults, but the larvae as well- a two for one punch! However, given that house flies have seen an immense amount of documented resistance to insecticides, this study may provide a window into why that may be the case. Let’s do a hypothetical experiment here. Let’s say we just applied commercial house fly bait to manage the adults, but the substrate we spread it on featured 100 house fly larvae. Based on this study, we can assume that 40 of those larvae will not make it to adulthood. However, the 60 that do make it to adulthood already will have some resistance to the commercial fly bait that we just applied. Therefore, since they were unresponsive to the insecticide as larvae, it could mean that they are similarly unaffected as adults. In addition, since they made it to the benchmark of adulthood, they could pass on those same traits of resistance to their offspring. Therefore, applications of the insecticidal baits on larval substrates could be a secondary contributor to house fly resistance by unintentionally selecting for the larvae that make it through to adulthood.
This study highlights the potential of insecticide resistance developing in a secondary pathway that was previously not considered. While more research is necessary, this plants the seed for further consideration on how these pests interact with the insecticides that we manage them with.
Article by Laura Rosenwald, BCE
References
Jimmy B Pitzer, Jessica D Navarro, Evan S Phillips, Decreased emergence rates of adult house flies (Musca domestica; Diptera: Muscidae) due to exposure to commercially available insecticidal baits during larval development, Journal of Economic Entomology, Volume 118, Issue 1, February 2025, Pages 391–396, https://doi.org/10.1093/jee/toae310
Insecticide Resistant Mosquitoes
Time-of-day Changes in Permethrin Susceptibility and Metabolic Gene Expression in Florida Aedes aegypti (Diptera: Culicidae)
Insects, like humans, exhibit circadian rhythms which are the natural 24-hour cycles that regulates bodily functions. The most common function associated with circadian rhythm is sleep, but there are a whole host of other functions regulated by this 24-hour cycle, including body temperature and hormone production. The major driver behind how the 24-hour circadian clock is controlled is photoperiod, or the light-dark cycle between day and night.
Fig. 1 Aedes aegypti, Dan Mendelowitz
The seasonal changes in light-dark period, such as shorter days in winter and longer days in summer, is what triggers things like diapause in insects and hibernation in mammals. In mosquitoes, circadian rhythm and photoperiod also control a lot of other biological functions including hunger, mating, oviposition (or, egg laying), and flight.
These cyclical changes in an insect’s internal biology can impact more than just their drive to feed. As it turns out, a mosquito’s ability to survive exposure to certain insecticides may also fluctuate over a 24-hour period. Previous studies working with a permethrin-resistant strain of Aedes aegypti, aka the yellow fever mosquito, documented greater insecticide resistance during the “light” phase of a 12-hour light / 12-hour dark cycle. Similarly, other studies focused on tracking the expression of genes responsible for insecticide resistance among mosquitoes were also influenced by photoperiod.
Aedes aegypti is an important disease vector for some of the most important mosquito-borne diseases around the world. Pyrethroids such as permethrin are a commonly used adulticides and represent an important tool in the fight against these dangerous insects, but resistance is a growing concern, particularly in states like Florida where mosquito seasons are longer and the need for constant control efforts is critical to protecting public health.
If a mosquito’s susceptibility to pyrethroids such fluctuates over a 24-hour cycle, then knowing when the population is most sensitive to that application would be invaluable information for improving adult mosquito control efforts. To hopefully provide answers to this question, researchers at the Florida Medical Entomology Laboratory located in Vero Beach, Florida measured changes in permethrin susceptibility of resistant and susceptible field strains of Ae. aegypti over a range of different light/dark cycles. In addition, every 4 hours over a 24-hour cycle they tracked the regulation of detoxification enzymes known to play an important role in permethrin susceptibility, and the expression of several detoxifying genes.
What the researchers found was that, overall, permethrin susceptibility was lower during the day compared to the evening for both strains. And that susceptibility was lowest at dusk (around 6:00pm) and highest between 2:00am and noon. They also found that gene expression increased for all detoxification genes in the resistant strain of Ae. aegypti at night (from around 6pm to 2:00am).
In other words, these data showed that susceptibility of Ae. aegypti did change over a 24h period, and that permethrin applications made between the hours of midnight and dawn (around 6:00am) may be the most effective at reducing adult Ae. aegypti mosquitoes.
Article by Mike Bentley, PhD, BCE
References
Sierra M Schluep, Tse-Yu Chen, Shelley A Whitehead, Eva A Buckner, Time-of-day changes in permethrin susceptibility and metabolic gene expression in Florida Aedes aegypti (Diptera: Culicidae), Journal of Medical Entomology, 2025;, tjaf013, https://doi.org/10.1093/jme/tjaf013
Tracking Red Imported Fire Ants
A New Nondestructive Method for Monitoring Red Imported Fire Ant (Hymenoptera: Formicidae) Populations in the Field
The red imported fire ant (RIFA) is an invasive species that has been spreading throughout the southern US, slowly moving further north and increasing their geographic distribution. Like any invasive species it is critical to know their range, and what areas they are more concentrated in. For anyone who has encountered these ants, you will know these are a public health issue as well as a pest of structures and agriculture. RIFA are known for their painful sting that can leave pustules or blisters that linger for days or weeks. Encountering a nest can result in a person getting dozens or even hundreds of these stings. Small animals like lizards can even be killed by these ants.
In this study, the researchers focused in on the question of monitoring populations not just for their spread, but to look at how well treatments were working. Granular insecticides are one of the most common ways to treat for RIFA. To be able to tell how well treatments are working, you need to be able to measure in some way the population before and after. The current available methods can be time consuming, challenging, and may require killing the ants you are sampling.
Current methods include things like counting mounds by probing them and seeing if they are active. That is tough because not all mounds are easy to find and there are varying definitions of “active”. Another method is counting foraging ants by pitfall traps or counting ants at non-toxic bait stations. Counting ants at baits is getting warmer to the new method they came up with, but so far has required photos, human error in counting, and lots of time.
The folks at UC Riverside and the USDA set out to come up with an easy, fast, and accurate method of monitoring RIFA populations. They set out a clear plastic basket-like container with Fritos in it to lure the foraging ants in. These Frito traps were left on the lawn for a few hours and then carefully picked up with long forceps and roughly shaken into a solid slippery-walled container to get all the ants out but leave the chips. This container had a grid on the bottom to take photos of, remember we want to compare current methods and accuracy. The actual method would skip this step. Then the container gets shaken into a funnel that pours them into a graduated cylinder with measurements on the side. See figure 1. They tapped the tube a few times to get the ants to go to the bottom and then they looked at the total volume of ants.
Fig. 1 A) RIFA sampling using corn chips and a miniature basket. B) Measuring the volume of RIFA workers in the field after pouring RIFA workers into a plastic funnel leading to a graduated cylinder. The graduated cylinder was tapped a few times to trigger ant compactionThey recorded the volume of ants to be compared to the counts with the photo taken. They used a software called ImageJ to get the counts of the ants in the photos and then used that to compare to the volume found.
Here’s some numbers to compare- it took 15 seconds - two minutes for the people and software to get the sample counts of these photos in the lab. Compare this to a few seconds to look at the volume measurements on the side of the tube.
The crazy cool discovery here is that they were able to match the number of ants taken with the photo to correlate with the volume counted number in the tube. Therefore, proving the concept that volume counts are a viable option for counting RIFA numbers. While not exact, the very ballpark estimate is that 1 milliliter of ants is somewhere around 100 ants. Please note this correction from the recording of the episode, I had misspoken and said 10 instead of 100.
To get the most accurate picture, you would plug in the volume to the linear equation they created and get a more exact number of ants. This shows a method that can be used in the field by any PMP. Since you are here looking at our written article, that linear equation is number of ants = 140.4(volume of ants in mL) - 16
The method is fairly accurate, requires no expensive equipment, and initial deployment could even be done by a homeowner or business, two hours before the PMP arrives so that the PMP can just do the collection. This simple, yet ingenious method could be easily and cheaply implemented to fire ant control programs giving PMPs accurate measurements to inform their treatment plans.
Article by Ellie Sanders, BCE-Intern
References
Siavash Taravati, Jung Ma, Kathleen Campbell, Dong-Hwan Choe, A new nondestructive method for monitoring red imported fire ant (Hymenoptera: Formicidae) populations in the field, Journal of Economic Entomology, Volume 118, Issue 1, February 2025, Pages 397–402, https://doi.org/10.1093/jee/toae278
Listen to the Episode!
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