Ibrahim & Spivak (2004) hypothesized that removal of brood infested with reproductive mites could explain part or all of what we describe as the SMR trait. However, Ibrahim & Spivak (personal communication) concluded that colonies of bees with the SMR trait control mites through a brood effect as well as through the behaviour of adult bees. — HARBO & HARRIS (2005)
[There is] an unknown physiological effect of either worker pupae or adult bees in some colonies that reduces mite reproduction (Camazine, 1986, Harbo and Hoopingarner, 1997; Harbo and Harris, 1999).
In 1997, a new line of bees was bred for the heritable trait, “Suppression of Mite Reproduction” or SMR (Harbo and Hoopingarner, 1997). In colonies of bees bred for SMR, the mites entered worker brood cells to feed and reproduce; however, the mother mite died, produced no progeny, produced only males, or produced progeny too late so that the mite offspring did not mate before the bee emerged as an adult and so were not viable (Harbo and Harris 1999). As a result, SMR colonies were able to reduce the population growth of the mites and so showed promise as a line of bees that could resist V. destructor. However, many SMR colonies suffered from poor brood viability and low honey production (M.S., personal observations), making them commercially undesirable.
Originally we speculated that the poor brood patterns were due to inbreeding of the SMR line, which causes homozygosity at the sex determining locus and the production of unviable diploid male offspring (Mackensen, 1951). However, the brood patterns did not improve substantially when the SMR and Hygienic colonies were originally crossed. Based on our later experiments (Ibrahim and Spivak, in press), we speculated that the poor brood patterns were somehow linked to the low reproductive success of the mites in brood. — Ibrahim & Spivak (2005)
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