Our first blog post as LASI Queen Bees documents a paper showing that hygienic behaviour reduces the growth of varroa populations and greatly reduces levels of deformed wing virus.
42 colonies in normal hives were studied in Sussex, UK. All were treated with oxalic acid in early January 2013 to reduce varroa populations. After treatment, the number of varroa per colony was determined by extracting mites from a sample of worker bees. Varroa levels were determined in the same way in December 2013. Colonies were broodless when worker samples were taken so that all varroa were phoretic on the worker bees.
Hygienic behaviour was determined in summer 2013 by making 4 freeze-killed brood (FKB) removal tests per colony. FKB-removal ranged from 27.5% (non-hygienic) to 100% (fully-hygienic).
Varroa populations increased 40.2 times (range 21 to 66), on average, from January to December 2013. The increase in the 8 colonies that were fully hygienic (> 95% FKB removal) was 19.4 times (range 7.4 to 28.2) versus 45.1 (range 21.1 to 65.4) in the 34 colonies with FKB removal below 95%. That is, varroa increase was 57% less in the fully hygienic colonies. Intermediate levels of hygienic behaviour did not reduce varroa increase.
Deformed wing virus (DWV) levels were determined from a sample of 50 worker bees taken from each hive in December 2013. DWV was detected in 34 of the 42 colonies. The number of viral copies was 10,000 times lower in the fully-hygienic colonies (> 95% FKB removal) than from the 34 colonies with FKB removal below 95%. Across all 42 colonies there was a significant negative relationship between FKB removal and DWV levels. Ten of the 42 colonies had workers with shrivelled wings, a visible symptom of DWV. Workers with shrivelled wings were not seen in any of the 14 colonies with FKB removal above 80%. This indicates that intermediate levels of hygienic behaviour are effective against DWV.
Al Toufailia, H. M., Amiri, E., Scandian, L., Kryger, P., Ratnieks, F. L. W. 2014. Towards integrated control of varroa: effect of variation in hygienic behaviour among honey bee colonies on mite population increase and deformed wing virus incidence. Journal of Apicultural Research 53: 555-562.
F. Ratnieks, April 2016