I always just chalked it up to the "there were more thunderstorms when I was a kid" phenomenon (that a large number of people will remember there being more thunderstorms when they were children, despite studies suggesting the number of thunderstorms has remained constant).

It's not really surprising to me though, at least in my neighborhood in Toronto there has been massive urbanization; there are no more wild fields. Parks and even hydro fields (land under power lines) are being mowed bi-weekly, destroying any chance of native plant biodiversity. I haven't seen a mosquito in literally years. But to be fair once you leave the city, not much has changed; insects are still plentiful.

I'd be wary to draw any conclusions from this study, since the areas the researchers set up their monitoring stations may have experienced a similar level of urbanization.