To start with some background, house flies are pests in a wide variety of settings. The context this paper is focused on is more of animal handling facilities which can face high infestation levels of house flies. They may treat for using granular products containing active ingredients such as neonicotinoids including one called imidacloprid. While many house flies will remain around the same area, they can actually travel quite far and that’s how people can wind up with these flies in residential areas from all the way over at an animal handling facilities. 

That being said, get ready to buckle in for some heavy science. I’ll try to keep it straightforward. We are looking at the stability of resistance of house flies to imidacloprid over time. Basically, asking if resistance persists in both lab strains and field collected strains of house flies.  I want to note here that behavioral resistance is a fly choosing to avoid the imidacloprid containing bait versus physiological resistance which is when a fly will consume the product, but won’t be killed by it. There were three parts to the experiment they conducted. First, reference colonies. They had colonies of flies collected in 2015 and in 2021 from a dairy farm, and five additional strains were created from the 2015 colony by selecting for behavioral resistance over the course of several generations to exhibit behavioral resistance in the form of reduced feeding on imidacloprid containing sugar sources. These five strains were further differentiated by leaving them unselected for 10 and 30 generations so that they wound up with ones who were resistant for a while and then were not exposed to anything. 

 

 

The second part was evaluating physiological susceptibility. If something is physiologically susceptible that means it has lower or no physiological resistance. They did this by placing flies in containers where they had access to water and to the imidacloprid/sugar mixture to feed on and there were controls with just the sugar. After a few days they assessed mortality to see what percent survived 

The final part was testing behavioral resistance. Which side note, has been found to be heritable through genetics. They put the flies in little cups with a choice between the imidacloprid feed and the standard feed. After three days of this, they looked at their survival. If the fly had avoided the bait they would live and if they had chosen it they would die off. 

Now to the results. There were differences in susceptibility between some of the strains. The most notable points where the wild-type 2015, the original unedited strain, which exhibited the highest level of susceptibility compared to the other types. There was not a significant difference in survival between the generations immediately post behavioral selection vs 30 generations post selections. This leads us to the point that behavioral resistance selection does not necessarily mean a difference in physiological resistance.   

The behavioral resistance did not decrease in continually exposed strains or those that had not been exposed in several generations. The 2021 collected wild-type exhibited high levels of behavioral resistance, but didn’t differ across subsequent generations, indicating that potentially behavioral resistance does not have a negative impact on general fitness so it will persist across generations.   

This is in contrast to the well-established concept that physiological resistance can lead to decrease in fitness. This leads me to what I think was the mic drop sentence of the paper which explained that once behavioral resistance to imidacloprid evolves, no rotation of modes of action would get rid of it. If you have a strain of flies that avoid the bait, they will continue to avoid it even if you take a long break in exposing them.  

If imidacloprid baits are not being used then there is likely a fixed portion of the population with behavioral resistance but not physiological resistance since that goes away after time. This makes things hard because if you use imidacloprid baits, the flies would be susceptible but won’t go near it due to behavioral resistance. They suggest a more comprehensive survey of resistance presence across a wider geographic area to determine how prevalent it is.

Article by Ellie Lane  

References

Hubbard, C.B., Gerry, A.C. and Murillo, A.C. (2023), Evaluation of the stability of physiological and behavioral resistance to imidacloprid in the house fly (Musca domestica L.) (Diptera: Muscidae). Pest Manag Sci. https://doi.org/10.1002/ps.7866 

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