published in American Bee Journal 131(8) 1991 p508-510 |
Brother Adam Kehrle, O.S.B., O.B.E., D.Sc., St. Mary Abbey, Buckfast, Sud Devon, England |
… This is not to dismiss the word in the sense that it is “merely academic”, as if it were of no practical importance. On the contrary, students of Brother Adam's work will find no dichotomy between what is “academic” and what is of the most important “practical” considerations. For his contributions, two universities, one in Sweden and one in England, have awarded him the degree of Doctor of Science.
It is therefore a matter of concern that in relation to the susceptibility of honey bees to acarine disease, which Brother Adam established so early on in this century, his findings appear to have gone either unheeded or ignored. The results secured in Europe, based on wide experience in the field and confirmed by instrumental insemination leave no doubt or room for uncertainty. For the beekeeper interested in practical results right now, his paper is worthy of close attention.
In a recent issue of Bee World, the official organ of the International Bee Research Association, appeared a leading article by an American scientist expressing a fear that in the course of the winter of 1989-90 more than 1 million colonies perished, primarily due to the ravages of the tracheal mite (Acarapis woodi). He likewise stressed: “Losses from Varroa jacobsoni were predicted, but losses from Acarapis woodi were not widely expected.” A few weeks later a letter reached me from a commercial beekeeper from Vermont, who reported that he lost half the number of his colonies in the spring of 1990, due to the tracheal mite, notwithstanding the use of menthol. He stated: “We have a bee capable of meeting our honey production requirements, but none able to withstand the diseases to which the honey bee is subject.” In actual fact the loss of colonies recently suffered throughout North America could have been largely prevented if the necessary precautionary measures had been taken in time.
In retrospect these reports remind me of the Isle of Wight epidemic, which according to official estimates, in the matter of about 12 years, caused a loss of more than 90 % of the colonies in the British Isles. The first outbreak of this disease was recorded in 1904 in the Isle of Wight. On reaching the mainland it spread like wildfire to every part of the British Isles. It reached Devon in 1913 and our own apiaries the following year. In the winter of 1915-16 we lost close to two-thirds of our colonies. The colonies lost belonged to the indigenous dark native variety, which had existed in this part of Europe since the termination of the last Ice age. Those that survived were mainly of Ligurian origin — the Alps of northern Italy bearing that name. The bees of this region are leather colored not bright yellow or golden. We had here a classic instance of a hopelessly susceptible race and in the Ligurian one manifesting, in identical environmental conditions, a high resistance to the tracheal mite. Moreover, before the demise of the native race every kind of disease of the honey bee could be found in our apiaries. With the eradication of the native variety, all the diseases, apart from acarine, vanished simultaneously. Needless to say, these findings proved a far-reaching turning point in our beekeeping and still more so in our efforts at improving the honey bee. Before very long we also found that the bright yellow or golden strains, irrespective of their origin, including a combination we developed ourselves, have proved invariably exceptionally susceptible to the tracheal mite. Also, we found that this extreme susceptibility will be transmitted by these bright golden drones and dominate when crossed with queens of highly resistant strains. Why this should be so, we do not know.
A short while ago an even greater menace than the tracheal mite began to threaten beekeeping in North America. However, Varroa jacobsoni has not as yet had time to manifest its ruthless killing ability here in this part of the world (United Kingdom). However, according to our findings, the tracheal mite can destroy a colony in less than nine months. But it may also take up to three years, before mass crawling sets in, depending on the degree and ability of the innate resistance at issue. On the other hand, an initial infestation of the Varroa mite may seem harmless and will usually take about four years to manifest its full virulence, but will then prove fatal to every colony without fail, unless periodically safeguarded by some remedial measure or other. Moreover, these remedial measures have to be applied at the appropriate time without fail, to prove fully effective. Indeed, we have to accept the fact that the Varroa mite will spell the doom of any form of let-alone beekeeping, as widely carried out up to now in all the main honey-producing countries throughout the world.
The Varroa menace can be held in check, but only by regular application of acaricides. These, in turn, pose the unacceptable risk of traces finding their way into the honey destined for human consumption. Therefore, as Prof. Engels of Tubingen University (Germany) stressed some years ago, beekeepers are at present compelled to use these toxic substances to keep their colonies alive, but their use can in no case prove a permanent solution. Only a fully resistant genetically endowed race or strain, resistant to the Varroa mite in any given circumstances, can prove the ultimate answer to this menace. Apart from Apis cerana, which does not cross with Apis mellifera, and a few races native to parts of Africa, south of the Sahara, no known races of the honey bee are able effectively to resist Varroa jacobsoni. Minor variations in resistance have been observed amongst the European races. But such minor variations may not be genetically based, but due to accidental circumstances.
On the basis of the findings and experience gained in breeding the honey bee since 1916; also the knowledge acquired to the genetic possibilities at hand, I feel confident that in the course of time a honey bee fully and effectively resistant to the Varroa mite can be developed. It will do away with a host of uncertainties, as well as the endless extra labor and expense, including the incidental brood diseases for which this parasite is seemingly responsible. This is indeed a challenge that modern progressive beekeeping cannot circumvent. However, both the tracheal mite and Varroa jacobsoni will in all probability never be totally eliminated. However, if we can reduce their incidence to a point when their presence has no bearing on the honey-gathering potentiality of a colony, our practical objective will have been attained.
In the case of the tracheal mite the hereditary characteristics on which susceptibility or resistance is based have up to now eluded discovery. In breeding for resistance the essential selections were, therefore, at all times restricted to the absence of any visible symptoms of this disease — this is in conjunction with a total avoidance of any remedial measures.
The consequent inevitable losses were the price such an endeavor demanded. By contrast, we are confronted by a somewhat different proposition in the case of the Varroa mite. The parasite is visible to the naked eye, apart from a number of known genetic traits, which impede the full reproduction potential of this mite, restricting an infection below a level affecting the honey-gathering ability of a colony. Nevertheless, considered from the strictly practical aspect, the selection for resistance will in all likelihood have had to be based on the actual degree of an infection — in this instance on the number of dead mites following the periodic control measures.
Considered from the wider aspect, in breeding the honey bee we are confronted by a series of problems not met with in the breeding of live-stock and plants We have constantly to bear in mind the influence of parthenogenesis, multiple mating and the extreme susceptibility of the honey bee to inbreeding. Indeed, inbreeding is the primary limiting factor whenever an intensification of a resistance to any particular disease is at issue. The best of strains can be quickly ruined by close inbreeding, particularly by instrumental insemination.
As experience has revealed, a resistance to a particular disease may vary individually in degree genetically from one extreme to the other. Moreover, resistance may break down in exceptional adverse circumstances. However, this is a universal phenomenon that is not restricted to the honey bee.
It is widely assumed that by close inbreeding of pure stock a particular genetic trait can most advantageously and successfully be intensified. Where close inbreeding is judiciously applied reasonably satisfactory results can thus be secured, but only at a snail's pace. More usually, before any worthwhile intensification has been achieved, a loss of stamina will have annulled any worthwhile advantage. Pure stock, however, forms the anchorage of any success secured by way of breeding.
This form of breeding has since the beginning of time — in regard to all sections of animate creation — been Nature's way of developing more vigorous genetically endowed races and strains, to supplant those that failed to match a particular exigency, according to her maxim of “the survival of the fittest”, within the limits of the genetic potentialities at her disposal. Her endeavors were of necessity restricted to her more immediate facilities, whereas the modem bee-breeder has a worldwide choice at his command. It was likewise Nature's sole facility of combating every kind of disease. The honey bee proved in no case an exception.
However, cross-breeding as here envisioned, as the exclusive means of securing a fully efficient genetically based resistance to Varroa jacobsoni calls for a whole series of exceptional measures, without which any attempt at a task of this kind can be regarded as futile from the very outset. Indeed, I believe very few beekeeping establishments have at their disposal the essential facilities for a task of this magnitude. This is not based on an assumption, but on a lifetime of practical experience.
On the other hand, as we have found, a properly conducted scheme of crossbreeding can reveal genetic possibilities of which we had no inkling before at any time. A practical instance will make this clear. From a cross between a queen of our strain mated to French black drones we secured in the F-3 a new color break we had never seen before. Moreover, though the French black strain was extremely aggressive, we nevertheless secured a new combination which proved by far the most good-tempered bees known to us. Also, this combination proved to be practically immune to the tracheal mite, notwithstanding the fact that the French breeder was extremely susceptible to this disease. In fact, of 12 colonies headed by French sister queens, only two survived. The 10 that died perished in the middle of the summer. However, to secure these results we had to raise no less than 1,200 queen cells of which only 200 queens, on emerging, were retained. These were mated to handpicked drones. Following a full year's test in a normal honey-producing colony, two breeders were chosen of the original 1,200 virgin queens. Apart from the case cited, we also found that crossbreeding, based on Mendelian findings, presents as yet a virtually unexplored section of modern beekeeping. The reasons are obvious.
Results obtained in the laboratory, secured in the absence of the specific colony influences, are unfortunately all too often given a universally applicable significance. The actual results secured are in no case questioned; their general validity merely assumed. Obviously, the actual reactions within a normal free-flying colony must determine the issue in every case. When in doubt, A. I. Root used to say: “Let the bees tell you.” I have here in mind the massive loss of colonies, occurring at the present in North America. These could have been largely avoided if the wide long-term results, based on practical experience, had been heeded in time.
The acarine epidemic eliminated in a brief few years no less than 90% of the colonies within the British Isles. The losses suffered at present in North America confirm that the epidemic, which raged at one time in a small section of Europe, was no myth but a stark reality — one which I witnessed personally. However, there is no doubt, Nature has in the intervening years eradicated in Europe most of the highly susceptible stock. However, Acarapis woodi is still found everywhere. Varroa jacobsoni has not as yet invaded the British Isles (1991). However, where this parasite is found, beekeepers have no option, but must treat every colony without fail, regularly and at the appropriate time — irrespective of the numerous uncertainties and drawbacks.
We have also, of course, given our attention to the possibility of combating the various other diseases of the honey bee by way of breeding. The results secured have proved most valuable. There seems no doubt that a properly conducted scheme of crossbreeding represents the most successful form of “genetic engineering”.
published in American Bee Journal 131(8) 1991 p508-510 |
Brother Adam Kehrle, O.S.B., O.B.E., D.Sc., St. Mary Abbey, Buckfast, Sud Devon, England |