The BeeHolder, January 2011

At the Bee Disease day in Gregynog I was quite horrified that some members were expressing delight at the sight of varroa under the microscopes. I hope it was delight at the journey into a microscopic world rather than admiration for the varroa mite itself.

Varroa mites are for me the epitome of revulsion. Not only do brothers incestuously mate with their sisters but oral sex reaches a new level of depravation; they ejaculate through their mouths!! And of course they kill bees. The male, understandably, looses interest in eating; in fact it can’t, and has a very short life. Since sex does not introduce any new genes, reproduction is akin to pathogenesis, virgin birth, cloning.

But somehow a genetic change must have happened for, as we all know Varroa has made the transition from the Asian bee to the European Honey Bee. The ranges of Apis cerana and Apis melifera have overlapped for many centuries and yet varroa is a comparatively recent pest to our European Honey bee. The Asian bee has learned to live benignly with both Varroa destructor and the related Varroa jacobsoni.

In the original host, Apis cerana, the mite enters a drone cell before it is capped. It will be a fertilised female, ready to lay eggs. With no eyes and only smell to go by it apparently gets a signal from the larva indicating that it is the right age to receive eggs; not too young to bear the parasites but old enough to give time for the young mites to mature before the drone emerges. Incidentally drones take that little bit longer to emerge than workers which is critical to the mite’s development. Things happen quickly: the mite lays about five eggs; the first to hatch is a male and he inseminates all the others that hatch, his sisters. The females then attach themselves to the drone and draw nourishment until the weakene

drone emerges. The four or so young mites jump out at the same time and scramble around looking for another drone cell to infect. They hitch rides on workers and often fall or are groomed off. By attaching to foragers and drones the mites spread from colony to colony since both of these occasionally 'drift’.

Varroa has been seen on a number of species: a bumble bee heavily infected is not an uncommon sight. Other insects infected are the sacab beetle and flower fly, Palpada vinetorum. But in these cases the parasite is merely sucking the haemolymph of its host; we have no evidence yet that it is actually breeding on these species. The original infestation of the European Honey bee was the same. At first varroa did not reproduce with colonies of the European Honey bee. That's because the mite failed to detect that vital signal from the bee larva that it was time to lay eggs. Varroa Destructor from the colder parts of Asia Korea, Japan and the mountains of the Philippines, first learnt to detect that vital signal probably sometime in the 1960ies. Only recently have we found that V jacobsoni has also learnt the signal If we can understand the nature of this signal we might find a way to the effective control of the mite.

Tony Shaw