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In this widely reported Plos One article, it is stated that, after roughly 3 decades of placing Malaise traps in a set of predetermined locations (counting and replacing them regularly), a sharp decline (roughly 75%) of insect population is observed. The idea is that the amount of insects caught in the traps is a proportionate representation of the insect population.

This seems to be a very important publication. It is getting reported in many popular articles as the most compelling evidence for a massive recent decline in insect biomass.

Maybe I didn't read the article thoroughly enough, but I saw no mention of the possibility that the local population (around where the traps are set) are adapting to avoid the traps. Since getting caught in a Malaise trap is lethal, genes that promote the behavior of avoiding those traps would thrive, in a local population of flies, at a higher proportion than their competitors. And since flies reproduce very quickly, 27 years seems (intuitively, to me) like a lot of time for this new local selection pressure, created by the experimenters, to make notable changes in a local fly population--particularly, their tendency to avoid Malaise traps.

Could the result of the linked article be explained by local flies adapting to the new selection pressure of Malaise traps?

When I first read about the methods with which the insect population drop was measured, this was the first possibility that came to my mind. But I can't find anywhere where this possibility is discussed. Could that be because it's so ridiculous that it's not even worth considering?

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  • $\begingroup$ Welcome to Biology.SE! I would guess that the selective pressure exerted by these traps is too small to have a significant effect — do you know how many insects are being killed relative to the entire population? In addition, do you have any reason to think that flying insects have an isolated local population? ——— You may also wish to take the tour and then go through the help pages starting with How to Ask questions effectively on this site. Thanks! 😊 $\endgroup$
    – tyersome
    Commented Feb 24, 2020 at 20:09
  • $\begingroup$ @tyersome Yes, thanks. Intuitively I feel like the effect should be so small that it's likely negligible. However, if I understand correctly, gene with even a 0.001% replication advantage, given enough generations, can eventually dominate a population. And 27 years is a tremendous number of generations for flies. I want to know if the dynamics were considered for this particular case. $\endgroup$
    – Bridgeburners
    Commented Feb 24, 2020 at 20:58

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From your own PlosOne link comes proof that the selection pressure was very weak: "Most locations (59%, n = 37) were sampled in only one year, 20 locations in two years, five locations in three years, and one in four years, yielding in total 96 unique location-year combinations of measurements of seasonal total flying insect biomass."

"We collected in total 1503 trap samples, with an average of 16 (4–35) successive catches per location-year combination"

"Between 1989 and 2016, a total of 53.54kg of invertebrates have been collected and stored, over a total trap exposure period of 16908 days, within an average of 176 exposure days per location-year combination."

Wikipedia says about houseflies: "12 generations may occur per year" https://en.wikipedia.org/wiki/Housefly#Life_cycle 27 years gives 444 generations. That is not a lot of time for evolution, especially with such a weak selection pressure.

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There are many other lines of evidence over many decades, using many techniques, all showing declines in insect populations.

Coming up with excuses for the one study misses the forest for the trees.

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    $\begingroup$ Thanks, but this does not answer the question. My question does not address whether insects are really declining or not, only whether this particular method of measuring change in insect population is susceptible to adaptation as a confounder. (The bold text is my question, but I can see how my title can be interpreted more broadly. Perhaps I should edit that.) $\endgroup$
    – Bridgeburners
    Commented Feb 24, 2020 at 19:33

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