It seems that varroa were first discovered in North America in 1987. [1] Glyphosate use at that time was around 4,500 metric tons. By 2014 we were up to 125,000 metric tons [2]. There was an exponential increase coming after 1996 when glyphosate resistant GMO crops became a thing. I don't have an opinion on this topic one way or the other, but there seem to be quite a lot of negative correlates since then, and this is just another one. Of course correlation doesn't mean causation, but you can't completely dismiss it.

[1] - https://en.wikipedia.org/wiki/Varroa

[2] - https://pmc.ncbi.nlm.nih.gov/articles/PMC5044953/