First, honeybees (apis mellifera) are non-native to North America.

As beekeeping history buff (my appeal to auth), the decline in my (unpopular) opinion is little to do with pesticides. It has to do with change on how bees are over-wintered and propagated, and with insufficient knowledge of beekeeping.

Current practice is to "split" hives after last harvest, and then over-winter them, cutting their strength in half. Prior to the 1960s, the practice was to "combine" the hives after harvest, and then over-winter them, doubling their strength.

This is significant, as splitting hives reduces the hive chances to survive significantly. Less workers, less collection for the remainder of the season, less housekeeping, less chances to resist pests & ilnesses, less heat generation during winter.

The reason for splitting is the promise of nearly doubling the hives in just one year.

Unfortunately the beekeeping expertise curve is very left-skew of the median. Most have less than 3 hives, less than 3 years of expertise. This additionally reduces the chances of the split, and now weakened hives' survival. Of course, when spring comes and they hives are found dead, no one will admit to killing them. It must have been the pesticides.

Anecdotally, from beek meetings some keepers are going "all natural" with no assistance to the hive from small hive beetle, wax moth, varroa & trachea mites, etc. Almost all those hives are now dead. But, not only dead also potentially re- and infecting other beeks' hives despite their best efforts. This is identical to the same thinking as anti-vaxers.

Sure; I know first hand pesticides can kill entire hives. But, in my opinion it is not the primary reason.