Thunder Bay Beekeepers' Association

American foulbrood is a highly infectious disease of honeybees caused by the spore-forming bacterium Paenebacillus larvae subsp. larvae. The spore is a resting stage of the bacterium, and can be present in old beekeeping equipment and remain infectious for 70 years or more if the equipment were from a colony that had succumbed to the disease. In the early 1900's it was rampant in North America, and was a serious threat to the beekeeping industry. A colony infected was usually doomed. (It was because of this disease that apiary inspection programs were started by many agriculture departments, and colonies found to be infected were killed and burned to prevent the spread of the disease by either robbing bees or equipment transfer by beekeepers unaware of the problem. The burning of diseased colonies continues to be the practise in many places.)

However, occasionally a colony was found to survive the disease, and the question arose whether there might be resistance to it. In 1935, twenty-five of these supposedly resistant colonies were gathered together in Iowa by researchers O. W. Park, Floyd Paddock and Frank Pellet. These colonies, as well as six control colonies, were tested by inserting a piece of comb containing 75 larvae dead of American foulbrood into the brood nest of each colony. The test comb replaced a like-sized piece of comb that was cut from a brood comb in each colony. By the end of the summer, seven of the supposedly resistant colonies were free of all symptoms of the disease. The six control colonies were all heavily diseased. In other words, some of these colonies did in fact demonstrate resistance to American foulbrood.

The next question was could this resistance be passed on to the next generation? To insure that resistant queens would mate with resistant drones, Park and Pellet raised queens and drones from these resistant colonies in an isolated citrus grove in Texas. There they mated, as bees do, in the air, away from their own hives, each queen mating with many drones. Twenty-seven colonies from that second generation stock were sent to Iowa for testing in the summer of l936. Of those, nine were disease free at the end of the season. So the resistance could be inherited.

In 1946, W. C. Rothenbuhler became a student of O. W. Park, and continued the study of resistance to American foulbrood. By means of single drone matings using artificial insemination of queens, he was able to study the mechanism of inheritance of the resistance exhibited. He found behavioural differences between the resistant colonies and control colonies. Those colonies resistant to the disease would uncap the dead brood, and then remove the dead individual from the cell very quickly. This was termed hygienic behaviour. The susceptible colonies largely ignored the dead brood and were called non-hygienic. Hygienic behaviour assists a colony in overcoming disease by removing the source of the infection.

Rothenbuhler and his students found that this behaviour seemed to be inherited in a simple Mendelian fashion-- one pair of recessive genes for uncapping behaviour, and another pair of recessive genes on a different pair of chromosome for the removal of dead brood behaviour. Both pairs of genes had to be present in a homogygous condition for a colony to exhibit hygienic behaviour and be resistant to American foulbrood. Hygienic behaviour was one of the mechanisms of resistance to American foulbrood, and although subsequent research has found that there may be other genes that modify the expression of those recessive genes, the fact remain that hygienic behaviour is heritable, and selecting for that behaviour can result in resistance to not only American foulbrood, but also other brood diseases such as chalkbrood, sacbood and European foulbrood. Recently it has been found that hygienic colonies can also detect varroa mites on immature bees in capped cells and do remove some of them.

It is important to test for hygienic behaviour if a beekeeper wants disease-free bees without resorting to the use of various preventive drugs. There are several ways to test for this behaviour, the first being to actually administer the bacteria responsible for the disease, as was done in the early tests. This could result in initiating problems where none existed before by spreading the bacteria to susceptible colonies that would then develop the disease. The Rothenbuhler era used a small block of freeze killed capped cells of brood inserted into brood comb in place of a like-sized piece that had been cut out. Hygienic bees were found to uncap and remove it in the same fashion as they removed the actual diseased brood, and with no chance of spreading disease. Lately, liquid nitrogen has been used to freeze circles of brood in combs, and that is fine except it is not something an average beekeeper is equipped to do. It is also costly and somewhat dangerous to the tester. He can freeze his fingers or toes badly! However, there is a simplified bioassay suggested by Newton and Ostasiewski in an April 1986 article from the American Bee Journal that is simple and easy for a beekeeper to use. It is known as the pin-killing method.

The pin-killing test is done as follows. Twenty-one sealed cells containing pupae or pre-pupae are killed by piercing them through the cell cap with a pin. If a colony is hygienic it will completely remove the killed brood in 24 hours. Non-hygienic or susceptible colonies will require three or more days to remove the dead brood, if they remove it at all. The speed of removal correlates well with the speed of removal of freeze-killed brood by hygienic or non-hygienic colonies (r=.956). Usually three groups of seven capped cells are selected and killed by the method described above, the site marked by leaving a pin inserted in a cell just to the left of the group of seven. Co-ordinates to the site of the pin are marked on the top and side of the frame, and numbered one, two and three. Twenty-four hours after killing the brood, each group of seven is checked. Note how many cells are uncapped, partially uncapped, and how many cells have the dead individual removed completely, or partially removed. All twenty-one need to be completely uncapped and completely removed in twenty-four hours for a colony to be considered hygienic. (Sometimes the pin doesn't kill the individual in the pierced cell, so if just one completely capped cell remains, uncap it to see if it is really dead). When a beekeeper first starts doing the test in unselected bees, he probably won't find many colonies that are completely hygienic, but he can pick the one that shows the best result if he wants to raise a few queens of his own. Selecting the best cleaners for a few seasons and using them as the breeders will certainly increase the percentage of colonies that display hygienic behaviour, and will result in increased resistance to American foulbrood in those colonies.

By Jeanette Momot 3/28/2010

To learn more, order a copy of Honeybee diseases and pests from CAPA publications.

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