Varroa mite alert from the Honey Bee Lab

Hope all of you had a relatively successful bee year with strong hives and significant honey production, and have prepared your hives for successful overwintering. I just wanted to take this opportunity to alert / caution you about possibility of high mite populations in the colonies this year due to an unusually long bee season. As you all are aware we had a long bee season this year (at least in the Willamette Valley) as a result of warm weather that prevailed for almost more than 7 months. Longer brood cycle (abundance of larvae) usually results in higher mite populations, as the mites get a greater opportunity to breed and increase their populations relative to bees. Most of you might agree that this year was a year with longest brood cycle seen in the recent past (I have been in Oregon only for the past 6.5 years, so can’t go beyond that number). It has been reported that mite populations could increase exponentially (up to about 50 fold increase) in years when the brood is present in colonies almost round the year (Martin 1998).

The economic threshold to treat Varroa mites in general for temperate areas is considered to be about 3% or higher in fall, but as economic threshold depends on several factors it is not ideal to always rely on this magic number. In Oregon during the past six years we have documented mite intensities ranging between 3% and 5% in Fall (August sampling). We observed significantly higher mite intensities this year (2015). The average mite intensity observed in backyard beekeeper colonies was 7%, whereas average mite intensity documented in commercial beekeeper colonies was about 3%. In few backyard beekeeper colonies we observed mite intensities as high as 32%, which is alarming.

If you treated your colonies for Varroa on time during July or August then probably you may have your mite populations under control, but still I urge you to monitor mites one more time before overwintering to make sure that the treatments that you used were effective and your current mite populations are not at damaging levels. If your mite levels are still high then please consider using an oxalic acid treatment if feasible when there is no brood (possibly during November).

If you did not use any Varroa mite treatments yet, then please assess the mite populations using alcohol wash or powdered sugar method as soon as possible and consider treating your hives with oxalic acid when there is no brood in the colonies. Oxalic acid was recently approved by EPA and is available from the bee supplier Brushy Mountain Bee Farm (http://www.brushymountainbeefarm.com/?gclid=CLzrqIrB98cCFUiEfgods-gJ6w).

Following are some consequences of inadequate or no Varroa mite control this fall:

  • Bee population may decline significantly or the colonies might totally collapse.
  • Colonies that survive the winter will start upcoming year / season with higher mite loads and hence could reach damaging levels soon by late spring or summer.
  • High mite infested colonies may contribute to higher mite drifting via robbing bees to other beekeeper colonies and your existing healthy colonies, as your mite infested dead colonies may be robbed by other strong colonies and aid in greater mite dispersal.

Also, please continue feeding protein to your colonies if pollen stores are not adequate in the colonies. Protein feeding not only helps with brood rearing, but also helps boost the immune system of bees. We have observed colonies to consume protein until October 25 in the Willamette Valley and few other locations in Oregon when the weather is still OK (temperatures around 55 to 60° F).

Following is a question relevant to Varroa mite biology that an Oregon beekeeper asked me few months ago.

Question: How many days is the female Varroa mite outside of the capped brood before it re-enters another cell for reproduction? Do the young female mites that emerge along with the new bees also take the same amount of time to re-enter another larval cell for reproduction?

Answer: The time a female Varroa takes to re-enter (re-infest) a new cell depends on the availability of older larva (ready to be capped) to enter, and also on the number of bees in the hive at that point of time. One study showed that on average female mites take about 4 to 6 days to re-infest new larval cells. In a lab study, female mites that were artificially reintroduced into new cells with appropriate aged larvae (ready to be capped) immediately after emergence from a cell were able to reproduce successfully without any problems. Young female mites that emerge along with the foundress mite (parent mite) need time to achieve full maturity and hence may take a little more time to enter a cell for reproduction than the parent mite. Research pertaining to these new young mites is scarce, hence providing an average time for infestation is difficult.

Regards,

Ramesh Sagili

Oregon State University