Pseudoscorpions have been in the fossil record for some 380 million years. There are currently 3,300 species of them worldwide, with territory ranging from the arctic to the equator, from sea level to above the tree line in the Rockies, though most of them prefer the tropics and subtropics. It is the Book Scorpion (Chelifer cancroides) who is of most interest to bee keepers. They are most frequently found in old books, eating the book lice that attack ancient binding glues. If you find them in your books, rejoice. They are rescuing your library! They also eat carpet beetle larvae and clothes moth larvae, small flies and (best of all) mites; all in all a good neighbor to have.
Book Scorpions are tiny - no more than 1/4 inch - brown, teardrop shaped (like a tick), with 8 legs and two pedipalps (pincers) used to grab food. The pedipalps are more than twice as long as the legs and look quite fearsome in photographs! The female produces 20 to 40 eggs that she carries beneath her abdomen. After the young house pseudoscorpions, which look like small adults, emerge, they stay with the female for several days, sometimes riding on her back. The entire brood then disperses. This process, from egg deposit to brood dispersal, can take 3 weeks.
The young house pseudoscorpions molt three times before adulthood; these stages are protonymph, deutonymph, and tritonymph. The developmental period is temperature dependent and takes 10 to 24 months. Adults do not molt and can live for 3 or 4 years.
There is a long historical record of Scorpions living symbiotically with bees. They were once simply a central part of life in a hive. Early bee keepers didn't think anything about their presence in the hive so there was no hue and cry raised when we changed to modern beekeeping practices and the scorpions moved on. It was just another "oh well...." These days it is felt that scorpions are no longer living in hives for two reasons. The first is that the insecticides used on mites will also destroy scorpions. The second is trickier; the modern equipment we use isn't a suitable environment for them. Book Scorpions live in cracks and crevices in certain wood where there is a rich environment of microfauna to feed their young until they are big enough to eat mites. Bee skeps were an excellent environment for them and they flourished. Today, with our modern, smooth wooden or even plastic bee keeping equipment there is simply no place for a crevice dwelling critter to hide.
In answer to your question about how they eat, yes, they do pull the mites off the bodies of bees. They also trap them when they are breeding and laying. An adult scorpion will eat up to 10 varroa a day. (I should mention they also have a strong preference for wax moth larvae.)
So why not rush out and scoop up a handful of book scorpions and throw them in the hives? Well, first, despite their world-wide territory, they are very hard to find. Second, it likely won't work, and third, even if it does work, it will take a very long time. Even if your hive happens to be made from one of the few preferred woods the scorpions like with enough deep cracks for them to live in, and you just magically happen to have a luscious carpet of microfauna to feed the baby scorpions, it will still be slow going given that it takes up to 2 years for a baby scorpion to mature, and then many, many generations to develop a colony of sufficient size to keep mites in check.
But if you can do it, it will be awesome.
Most of the modern research on pseudoscorpions and bees is being done by Torven Schiffen, a German biologist and beekeeper. Google him and learn much more. His YouTube video of a Chelifer cancroides in action against varroa will make you very, very happy.
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