Inside: Hairy-footed pollen mites eat the food of larval bees and then hitch a ride on an adult bee to flowers with pollen or a new nest with pollen. The mite population expands until an infested bee can barely fly.
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A nasty case of pollen mites
I try to remain vigilant for mason bee mites and I use recommended control measures to limit their population on the few mason bees I have. But earlier this year I saw my first terrible case of hairy-footed mites on the backs of two newly emerged mason bees.
The bees in question emerged not from my mason bee houses, but from a little moisture drain on the bottom of a bedroom window. I heard the window buzzing, and I wondered what was going on. After several hours of commotion, two masons emerged into the space between the window and the screen.
The masons nested in a drain hole
The adult female must have crawled into the tiny drain hole on the outside of the window frame last year. This spring, the newborn bees emerged through the other end of the hole. I thought the whole thing was adorable until I looked closer: each of the bees was covered with what looked like sticky brown fluff—an infestation of hairy-footed mites.
The infestation was so bad that I suspect mason bees have been nesting in those drain holes for a couple of years or more. I tried to photograph them through the glass (which didn’t work so well) but you can see the coating of mites, which looks something like a brown shearling vest. One bee was rising on its hind feet trying to fly, but the weight of the mites was overwhelming.
The pollen mite is a kleptoparasite
We call the hairy-footed mite (Chaetodactylus krombeini) a pollen mite. A special nymph stage, called a hypopus, rides around on the back of an adult bee until it finds a source of pollen to eat. But frequently the adult bee unwittingly takes the mite home.
The mites reproduce within the larval chamber where they eat the pollen provisions the bee left for her own offspring. With no food stores left, the bee larvae die. Sometimes the mites eat the bee egg or larva as well. If you open a mite-infected tube, you can often find a mass of orange-colored debris filling a cell—a sure sign they were busy eating and reproducing for a long while.
Surviving bees carry the mites to new nests
In the spring, surviving bees crawl through the nesting cavity where they pick up many of the phoretic nymphs. The nymphs instinctively grip the bee and do not let go. From there, the bees unwittingly carry them to flowers or into new nesting cavities.
The mites are native to North America and can infect many of the bees in the Megachilidae family, which includes the mason bees, leafcutters, and carder bees. They prefer damp environments—one reason they are proliferating in the coastal Pacific Northwest.
Honey Bee Suite