This paper looks at the effects of mosquito repelling Thermacell devices that use pyrethroids. Pyrethroids are a class of insecticides that are derived from pyrethrum which is from chrysanthemum flowers. There are a wide variety of pyrethroid products available and in particular this study looks at prallethrin which works in the mosquito repellant devices by turning into a gas that can spread through the air and affect flying insects. Now this aims to work on mosquitos but since it is in the air, any other insects, including beneficial pollinators, could fly by and potentially be affected.   

Honey bees are a common pollinator and work well as a study organism. In case you weren’t convinced that bees are really cool, one of my favorite bug facts is that bees are very trainable. They won’t sit or roll over but you can get them to feed at particular places. This research took advantage of that fact and based on other studies’ methods of training, got the experimental groups to learn where a particular feeder was and prompted them to forage there. Foraging behavior is a good metric for determining if bees are affected by the product because foraging relies on flight and dance communication of individuals and since pyrethroids affect the nervous system of an insect, these behaviors would not perform well if affected.   

The experiment began with setting up a foraging area and training the bees to forage there by placing the feeders close to the hive and stepwise moving them away to about 100 meters final distance. The researchers marked bees as they foraged at the feeder with color/number coded plastic discs so they could monitor visits from each bee.

 

They split the feeder into two locations equidistant to the hive and one had the repellant device placed about 2 meters away. This is within the effective range of the device. The observational times and active device time was between 4pm and 8pm which is typical of when users may deploy the device in residential areas like their patio in the evening. The observations were recorded by camera and the marked bees were able to be tracked from the feeders and to the hive. There was a concurrent deposition study performed to make sure the repellant devices were working properly which they were, exactly as intended.  

They found no significant difference between the control and treatment feeders and the various metrics of foraging to them. The different metrics they looked at between the footage from the hive and the feeders were foraging frequency, waggle dance propensity, waggle dance frequency, and feeder persistency. The waggle dance, if you are unfamiliar, is the series of movements honey bees perform at the hive to communicate the location of and other information about a feeding source. Other bees will go to the described location to forage. Because this was no different between the control and treatment, the conclusion is that the pyrethroid based aerial mosquito repellant device has no observable effect on honey bee foraging behavior. This means that it is potentially safe for bees and the general public can use it without worry of affecting their local pollinators.

Now, I do want to clarify that this may not extend to every product within the pyrethroid class, but in lab tests, all were more toxic to mosquitos than to honey bees. So overall, this device may be a good option for residential areas for those especially concerned about their bees.

Article by: Ellie Lane

References

Margaret J Couvillon, Bradley D Ohlinger, Connor Bizon, Lindsay E Johnson, Laura C McHenry, Benjamin E McMillan, Roger Schürch, A volatilized pyrethroid insecticide from a mosquito repelling device does not impact honey bee foraging and recruitment, Journal of Insect Science, Volume 23, Issue 6, November 2023, 11, https://doi.org/10.1093/jisesa/iead079

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