Varroa Behavior

There are several methods available to check for the current "mite load" (population) in a hive, but a simple visual inspection by looking on the backs of bees and not seeing mites can be very misleading.

Since mites do not spend much time on the backs of bees, they are rarely seen on the back side of a honey bee. If and when there are noticeable mites visible on bees, it's likely a major problem and may result in a colony loss.

While yes, mites do hitch a ride on bees, so they can occasionally be spotted on the backs of bees, but they quickly move to the underside of the bees and work their way into the slits of the abdomen called "spiracles". So unless you happen to accidentally spot one on a bee's back, or catch a bee and check the underside in the spiracles of the bee, where their tracheal (breathing tubes) are located, you are not likely to see many mites... even when there are a good deal of mites present in the nest.

Knowing how and why mites use bees as a vehicle to get around in the hive makes it understandable that they are not easily seen. When first coming in to a hive, the mites typically arrive from the outside on the older age foraging bees that may pick them up from a flower or perhaps while Robbing another hive. Or bees coming that are immigrating from a "mite bomb", a hive dyeing from varroosis - death by varroa transmitted viruses. Surviving bees will look for other colonies to join. (See link below for more on mite immigration)

Once inside the hive, they then look to transfer to the younger age "nurse bees" so they can travel to the area of the nest where the nurse bees are feeding young. The heart of the nest is the prime target for the mite's "breeding grounds" and source of food. The mites have evolved to recognize these different age bees by there scent and take advantage of any opportunity to switch from one to the other.

Studies have shown that the mite may spend as much as 20 minutes on a single bee while waiting for the opportunity to change bees or get off a bee and into the comb with a larvae. During this time, they are able to go undetected by masking their own scent to match that of the host bee they are traveling on at a given time. Similar to how the little green Anole lizard in the garden will change it's color to match it's surroundings when moving from a green to a brown surface to better camouflage into the background.