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Termite Tunneling, Horse Flies, and Cockroach Salmonella
Fairfax, VA – January 1, 2025
In the January 2025 episode of NPMA BugBytes, cohosts Ellie, Laura, and Mike were joined by special guest Aly Silva Mulgrew of Plunkett's Pest Control! We covered new research on termite bait finding, visual cues horse flies use to find hosts, and salmonella presence in german cockroaches.
Featured Article Summaries
Termite Tunneling
Subterranean Termites (Coptotermes Formosanus [Blattodea: Rhinotermitidae]) Colonies Can Readily Intercept Commercial Inground Bait Stations Placed at Label-Prescribed Distances
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Article by Ellie Sanders, BCE-Intern
References
Kaitlin Gazdick, Sang-Bin Lee, Nobuaki Mizumoto, Thomas Chouvenc, Nan-Yao Su, Subterranean termites (Coptotermes formosanus [Blattodea: Rhinotermitidae]) colonies can readily intercept commercial inground bait stations placed at label-prescribed distance, Journal of Economic Entomology, 2024;, toae259, https://doi.org/10.1093/jee/toae259
Horse Fly Visual Cues
Brightness-dependent Visual Attractiveness of The Human Body for Horse Flies (Diptera: Tabanidae): A Field Experiment
Most bloodsucking insects rely on a similar set of cues to locate a host. These usually include carbon dioxide, sometimes body heat, and to some degree, vision. When it comes to female horse flies, finding a host mostly involves the use of visual cues, looking for dark objects that offer a contrast from a lighter background the host may be in front of.
One very cool and unique aspect of the horse fly’s visual capabilities that sets them apart from other blood suckers is the horse flies’ sensitivity to the polarization of light. By that I mean horse flies can differentiate between polarized light, such as the light that would reflect off water, and unpolarized light like the kind that your flashlight gives off. And not only are they able to distinguish between the two types of light, but horse flies are attracted to polarized light versus unpolarized light. This attraction makes biological sense for two reasons. First, horse flies lay their eggs near bodies of fresh water because the larvae need water or muddy environments to develop. So, they need to be able to locate water to find good places to lay their eggs. And second, the light that reflects off dark objects is also polarized light. Adding to the horse fly’s ability to differentiate hosts from non-host objects.
Horse flies are more often associated with farm environments, but these painful biters can become a serious nuisance around urban environments. Especially when agricultural environments or ideal larval development sites are located within flying distance of these urbanized areas. Previous studies have tested the visual preference of horse flies to dark or light-colored horses, confirming that the flies largely preferred black or brown animals to white or spotted animals. But data was limited to how brightness would translate to human shaped hosts.
To test if horse flies would exhibit similar contrasting brightness preferences to human shaped hosts, researchers Attila and Matúš used mannequins as their human stand-ins. In field experiments, the researchers used one black and one white shiny plastic mannequin in an open grassy field to measure the horse fly’s preferences. The mannequins were placed in direct sunlight, spaced about 5 meters apart, and covered in a scentless glue that would trap the flies to the mannequins and allow researchers to count and collect any hungry flies that landed on the test subjects. Each week when experiments were conducted, researchers would collect any flies and reapply a fresh layer of glue. To more accurately assess the horse fly’s landing preferences, the researchers subdivided the mannequins into 4 zones: head, upper limbs, torso, and lower limbs. This would allow them to track if the flies preferred one landing zone over another.

What Attila and Matúš found was that nearly 86% of all horse fly’s trapped were recovered on the black mannequins, which is why they hypothesized would happen. This result aligns with how horse flies responded in previous studies using similar horse shaped models. However, when they analyzed the landing zone data, they found that 56% of all horse flies were recovered from the legs of the human-shaped mannequins. What’s interesting about this result is when the same study was conducted using horse-shaped models, the distribution of trapped flies appeared to be random, and no body part was preferred. There were earlier studies conducted in the mid-90’s using human volunteers to track feeding site preference on the human body that also observed similar results, reporting that just over 50% of all flies were caught on the lower limbs as well.
So, what does this all mean for structural pest control? I think the take home message here is that we may be getting closer to building a better horse fly trap. The unavoidable challenge of dealing with flying pests is that it’s nearly impossible to prevent them from accessing a property with anything short of physical exclusion. So, traps often play an important role in reducing adult populations around a structure. The data from Attila and Matúš’ study show that horse flies attracted to human shaped hosts prefer a dark surface and their preferred landing zone may be in the 3-4’ range off the ground. This data could be used to improve upon existing trap designs, suggesting a dark trap set at the 3–4-foot range may perform better. And the addition of another host cue such as CO2 could even further enhance the performance of these traps. Having said that, this is all hypothetical and field studies would need to be conducted to confirm whether any of this would be the case. Regardless, farm pests such as horse flies are likely to become a greater problem as urban sprawl continues to occur and homes continue to be developed closer and closer to agricultural areas, meaning that finding viable solutions to these painful biting pests is something that will need to be worked out sooner rather than later.
Article by Mike Bentley, PhD, BCE
References
Attila Balogh, Matúš Kúdela, Brightness-dependent visual attractiveness of the human body for horse flies (Diptera: Tabanidae): a field experiment, Journal of Medical Entomology, Volume 61, Issue 6, November 2024, Pages 1368–1372, https://doi.org/10.1093/jme/tjae104
German Cockroaches and Salmonella
Differences in Salmonella Typhimurium Infection and Excretion among Laboratory and Field Strains of the German Cockroach Suggest a Genomic Basis for Vector Competence
One of the universal truths about working in structural pest management is that pest cockroaches are gross. As constant explorers of sewers, rotten food, and other disgusting things, they frequently are the reason for the season in terms of transmitting diseases. However, new studies have begun to suggest that cockroaches could potentially be active biological vectors of diseases, much like mosquitoes.
Salmonella enterica is a bacterial species that comes in many varietals, including one called Typhimurium. This serovar is responsible for causing gastroenteritis in humans, but now new research is showing how intimately connected it is to Blattella germanica, or the German cockroach. Recent research demonstrated that the bacteria appear to thrive in the guts of German cockroaches and may be an important stopover point for S. Typhimurium in its spread. In other words, the guts of these German cockroaches may be acting as a kind of incubator for these bacteria, promoting their growth. The cockroaches then are thought to potentially spread the bacteria to humans through their feces. However, these previous studies were only conducted on inbred lab strains of German cockroaches. The researchers wanted to understand the real-world implications of this research. In other words, do field-collected cockroaches differ in their abilities to become infected and disseminate S. Typhimurium?
They took four different strains of field-collected cockroaches. Three of these strains were recently collected, and one was the old-Orlando standard: the Orlando strain. The Orlando strain has been in maintenance in the lab for over 40 years at this point, but continues to act as a benchmark for studies such as this. The researchers took adult male German cockroaches from each of these groups, and exposed them to S. Typhimurium through a feeding assay. To ensure that the results from the experiments were due to differences in the cockroach strains, the researchers determined that all the strains were feeding on the bacterial medium equally. This meant that all differences in bacterial loads within the cockroaches were due to differences in the strains of cockroaches.
A few of these cockroaches were selected after 24 hours to determine the amount of S. Typhimurium in their systems. These cockroaches were ground, and then plated on a special agar, or bacterial growth material, that only allows for the growth of S. Typhimurium to determine the level of infection in each cockroach. The researchers counted the bacterial growth, and analyzed for both the prevalence of infection within the cockroach strain and the intensity of infection, which was the amount of colonies from one adult male cockroach. They found that all the cockroach strains were able to host S. Typhimurium, but that the prevalence of the infection ranged from 63%-94% across the strains. Interestingly, those cockroaches that were infected, ranged in infection intensity, meaning that some cockroaches are more susceptible to infection than others.
The researchers additionally examined how much of the bacteria that the cockroaches were excreting following the exposure to S. Typhimurium. Following infection, a separate group of cockroaches were individually placed on filter paper for 24 hours. Their excretions over the course of those 24 hours were extracted from the filter paper, and then plated on the same type of agar to determine the amount of bacterial growth excreted by each cockroach. While all of the strains had some level of infection in their excretions, it was not correlated with the levels of bacteria that were measured inside the cockroaches themselves. In fact, the strains that featured the highest levels of bacteria in their guts were those that excreted the least amount of bacteria.
Lastly, the researchers also examined the rates of necrophagy, or the consumption of their fellow brethren, as this behavior is a known contributor to the spread of S. Typhimurium between fellow roaches. While the roaches used for this portion of the experiment were uninfected to act as a control, the researchers did find that there were differences in the rates of necrophagy between the different strains. This may point to a behavioral explanation for differences in infections between field populations.
While this study continues to confirm what we already know- that cockroaches are indeed, gross, this study perhaps adds a couple more items to the list. Overall, it emphasizes that not all field populations of German cockroaches are created equal, and that some populations appear to be much more well-suited as potential vectors of gastrointestinal diseases than others. While we don’t quite understand all the molecular underpinnings behind how German cockroaches may act as actual vectors of disease and not just mechanical vectors, this study emphasizes the importance of pest management professionals in the fight against diseases that threaten public health.
Article by Laura Rosenwald, BCE
References
Bashar Ismael, Morgan Wilson, Dini Miller, Jose E. Pietri, Differences in Salmonella Typhimurium infection and excretion among laboratory and field strains of the German cockroach suggest a genomic basis for vector competence. Infection, Genetics and Evolution, Volume 123, 2024, 105624, ISSN 1567-1348, https://doi.org/10.1016/j.meegid.2024.105624
Listen to the Episode!
Have questions or feedback for the BugBytes team? Email us at training@pestworld.org, we'd love to hear from you!