Varroa destructor: Revolution in the Making
A guest editorial published 2001 in Bee World 82(4): 157-159.
Keith S. Delaplane, Professor, Dept. Entomology,
University of Georgia, Athens, GA 30602 USA
Scientific revolutions don't come around often. These rare events are, in the famous words of Thomas S. Kuhn[i], "paradigm shifts" - the dethronement of a reigning paradigm and its replacement by an entirely new way of seeing things. This is what happened to the ancient and venerable Earth-centered model of the solar system which was ultimately demolished and replaced by Copernicus's heliocentric model. Einstein's relativity, while not supplanting completely Newtonian physics at the planetary scale, nevertheless blew away Newton when it came to probing the Universe's superstructure. Both of these were scientific revolutions, a reminder that the "march of science" is not the smooth, uninterrupted accumulation of knowledge that we like to imagine, but rather a fitful process with all the human drama one would expect of fringe visionaries, defensive dogmatists, and trammeled careers. If there is an illusion of uninterrupted progress, it is with the generations of scientists who labor between the revolutions, filling in the knowledge gaps in the current paradigm.
Now I would be grossly overstating things if I were to suggest that the recent shake-up in the taxonomy of Varroa mites were a scientific revolution. But for that fraction of the world's scientists who work on the parasitology of Apis mellifera, it may be as near as we get. For even though the news was non-controversial that Varroa jacobsoni is in fact a complex of at least two species (and I am unaware of any trammeled careers), the discovery of V. destructor does have some far-flung implications for apiculture and bee science in general.
The first, and least important, is the small but inescapable chagrin we scientists feel who have published for years on Varroa jacobsoni, only to find out that we had tagged the wrong beast. One can imagine the wry response of rough-handed, smoky-smelling beekeepers: "What? You didn't even have the right mite?"
But more importantly, when Anderson and Trueman[ii] unveiled V. destructor in 2000, they effectively forced beekeepers and bee scientists out of a familiar way of thinking, a paradigm if you will, that had its start nearly a century before with the scientific description of V. jacobsoni by Oudemans. Working in the Netherlands with mites sent him from the Indonesian island of Java, A.C. Oudemans described in 1904[iii] an ectoparasitic mite on Apis cerana (then called A. indica) and named it Varroa jacobsoni after its providential collector, E. Jacobson. With no economic interests at stake the original taxonomy of V. jacobsoni was untouched when over half a century later Varroa mites hopped opportunistically onto Apis mellifera and began their worldwide range expansion. With economic interests inflamed and funding marshaled, scientists turned a keen eye onto this mite from Asia and for three decades the name V. jacobsoni filled the pages of apicultural journals. It was V. jacobsoni that was named beekeeping's greatest threat, and it was V. jacobsoni that was responsible for the collapse of feral populations of Apis mellifera the world over. A battery of control measures, mostly chemical, was developed and beekeeping changed in a matter of years from a pesticide-free agricultural enterprise to one that was virtually pesticide-dependent. Recognizing this unhappy state of affairs, bee researchers and educators turned to integrated pest management (IPM) and demonstrated the promise of numerous cultural and genetic mite-control practices. But in spite of this output of ingenuity by scientists, pesticide companies, and progressive beekeepers, profits continued to contract in the beekeeping industries of many countries. The long story was told repeatedly in the beekeeping media and even captured the attention of the popular press. The problem was pretty black and white: one mite had shaken beekeeping everywhere.
But the simple idea of "one mite - one problem" was getting muddy, even before Anderson's and Trueman's pivotal 2000 paper. It had become apparent that the Varroa mite, or at least its effects, were far from uniform around the globe. Its depredations on A. mellifera were severe in western Europe and North America, for example, but less so in eastern Russia and South America. It was natural for bee scientists to first assign this variation to differences in resistance among bees, but it became obvious that genetic differences in the mites themselves were at least as important.
During the 1990s many workers contributed to the "revolution" that was heating up, but it was Anderson and Trueman who dealt the old paradigm its death blow. The mite we were all dealing with, we learned, was not Varroa jacobsoni at all. This much-maligned species turns out to be a benign homebody, still restricted essentially to its original host A. cerana in Indonesia as Jacobson found it, and not the culprit to worldwide calamity we had thought. For this we needed a new culprit, and Anderson and Trueman found it. They not only found it and named it, they named it in a glorious flourish of melodrama, appropriate in this case and all too rare in the stuffy halls of academe - Varroa destructor. Let your tongue roll on that one. Here's a Linnaean name anyone can appreciate.
Yes, V. destructor was there all along, a so-called cryptic species, and although both are parasites originally of A. cerana and their ranges overlap, V. destructor predominates on mainland Asia whereas V. jacobsoni predominates on the Indonesian archipelago. It was V. destructor that spread all over the world, with two genotypes on Apis mellifera identifiable today - the Korea genotype known to be in Europe, the Middle East, Africa, Asia and the Americas, and the Japan/Thailand genotype documented from Japan, Thailand, and the Americas. The Korea genotype is not only the most widespread, but the most virulent.
From a beekeeping perspective all this would be so much rigmarole were it not for one more realization: of the 20 known genotypes of Varroa, only three are known to infest colonies of Apis mellifera. One of these, the Javanese strain of V. jacobsoni described by Oudemans, can enter brood cells of A. mellifera but cannot reproduce there. Only the Korea and Japan/Thailand genotypes of V. destructor can reproduce in colonies of Apis mellifera. Put another way, Apis mellifera is 100% resistant to 90% of the known Varroa genotypes. Identifying and manipulating the factors that confer this resistance would enable apiculturists to provide resistance to the remaining 10%. These resistance factors are at least partially under genetic control and respond to selection. Most recently, John Harbo and Jeff Harris[iv] have demonstrated that Varroa nonreproduction is a heritable trait in North American A. mellifera. Queens selected for this trait are now available commercially from a number of bee breeders in the United States.
It is here that the recent work on Varroa taxonomy begins to take on significance of revolutionary proportions. A Varroa mite that cannot reproduce is essentially rendered a transient colony invader. More poignantly, a population of Varroa mites that cannot reproduce becomes extinct.
The biological world is complex and full of unexpected turns, and utopian editorials have a way of coming back to embarrass their authors. But I think I'll risk it on this one. No other Varroa control measure holds such promise for rendering this mite innocuous, and this fact won't be lost on either the scientific nor beekeeping communities. I suppose it is not revolutionary, chronologically speaking, to turn the clock back - back to the management practices, relative ease, and beekeeping pleasures of the pre-Varroa years, but if we hesitate to call it counter-revolutionary we could at least call it progress, manifest and indisputable.
[i] The Structure of Scientific Revolutions, 3d ed., Univ. Chicago Press
[ii] Experimental & Applied Acarology 24: 165-189
[iii] Entomologische Berichten 18: 156-164; 1904. Notes Leyden Museum 24: 216-222
[iv] Journal of Economic Entomology 92: 261-265