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The suppression of mite reproduction (SMR) is a genetically inherited trait that results in Varroa-tolerant bees. This characteristic is one of several found in Africanised bees that have potential use in selection programmes. The trait is widespread in the US honeybee population and is readily available in the gene pool.
Research undertaken by Jeff Harris and John Harbo at the USDA Honey Bee Breeding Laboratory in Baton Rouge shows that female Varroa in a colony do not all attempt to reproduce at the same time. In general one-third of the mites can be found on adult bees and the rest in the brood cells. Also 15-25% of mites that enter brood cells do not reproduce: these mites either die before laying eggs, live but do not lay eggs, produce only a male and no females, and/or produce progeny too late to mature before the bees emerge. One or all of these categories can be found in any one colony.
The number of non-reproducing mites in a colony is measured by examining 30 singly infested brood cells and recording the reproductive success of each female found there. Several environmental variables affect the percentage of non-reproducing (NR%) mites. These include temperature and humidity (increase NR%), season (higher NR% in summer) and climate (larger NR% in the tropics). NR mites often have no sperm (have not been mated) and in colonies selected for NR over 50% of mites are found dead ‘entrapped by the pupa cocoon’.
It takes six weeks after requeening a colony with an SMR queen to see results. This is called ‘delayed mite suppression’ or SMRd. Mite suppression occurs immediately in some populations and is known as SMRi. To show how SMR queens affect change in a colony the researchers performed several queen exchanges between control and SMR colonies, and found that mite populations became more or less reproductive dependent upon the queen received. The researchers say, “We are confident that honeybees will become resistant to Varroa mites. In the future, bees will need fewer chemical treatments to control mites and eventually they will need none”.
A full description of this work appeared in the May 2001 issue of Bee Culture and on the ARS website msa.ars.usda.gov/la/btn/hbb/jwh/SMRD/SMRD.htm
Extract from Florida Extension Beekeeping Newsletter, April 2001 sent to B&D by Charles Frederic Andros
[Bees for Development Journal #59]