Keeping your colonies alive is not an easy prospect here in the suburbs of Maryland. We have less wildflowers than we used to, the region has a limited nectar flow, and there is an abundance of beekeepers. The more beekeepers the better, right? Well, not exactly. Even if you treat your bees for mites, fellow beekeepers within a 3-5 mile radius may not, and when their colony gets overwhelmed with mites, the bees become weak.
Weak hives can't prevent honey robbing, and strong colonies within that radius will rob a weak colony of all their stores within hours. If it's your colony that robbed that weak one, you inherited their mites along with that honey. Even if you treated earlier, you now need to treat again. Bad thing is, you may not even know that your bees were robbing a mite-infested hive.
This is why routine mite treatments, several times a year, is critical. If you have more than a dozen hives, it can get downright time-consuming and labor intensive. It's important to keep a record of each time your hives receive mite treatments. Recently, I've tried to flag my treated hives with these small survey flags. The flags help me to keep aware of which hives have received treatments until I find time to log in the information.
The Honey Bee Health Coalition has some great information about various Varroa Mite treatments.