Varroa destructor, which is the single biggest contributor to the decline in health of honey populations worldwide. Originating in Asia, the invasive Varroa mite is wreaking havoc on honey bee colonies, both by feeding on adult and immature bees and by serving as a vector for five debilitating viruses.
Dr. Samuel Ramsey presented to HCBA in 2017. He has a very informative and popular 3 minute video on the most recent science about the mite.
Your hive should be inspected for mites in the spring and the fall. Here is a video on how to inspect your hives for mites.
There are a few ways of treating for Varroa, a simple way is using formic acid pads. Here is a video on how to do it: