Can the apparent drop in insect population be explained by local insects evolving to avoid traps?
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.