This report, just out, tells us what we basically knew, but maps it out with clear figures. They monitored 160 hives in 3 different states. Applied no treatment and tracked the colony progress over seven months. 

I realize that some people will dismiss the work outright, because it was performed by Monsanto personnel, but the numbers are pretty clear and they are not advocating any particular course of action other than “control mites or lose hives.” (Truth is, they hope to bring an RNA based virus or varroa control to market, but as yet that course of action simply isn’t feasible)

Three commercial beekeepers participated in the trial. At each site hives were re-queened with
queens of the same age and genetic background, and equalized to have 7 frames covered with
bees and 3–4 frames covered with capped brood. Queens were purchased from Kona Queen
Hawaii, Inc

With no available treatment against Varroa, its levels can exceed 8 mites per 100 bees, causing
hives to collapse. At lower mite infestation rates, replication of bee viruses takes an active role
in the collapse. According to our model, approximately 70% of hive collapse is caused by
Varroa and bee viruses. Control of mite and viral levels may mitigate colony loss, resulting in
levels more acceptable to the apiary industry.

Kielmanowicz, Merav Gleit, et al. "Prospective Large-Scale Field Study Generates Predictive Model Identifying Major Contributors to Colony Losses." (2015): e1004816.

http://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1004816

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