Pestology Blog
Mosquito Swarms, Blacklegged Tick Management, and Silver Bullets for Pest Control
Fairfax, VA – December 1, 2024
In the December 2024 episode of NPMA BugBytes, cohosts Ellie, Laura, and Mike were joined by special guest Court Parker of Bug Busters Inc. In this episode, Ellie reviewed an article about using silver nanoparticles against biting midges, Laura discussed a study about mosquito swam behaviors, and Mike covered research showing that a single fall treatment may be enough to control blacklegged ticks.
Featured Article Summaries
Mosquito Swarm Navigation
Mosquitoes Integrate Visual and Acoustic Cues to Mediate Conspecific Interactions in Swarms
Many insects use swarming to try and find a mate, as it increases the potential for males to run into a female. Male mosquitoes will swarm in complex aggregates that serve as mating arenas. However, the whole mating success of mosquito species hinges on one key feature: the fact that they’re able to not smack into each other while flying.
Therefore, the researchers in this study decided to ask a simple, but very complex question: are mosquitoes able to recognize different types of cues to track their fellow mosquitoes and avoid collisions? Previous studies only focused on the ability of male mosquitoes to recognize and react to acoustic cues by the females, but this study sought to be more encompassing with both acoustic cues and visual cues, while measuring the mosquitoes in both a tethered and free-flying environment.
Using Anopheles coluzzii, the researchers first tested whether the mosquitoes would respond to swarm-like visual scenes that were produced using a two-dimensional cylindrical LED panel, which was essentially mosquitoes being subjected to a VR headset. The picture was a “moving starfield” that rotated, and the mosquitoes wing measurements were recorded to determine their “steering response”. Both males and females responded to the “movement” of the starfield, suggesting that this could be a stabilizing feature when attempting to orient themselves in the swarm.
Next, the researchers integrated acoustic cues along with visual cues. In this experiment, the mosquitoes were again placed in front of the LED panel, while a speaker played the tone of either a male or a female directly in front of them. When the mosquitoes were only faced with the visual cues, there was no difference in steering behavior. However, when the acoustic tones were played in, it became a different story. Males, unsurprisingly, had a strong response in the direction of the moving object when presented with a female tone. Males did not have as strong of a response when presented with visual cues along with the dulcet tones of another male, which suggests that they slightly alter their steering behavior based on the object’s location. The direct opposite could be said for females. Female mosquitoes, while not as dead-set as the males, only slightly steered towards the visual object when presented with a male tone. They did not react in the presence of female tones.
The researchers also examined whether there was a distance component to this response and subjected the male mosquitoes to moving objects of different sizes. Male mosquitoes appear to have a size-dependent threshold, meaning that they only will react to moving objects only when they are in close proximity.
Since the researchers were seeing such drastic results, they decided to further examine how exactly these male mosquitoes would respond to certain situations beyond just their steering response. By measuring their wingbeat frequency and their wingbeat amplitude, they were able to determine what kind of aerodynamic thrust forces may be necessary to avoid collisions. The researchers determined that wingbeat responses decreased as the object came into front visual view of the mosquito and then increased again once the object had moved on. Interestingly, this pattern occurred even in the presence of acoustic cues, which indicates that this is a purely visual behavior.
Lastly, the researchers wanted to examine this behavior in a real-swarm system. Using videography, the researchers recorded male swarms in the lab and analyzed the results. They found that male mosquitoes had a clear “repulsion zone” and accelerated away from their neighbors once they reached an average distance of 1.55 centimeters of each other but interestingly don’t sharply alter their paths. So instead of making sharp turns, mosquitoes will just speed up out of the way to avoid collisions.
Overall, this study highlights that mosquitoes use both visual and acoustic cues to avoid collisions in these swarming scenarios. In fact, in some cases, visual cues alone were able to influence the mosquitoes flight behavior, and both males and females are responding to both the acoustic and visual cues. Not only does this study add immensely to our understanding of sensory ecology of mosquitoes, it also potentially presents a new means of improving our current lure traps. By including both visual and acoustic cues that would draw in swarms, we may be able to have another means of managing these public health pests.
Article by Laura Rosenwald, BCE
References
Mosquitoes integrate visual and acoustic cues to mediate conspecific interactions in swarms. Gupta, Saumya et al. Current Biology, Volume 34, Issue 18, 4091 - 4103.e4
https://doi.org/10.1016/j.cub.2024.07.043
Blacklegged Tick Treatments
Late Fall Synthetic Acaricide Application is Effective at Reducing Host-seeking Adult and Nymphal Ixodes Scapularis (Ixodida: Ixodidae) Abundances the Following Spring
Lyme disease is the fastest growing vector-borne disease in the United States, and is now the most common tick-borne illness in the country. Lyme disease is a bacterial infection spread by the tick Ixodes scapularis, commonly known as the deer tick or blacklegged tick. There are several reasons for the recent… “UP-TICK” ….in Lyme disease cases in the US, including climate-change induced range expansion of the blacklegged tick which in turn puts more people at risk of infection, and an increase in overlapping habitat of both deer mice and whitetail deer which can both serve as reservoir hosts of the Lyme disease bacteria.
Ixodes scapularis, Photo by: CDC/ James Gathany; William L. Nicholson, Ph.D.
While tick management hasn’t historically been a widespread service offered by most pest management professional’s, the growing public health risk of tick-borne disease cases in the US over the last decade or two has led to more and more PMPs adding backyard tick control to their seasonal service menu. These tick-control services typically involved the application of a liquid or granule pyrethroid in both the fall and the spring to the surrounding vegetation along the perimeter of a property line where adult and larval life stages of many tick species are known to rest when not feeding on a host. Liquid applications were commonly done using a backpack blower or truck-mounted sprayer. The logic behind the combined seasonal treatment is that late fall applications would target all life stages, and the spring application would target host-seeking nymphs the following season.
The combined approach of a backyard pyrethroid application in both the late Fall and early Spring has shown to be effective at reducing tick activity. But, springtime applications could come at an unintended cost since this is when many non-target insects would also be emerging from their winter dormant state. Fall time applications, however, would be less likely to impact non-targets since this is the seasonal period when beneficial insects would have already migrated or would already be dormant. But, could a single Fall application be enough to reduce tick activity the following year? And, if a Fall application could work, what application type would be the better tool for the job?
Researchers Scott Williams and Megan Linske at the Connecticut Agricultural Experiment Station set out to investigate those questions in a series of field experiments measuring the effects of lambda-cyhalothrin applications across two consecutive seasons. Using residential backyards as their study sites, the researchers measured the effects of backyard pyrethroid applications using three different delivery methods: truck-mounted high-pressure sprayer, gas powered backpack blower for liquid delivery, and backpack blower for granule delivery. Additionally, they made applications for each delivery method at three different treatment times: Fall only, Spring only, or Fall + Spring. Ticks were sampled at treatment sites using a tick drag method to measure tick activity before and after treatments.
And here’s what they found. When comparing delivery types, their study showed that FALL-Only pyrethroid applications using the truck-mounted sprayer reduced adult host-seeking tick activity in the Fall and following Spring by 100%. Similarly, the backpack delivery of granules in the Fall also reduced adult tick activity the following Spring by 94%. When comparing different treatment times, the researchers found that no nymphal host-seeking activity was recorded in the following spring after Fall only, spring only, or Fall/Spring combination treatments were made using the truck mounted sprayer or backpack blower. Lastly, they found that NO adult activity was detected AT ANY TIME post treatment with truck mounted sprayer.
To sum this all up, Williams and Linskey’s study successfully demonstrated that a single high-pressure sprayer application of lambda-cyhalothrin in late fall was sufficient to successfully eliminate multiple stages of host-seeking deer ticks through the following spring. This means a single application could provide effective tick control, reducing the need for additional spring applications and therefore reduce the risk of pyrethroid exposure to non-target insects in the spring.
Article by Mike Bentley, PhD, BCE
References
Scott C Williams, Megan A Linske, Late fall synthetic acaricide application is effective at reducing host-seeking adult and nymphal Ixodes scapularis (Ixodida: Ixodidae) abundances the following spring, Journal of Medical Entomology, Volume 61, Issue 4, July 2024, Pages 965–974, https://doi.org/10.1093/jme/tjae044
Silver Nanoparticles and Biting Midges
Tiny Silver Bullets: Silver Nanoparticles are Insecticidal to Culicoides Sonorensis (Diptera: Ceratopogonidae) Biting Midge Larvae
How often in pest control do we hear the wish “if we just had a silver bullet” or the pragmatic statement that there are no silver bullets in pest management. Well, yes and no. In a metaphorical sense, there is no one size fits all answer to managing pests and an integrated pest management plan is always needed. But in a more fun sense, these researchers discovered an actual silver bullet.
Biting midges are pests of livestock and humans and are known for their painful bites and ability to vector livestock diseases, making them prevalent medical and veterinary pests. As larvae they live in water, similar to the mosquito lifecycle that you may be more familiar with. This is an important fact to the method of control.
Silver nanoparticles, which are extremely tiny bits of silver smaller than the eye can see, have been shown to be lethal to microorganisms and aquatic insect larvae like mosquitos and non-biting midges. The silver nanoparticles are attached to a protein-based material derived from grains which served as a vehicle for the silver. This means that you can’t necessarily just throw your silver jewelry in a puddle to achieve the effects.
In this research they tested a few different ideas, one was testing a new type of grain, sorghum, for the silver nanoparticles to attach to, and then looking at the effects of the silver particles with or without that new type of grain. They took larvae of the biting midges, Culicoides sonorensis, and put the solutions of the particles in the water they were living in. The particles would sometimes be eaten by the larvae as the method of delivery. But the particles are hard to track since they were so small.
The sorghum particles were not found to be harmful on their own. The silver particles disrupted other microbiota in the water. The larvae ate the combo grain/silver particles and mortality occurred after consumption. The mortality of the larvae was much higher when the silver alone reached a concentration of 100mg/liter and the silver/grain hybrid at 200mg/liter. This was all assessed after 7 days of exposure. They found that the larvae were being killed by the silver particles and not by starvation or alteration in gut microbes. This was a big concern initially, as the silver could certainly be killing off critical gut bacteria or environmental bacteria.
Some cool things are that the silver particles are pretty environmentally safe, the grain particles themselves are not toxic unless bound to the silver. The combo silver particle with grain was less toxic to the biting midges than it has been seen in mosquitos, but more toxic than it is to other non-target species. There have been some other studies looking at non-target fish and other non pest aquatic insects that were not as affected. So, this silver is more specific which is a good thing in pest control. More studies on non-target organisms would need to be done before this could be viable for use in the field.
While we may be far away from a spray or granule formulation to apply to breeding bodies of water, this shows a promising start to a new potential control method that relies on a simple element or opens the door for other insecticidal products being bound to grains as a method of delivery.
Article by Ellie Sanders, BCE-Intern
References
Cameron J Osborne, Amie E Norton, R Jeff Whitworth, Kristopher S Silver, Lee W Cohnstaedt, Tiny silver bullets: silver nanoparticles are insecticidal to Culicoides sonorensis (Diptera: Ceratopogonidae) biting midge larvae, Journal of Medical Entomology, Volume 61, Issue 6, November 2024, Pages 1427–1434, https://doi.org/10.1093/jme/tjae107
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