Selective breeding can bring mite reproduction to zero
in worker brood. And because the trait is additive, less-selective breeding
still produces results. For instance, if an SMR queen is mated to a
male bee, or drone, without the SMR trait, the colony will still benefit
from an intermediate level of around 50 percent resistance to Varroa.
The trait, which may be controlled by only two genes, can be bred into
any population of bees. So, beekeepers can add desired mite resistance
while maintaining genetic diversity and other good qualities in their
And what are beekeepers saying about SMR? "Beekeepers are happy
that we're finding genetic resistance to Varroa, but most are
cautiousand they need to be when it comes to controlling Varroa.
Some are unsure whether they can completely eliminate dependence on
chemical mite treatments. But those who monitor their mite populations
will be able to determine the effectiveness of their SMR bees,"
Another Pair of Genes Coming Soon
Harbo and Harris are studying a second trait in bees that offers mite
resistance. They've been measuring the potential traitdubbed "percentage
of mites in brood" or P-MIBfor as long as they've been working
on SMR, but they've just begun evaluating it in breeding studies. Mites
reproduce only in brood cells, so selecting for low P-MIB would diminish
the rate of mite reproduction.
"It would complement SMR well because it affects mites while they
are outside, rather than inside, a brood cell where SMR has an effect,"
says Harbo. The two traits together may enhance resistance to Varroa
The scientists have set their sights on creating absolute resistance
to mites. Their objective: to assemble a hybrid bee that is completely
resistant to mites, including tracheal mites, and has desirable commercial
qualities, like good honey production. Given what is known about the
SMR trait and the heritability of other traits, Harbo believes that
producing such queens is feasible.By Erin
K. Peabody, Agricultural Research Service Information Staff.
This research is part of Plant, Microbial, and Insect Genetic Resources,
Genomics, and Genetic Improvement (#301) and Crop Production (#305),
two ARS National Programs described on the World Wide Web at www.nps.ars.usda.gov.
John R. Harbo and Jeffrey
W. Harris are in the USDA-ARS Honey
Bee Breeding, Genetics, and Physiology Research Unit, 1157 Ben Hur
Rd., Baton Rouge, LA 70820; phone (225) 767-9288, fax (225) 766-9212.
A Queen Gets Her Mate
In nature, a queen bee will mate with as many as 20 drones. As a result,
the progeny of a single queen (a normal bee colony) make up a genetically
diverse population. Because a honey bee is a social animal, the traits
of special interest, like honey production and mite resistance, are
often group effects measured at the colony level. These measurements
reflect the interactions of a diverse group.
While colony diversity probably serves a role in adaptation, it poses
a challenge to breeders hunting for rare inherited traits. To find these
traits, Harbo and other bee breeders use a technique known as single-drone
insemination. Using a special instrument, the scientists inseminate
a queen with the genetically identical sperm of one drone. Her progeny
are then very closely related and are more likely to express genetic
uniformity at the colony level.
Harbo says the technique was a critical part of their SMR work. "With
single-drone insemination, 1 in 10 colonies had very low levels of mite
reproduction. With natural mating, I estimate that this level of resistance
would be seen in 1 of 1,000 colonies."By Erin Peabody,
"SMRThis Honey of a Trait Protects Bees From Deadly Mites"
was published in the May
2004 issue of Agricultural Research magazine.