Yes, there are about 250 native species of dandelions in the UK. That is how the mites are transferred. They hitch a ride when a bee leaves the hive, drop off at a flower and wait for another bee to be taken to that hive. But the thing is, there are about a 1000 different kinds of mites (that we know off) and most of them are beneficial to the bees, not harmful. There are 3 big groups. The ones like the Varroa who will feed on the bees, there is a group that will neither harm nor help the bees, and is only after stealing the food the bees bring in, and there is a group who will clean the hives, eating the waste and harmful organisms. A healthy nest of bees isn't really at risk of the harmful mites. It's only when they get stressed or sick that there is the risk of the Varroa mites to wipe out the colony. So it comes down to human intervention for the most part. When a bee-keeper sees a Varroa mite, he will treat the hive, and by doing so, also kills off the mites that keep the hive healthy and disturbing the balance. As a result, the bees, who are already living in an unnatural population density, get even more weak, since there is nothing left to keep the nest clean. The Varroa mites, if they find their way into the hive again, have now an easy prey and can easily wipe out an entire hive.