Posted 01 Dec 10
Is the Susceptibility to Chalkbrood Heritable?
Professor Dr Kaspar Bienefeld, Länderinstitut für Bienenkunde, Hohen Neuendorf.
E -Mail – Kaspar.Bienefeld@rz.hu-berlin.de
Translated from Schweizerische Bienen Zeitung, 07/2010 pp 22 – 24.
by A.E. McArthur MIL
On the basis of the information gained within the scope of the Breeding Value Assessment program carried out in Germany, it can be demonstrated that genetics play an important role in chalkbrood susceptibility. The susceptibility is probably a function of the hygienic behaviour of the colony relative to the brood.
Chalkbrood is a disease of the honey bee, which is caused by the fungus Ascosphaera apis. Various causes for the disease have been discussed; low temperatures in the brood nest, damp apiary locations, inbreeding and the different resistance of the bees themselves. Without exception in every reference book carries the recommendation that the queen be replaced. However up until the present time there has been little concrete evidence to support an inherited factor relative to chalkbrood susceptibility. This conclusion is actually based on two observations:
Both of these observations are able to be explained by non-genetic influences:
Monitoring the Incidence of Chalkbrood during Performance Testing by Beekeepers
For many years now the particulars relative to the appearance of diseases have been within the competence of the Data Collection Unit relative to the Breeding Value Assessment. (www.beebreed.eu). Since the start of the records, chalkbrood has taken a dominant position in them (Fig 2). Last year (2009) very many incidences of Nosemose were reported. We will watch the development of Nosematose closely, however I would like to concentrated on chalkbrood in this article. A total of 32 072 items of information collected in the years 2003 – 2009 were evaluated. Chalkbrood was confirmed in 607 of those colonies (1.9%). There are clear differences between the various Federal States (Fig 1). However there is no evidence of a North – South or an East – West trend. The differences can be explained by virtue of different environments, different qualities of recording by breeders, but also due to the presence of different susceptible strains of bees. However in order to answer the question relative to the genetic background for resistance to chalkbrood, the incidence of chalkbrood in related colonies, which are kept in very different environments must to be evaluated. This information is available in the Breeding Value Assessment Databank of the Institute for Bee Research in Hohen Neuendorf (www.beebreed.eu). Colonies whose queens were mated in the same mating station, have common ancestry, are distributed among the apiaries of many beekeepers and the size of such paternal half sister groups is markedly larger than those of full sister groups. This is the ideal situation to assess the significance of genetics relative to chalkbrood resistance. Only brother/sister groups which embraced at least 50 colonies and also which were tested in at least 2 associations were considered in the evaluation illustrated in fig 3. There is a highly significant difference between the 73 paternal half sister groups considered relative to the incidence of chalkbrood. In siblings investigated over many apiaries, in general 40% of these colonies indicated no trace of chalkbrood, while two brother/sister groups 8.5% and 9.2% respectively of these colonies indicated chalkbrood symptoms. These two paternal half sister groups were derived from colonies which had, with 316 and 1179 offspring colonies, a strong impact on the whole Carnica population. The colonies were tested in 6 and 8 different associations respectively. With such a large sample size off colonies there is a negligible probability, that by chance all the colonies of the sibling groups tested were located in environments unsuitable for honey bees. The results shown in fig 4 are clear evidence that genetics play a very important role in the susceptibility of honey bees to chalkbrood.
The Relationship between Susceptibility to Chalkbrood and other Characteristics
Colonies affected by chalkbrood clearly produce less honey (statistically significant) and demonstrate significantly lower hygienic behaviour relative to killed (using the needle test) brood (Table 1). This is possibly a consequence of the disease. This means that chalkbrood damages the colonies such that as a result of the disease they exhibit a lower level of hygienic behaviour consequently suffering more from chalkbrood and thus produce less honey. It could also be that the genetics for weak hygienic behaviour is the factor which makes the incidence of chalkbrood possible in the first place. The latter is indicated by the significantly higher Breeding Values relative to resistance to Varroa in colonies which are free of chalkbrood (Table 2). The Varroa Breeding Value also embraces hygienic behaviour. Quite generally the Breeding Values are independent of environmental influences and also take into account the test results of related colonies. From this it can be deduced that the promising possibilities noted in the selection aims for the fight against Varroa could also be of decisive importance for resistance to chalkbrood. Other “in House” research (with American Foul brood) indicates that the bees probably do not recognise the individual pathogens, but that the hygienic behaviour is stimulated by the scale of the injury to the brood, irrespective of what is causing the injury. Hygienic colonies relative to chalkbrood recognise the damage to the brood at an early stage, such that the chalk-like mummies are less likely to occur. Due to this factor the disease vector cannot spread.
Inbreeding and Chalkbrood
Contrary to the widely held view, no connection could be found between inbreeding and the incidence of chalkbrood. In actual fact reading from fig 4, the opposite of this trend appears to be the case. When inbreeding of the queen and workers increases, susceptibility to chalkbrood diminishes. This result is surprising, because inbred colonies tend to be weaker, which has a limiting effect on temperature control and due to this should promote the spread of chalkbrood. On the other hand queens, which have higher Inbreeding Values often arise from especially involved breeders, who participate in a Varroa tolerance- breeding program. It is often the case among these breeders that deliberate related mating is carried out, which leads to inbreeding. Skilled execution of this procedure can accelerate the establishment of particular characteristics. The breeding progress, also in the case of chalkbrood relative to hygienic behaviour, achieved at the cost of increased inbreeding appears to be helpful.
The anticipated negative influences of inbreeding in the case of chalkbrood therefore did not materialise. It should however be considered that with an average of3% the level of inbreeding in the German Carniolan population is relatively small.
Hereditability computations and Breeding Value Assessment factors for chalkbrood resistance will be developed within the scope of a research project sponsored by the Federal German Ministry for Food, Agriculture and Consumer Protection in the next two years.
Heritability computations and Breeding Value Assessment for the so-called - “Yes/No” characteristics ( e.g. unwell – well) are much more complicated, than for continuously variable characteristics, like for example, honey yield. There are already similar models available; however these must be modified to accommodate the peculiarities of the honey bee. An exact recording of the incidence of chalkbrood during the Performance Tests by the breeders is an essential component in achieving this. Successful Breeding Value Evaluation relative to chalkbrood will not only help to achieve a higher genetic gain for this brood disease, but the struggle against other brood diseases will also benefit.
This Evaluation and Breeding Value Assessment was supported by the Federal States; Brandenburg, Sachsen-Anhalt, Sachsen, Thüringen and Berlin. The German National Beekeepers Association also supported the Breeding Value Assessment financially. The development of the Breeding Value Assessment for chalkbrood is being financed by the Federal German Ministry for Food, Agriculture and Consumer Protection. (Project Support Number 2808UM003)
The average incidence of Chalkbrood observed during Performance Testing by Breeders in different Federal States. The percentage values listed in the red boxes are calculated from less than 1000 submissions. As a result of the high volume of information input from Lower Saxony it was possible to differentiate between The Hannover and Weser –Ems associations.
The development of the reports of different diseases in colonies during the Performance Tests by Bee Breeders conducted between 2003 and 2009.
Characteristic Chalkbrood - Symptoms?
Honey (Kg) 36.2 +/- 0.69 48.1 +/- 0.16
Hygenic Behaviour 52.6 +/- 0.91 62.9 +/- 0.28
Average honey yield and hygienic behaviour relative to killed brood (needle test) in colonies which indicated chalkbrood symptoms (yes) or (no) chalkbrood symptoms. In both characteristics; honey yield and hygienic behaviour, the chalkbrood- free colonies produced significantly better results.
Breeding Value for Chalkbrood - Symptoms?
Honey 98.1 +/- 0.40 100.6 +/- 0.06
Varroa 95.1 +/- 0.45 101.1 +/- 0.08
Average Breeding Value relative to honey yield and Varroa.
The Varroa Breeding Value was calculated from the partial results from hygienic behaviour and the Varroa infestation development. With both characteristics: The inheritance factor (breeding values) for honey yield and hygienic behaviour were significantly higher in the chalkbrood-free colonies.
Amount of chalkbrood infected progeny %
Amount of parental half sibling groups with different amounts of chalkbrood infected colonies. In the calculation only sibling groups, which were greater than 50 colonies strong and had been tested in at least two associations were considered.
Inbreeding and Incidence of Chalkbrood.
The Inbreeding Rate (QK 0-5%, K 5-15%, K > 15%) of the queens is represented by different colours and the Inbreeding Rate (WA 0-5%, A 5-15%, A > 15%) of the worker bees in the corresponding colonies is represented on the X- axis.