Why Do Varroa Mite Populations Sometimes Increase in the Fall?
The Varroa mite is public enemy number one to honey bees in the United States. Like little vampires, they suck the bees’ “blood” (hemolymph) and transmit viruses that can harm or kill them.
According to Kirsten Traynor, a postdoctoral researcher in entomology at UMD, “We know that Varroa acts as a vector for viruses. The mites are basically dirty hypodermic needles.”
Now scientists from the USDA are tracking the mites by observing the comings and goings of foraging honey bees that they suspect may have mites on them. The researchers are blocking access to honey bee hives with PVC pipes that have sliding wire-mesh doors in order to separate incoming bees from outgoing ones. The apparatus also allows the scientists to inspect the bees for mites as they come and go.
The team’s investigations in Bismarck, ND this June are actually a follow-up study to one they completed last year at two Arizona sites. Findings from that study suggest that bees can bolster their hives’ existing mite population by carrying in Varroas from other colonies — an influx that most often occurs in the fall, especially November.
Varroa populations usually grow slowly because females produce only three to five offspring. If mite populations in colonies are low, then they should remain that way for at least a season.
However, sometimes Varroa numbers soar to potentially hive-wrecking levels during the fall, which suggests that bees are picking them up from other hives that have been infected. At the Arizona hive sites, this influx of migrating mites correlated to population increases of 227 to 336 percent, starting in November.
Beekeepers who spend time and money on managing mites may suffer losses because other beekeepers do not employ mite-management techniques, allowing their hives to become infected and to infect other beekeepers’ hives in the fall.
“Many backyard beekeepers don’t have any Varroa control strategies in place,” said Nathalie Steinhauer, a graduate student at the University of Maryland who was not involved with this study. “We think this results in colonies collapsing and spreading mites to neighboring colonies that are otherwise well-managed for mites. We are seeing more evidence to suggest that good beekeepers who take the right steps to control mites are losing colonies in this way, through no fault of their own.”
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