I try to remain vigilant for mason bee mites and I use recommended control measures for limiting their impact on the few mason bees I have. But earlier this year I saw my first really bad 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 was perplexed as to what was going on. After several hours of commotion, two masons emerged into the space between the window and the screen.
The adult female must have crawled into the little drain hole on the outside of the house and, the following spring, the new-born bees emerged through the window frame. I thought the whole thing was adorable until I took a closer look: each of the bees was covered with what looked like sticky brown fluff—in other words, an infestation of hairy-footed mites.
The infestation was so bad that I suspect mason bees have been nesting in those drain holes for more than one season. 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 of the bees was rising up on its hind feet trying to fly, but the weight of the mites was overwhelming.
The hairy-footed mite (Chaetodactylus) is also known as the pollen mite. A special nymph stage, called a hypopus, rides around on the back of an adult bees 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, 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 mite debris filling a cell—a sure sign they were busy eating and reproducing for a long while.
In the spring, surviving bees crawl through the nesting cavity where they pick up a great many of the phoretic nymphs that are designed to grip onto the bee and not let go. From there, they are carried 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.