“Pollen mites”, also known as the “hairy-toed mite” (Chaetodactylus krombeini) are pollen scavenging mites that are widespread and commonly occur in nests of the Blue Orchard Bee (a.k.a. “mason bees”) and other solitary bees. They normally feed on leftover pollen in the nest but sometimes they bite the young larval bee in probing for their food and this kills the bee. In large numbers they can result in the death of many larval bees, which reduces the pollinator population the following spring.
The mites enter the nest by hitchhiking on a female bee. They hop off and begin to feed and lay eggs. The eggs hatch into a new generation that feeds and continues to reproduce. In a few weeks there can be thousands of mites. The mites develop into two final stages as the season progresses, either a new hitchhiking stage that will climb on the emerging bees the next spring or a dormant stage that looks like a tiny pearl. The pink hitchhikers are the mites most people experience on their bees. If they are numerous they can prevent the adult bee from flying, another way they can kill bees.
The mites can be easily controlled by taking advantage of the fact that the first new generation, when very young, can not survive dry conditions. They thrive in moist, cool conditions, especially in the Pacific Northwest. In the Rocky Mountains, where it is drier, they are much less common for this reason. Therefore, to control them, you have only to put the bee nests under dry, warm conditions when the mites are susceptible. The developing bees actually prefer these conditions, so you can also help the bees at the same time.
Take down the nest boxes by the end of May (normal year) and bring them inside to a warm, dry place. An attic that gets warm during the day may work. A more serious approach is to make a warm room with a heater and a dehumidifier, so the temperature is about 85 Deg. F during the day (room or outdoor temperature at night), and humidity in the 25-30% range. Even just keeping the bees in your house is better than leaving them outdoors.
You must take the boxes in EARLY! This is to make sure the mites have not developed to their second, dry-resistant stage. The bees may still be flying and apparently building nests but they will be laying few eggs and mostly males, which are dispensable; controlling the mites is more important at this stage. You will know the actual productivity of the bees and how this relates to mite control, by plotting the cumulative number of complete nest caps from several nest boxes over the spring season. When the curve begins to level off (but before it is flat), it is time to take the boxes down.
Keep the bees in the warm/dry cycle until at least mid-August. By then, they will have matured into next spring’s adult bees and most of the mites will have been killed. Open a few nests and cut open some cocoons to verify the adult stage of the bees. Look for live mites; you should see few or none. (The straw insert system or corn eco stacked trays enables you to do this.) If you find immature bees, keep the boxes in warm/dry conditions for another week or two.
When all the bees are adults, remove the inserts (or cocoons) from the nest boxes, keep them for a few days at a cool temperature, then store them in the refrigerator until the following spring. Keep them in a cardboard, wooden, or metal box (not plastic) to assure they do not get moldy.