The honey bee is perhaps one of nature’s neatest and cleanest organisms. These characteristics are especially important in social insects. Numbers of individuals crowded together for extended periods is risky because if one becomes infested with a disease or parasite, the chances of others likewise being affected are very great. In human history, this tendency is well exhibited in great disease epidemics, such as those associated with the “Black Death,” or bubonic plague. Sanitation in humans was found wanting in most instances where plague established a foothold, brought on by rat populations and their fleas that spread the bacterium, Yersinia pestis.
Like humans, honey bees are not necessarily equal when it comes to keeping a tidy house. We now know this tendency is ruled by genetics and thus determined by chance in bee populations. It was Dr. Walter Rothenbuhler, retired from The Ohio State University, who first used the term “hygienic behavior,” for this trait, according to Drs. M. Spivak and M. Gilliam . They have written a detailed review of the subject in Bee World, published by the International Bee Research Association. Ideas leading to the current knowledge of hygienic behavior proceeded through several stages, according to Spivak and Gilliam in “Determining Variation in AFB Resistance in Bee Populations.”:
Resistance to American Foulbrood (AFB) consists of the colony’s ability to detect and remove brood before the causative organism (Bacillus larvae, now renamed Paenibacillus larvae larvae) reaches the infectious spore state. Early removal of diseased larvae, which contain noninfectious rods only, prevents spread of the disease, but removal of spore-infected larvae also contributes to transmitting the disease.
Light cases may sometimes be overcome, but heavy cases are not. Spread of the disease is not so much determined by the number of spores, but by the extent to which general contamination is produced by removing diseased brood. Since diseased brood is removed whether or not a colony recovers from the disease, it is evident that colony resistance does not depend entirely on this behavioral characteristic.
Determining whether AFB resistance was behavioral or physiological. Dr. Rothenbuhler and V. Thompson subsequently showed that although resistance was behavioral in adult worker bees, it was also physiological or inherent in larvae.
Determining the genetics of this trait in worker honey bees was a major focus of Dr. Rothenbuhler and his students at The Ohio State University until the early 1960s. The results are summarized by Spivak and Gilliam as a two-locus process of uncapping a cell containing dead brood and removing the contents. Both characteristics are thought to be recessive and found at different locations (loci) on the chromosome. Thus, two recessive genes are needed at two separate locations on the chromosome before workers show both traits. There could be intermediate populations, therefore, that may show one trait (uncapping), but not the other (removing) with respect to affected larvae.
Dr. Rothenbuhler appears to have been remarkably prescient in coining the term “hygienic behavior.” Since his landmark experiments on AFB resistance, this trait has been determined to be responsible for a number of other phenomena observed in honey bees. According to Spivak and Gilliam, these include:
Resistance to chalkbrood. This disease, first discovered in the United States. in the 1960s, is now found throughout North America. Since the causal organism is a fungus (Ascosphaera apis) that attacks brood, removal of affected larvae by hygienic bees would seem to be a natural defense mechanism. Both M. Gilliam and S. Taber have been responsible for determining that hygienic behavior indeed confers chalkbrood resistance. The practical results from their research include testing for hygienic behavior, feeding homogenized chalkbrood mummies as a screening tool for resistance, requeening colonies found susceptible to chalkbrood, and preventing symptoms by eliminating stress and ensuring optimal nutrition.
Resistance to European foulbrood (EFB) . This concept has much less research behind it. However, the same mechanism found in resistance to AFB or chalkbrood would appear to apply for this disease as well.
Resistance to Varroa mites: Research in this area is continuing and seems to hold great promise. It has been shown that the native host of Varroa (Apis cerana) routinely removes infested brood more efficiently than many populations of Apis mellifera. Spivak and Gilliam conclude that removing infested pupae may theoretically limit the growth of mite populations by prematurely releasing young mites that can’t complete development. It may also damage the mother mite, and/or extend her time being carried by adult bees during what is called the “phoretic” stage.
Spivak and Gilliam credit Steve Taber, retired researcher at the USDA Tucson Bee Laboratory for helping to popularize the hygienic behavior concept. Among other things, he helped establish one of the current techniques for assaying hygienic behavior, which uses a small section (2 X 2.5 inches) of freeze-killed brood. Originally, full frames were used by those developing the test, and in other variations, larvae were killed by cyanide or piercing them in the cell with insect pins. Dr. Jerry Bromenshenk and colleagues at the University of Montana pioneered a further development using liquid nitrogen in a round pipe or container to freeze a part of the comb in the field.
The results and discussion above reveal that hygienic behavior is highly desirable in honey bee populations for many reasons. Unfortunately, it is employed by too few breeders, Spivak and Gilliam conclude. However, there has been some progress, including development of a commercial bee called DR (Disease Resistant) by Taber Apiaries in California, employment of the technique in Argentina to combat a serious AFB problem, and other programs used by USDA and University researchers and beekeeper cooperatives . Finally, selection for hygienic queens is often the byproduct of the one common solution beekeepers often employ for many problems–routine re-queening.
Although screening reveals the genetic tendency to exhibit colony hygienic behavior, this cannot be considered proof that populations are in fact resistant to specific maladies, Spivak and Gilliam say. It is important to go the next step and challenge colonies with the actual pathogen or organism in question.
Hygienic behavior is heritable and it can be selected for, Spivak and Gilliam conclude. Unfortunately, most colonies exhibit low levels because of its recessive nature. Thus, selection for the trait should become a routine part of any bee breeding operation. Commercially available lines of hygienic stock would help many beekeepers overcome a multitude of problems, especially those associated with mites and disease. This would reduce operating costs by minimizing use of chemicals, with their concomitant possibilities of bee, colony and product contamination. Finally, standard queen rearing and breeding techniques can be used to produce many hygienic queens from a few mothers using any race of honey bees.
Research has clearly demonstrated the benefits of hygienic bees. It provides many benefits with no demonstrable negative effects Spivak and Gilliam conclude . The good news is that it is increasingly being viewed as a component of many honey bee breeding programs. Dr. John Kefuss in France is also involved in research using this technology for varroa control, specifically called “Varroa sensitive hygiene.” He recently published updated information on developing resistant honey bees in a commercial environment.