[henearkr]

No. There is no suspected cause for any such "anomalously high insect population in the past", while we have a lot of suspected causes, verified by the way (so rather culprits than suspects) for the insects decline. The "anomalously high" "is possible" but is actually pure baseless speculation. Also, there is no such observation in the available litterature, scientific or layman.

You know, it is just a matter of knitting all of our knowledge bits together. Lots of nasty chemicals in the environments, especially design to harm insects. Add to that anormal temperatures and rain levels.