Varroa Mite Researchers Talk High Infestations in Bee Colonies

Female varroa destructor on the head of a bee nymph. (Photo by Gilles San Martin.)

In 2016, The New York State Beekeeper Tech Team (which is attached to Cornell University’s Dyce Lab Beekeeping Resources) found that 90% of the 19,000 colonies they monitored had varroa mite infestations. See our report on the results here.

American Vegetable Grower reached out to the research team to learn more about the study. Take a look at what Mary Kate Wheeler, Agricultural Economic Analyst at Cornell University, and Paul Cappy, Apiculturist, Division of Plant Industry, New York State Department of Agriculture & Markets, had to say.

AVG: Would you give a little perspective on your finding about 90% of hives having varroa mites present? Was this a surprising percentage for you?

Wheeler: We know that varroa mites are extremely prevalent in New York, and their populations peak naturally in the fall. Most beekeepers will not be able to eliminate varroa mites from their operations entirely. There are, however, a number of different management practices that beekeepers can use to limit varroa population growth and maintain low mite levels.

Cappy: Ninety percent of the colonies have varroa mites. This is not surprising since this parasitic mite was found in the U.S. in 1987 and is found in almost all colonies. The mites maybe below the level of detection so that is why 90% represents the number of colonies found with mites. Mites are normal in most colonies

AVG: That 90% figure is grabbing a lot of attention. Is there a more important segment of the report you think should get more attention?

Wheeler: Our finding that varroa mites were present in 90% of colonies last September is important, yet the true surprise was observing extremely high varroa mite levels (above 3 mites per 100 bees) in 62% of colonies. Colonies with mite levels above this 3% threshold are at a high risk of dying within the next year if the beekeeper does not intervene. Our findings suggest that beekeepers were not effectively managing varroa mite levels in their colonies prior to our September 2016 sampling date.

In response to this finding, the NYS Beekeeper Tech Team met individually with each beekeeper in our program and worked with the beekeeper to develop a customized varroa management plan for 2017

Our report focused on varroa as one of the primary threats to honeybee health that beekeepers have the ability to control through their management practices. The two big messages we want people to hear are:

  1. Varroa is a serious threat to honeybee health in New York State (NYS)
  2. Beekeepers can improve colony health by planning ahead and adopting integrated varroa management strategies that include regular monitoring and treatment.

Cappy: The one area I personally think is important is summed up on the Executive Summary page. Bullet number three is the yearly colony loss, and historically this loss was from 5% to 15% prior to 1975. Now, with all the health issues the colony, losses in each bee operation can vary up to 100% die off in all three beekeeper type operations. The more stress occurs from the different pests and pathogen loads on a colony, the losses increase from 15% up to 90% to 100%.

The additional stress factor is the weather or the environmental factor that is a completely separate factor influencing the colonies health to withstand all the stress honey bee colonies are under all year long. The weather influences the nutrition of the honeybee and it can limit the development of the bees immune system. The end result is the colony losses are blamed on a combination of pest and pathogens that will finish killing the colony. However, a summer drought and the stress of the cold, damp winter months can start to directly weaken the bees’ health. The combination of the individual pests and pathogens will have the final impact by killing the honey bee in its weakened state so the colonies are shown to die from pests and pathogens. The expression is “too many straws on the camel’s back” and the camel is sick to start with from all the stress coming  from the environment.

The western New York drought for four of the five months killed 40% to 70% of the bees in the fall of 2016, and not from starvation but malnutrition. By spring of 2017 colonies died at twice the rate of the previous winter months over most of the state. The higher winter loss rate was possibly from pathogens like viruses or Nosema disease left over after killing only the varroa mites in the fall. These losses may be in part from the environmental factor of limited rain fall in 2016.

AVG: Your spring 2017 report says that 55% of hives had varroa mites. Why is there such a difference in the amount?

Wheeler: The lower varroa prevalence that we recorded in June 2017 could be related to the natural population cycle of varroa mites, and it might also reflect intensifying varroa monitoring and treatment practices among beekeepers in the Tech Team program. We will know more at the end of the year, when we collect management data from program participants.

Cappy: The answer to question three 55% of the spring colonies had mites. This is do the fact that a limited number of new bees are raised in the winter months. New varroa mites are raised in the same cells as the worker bees are being raised. Fewer new bees means fewer new varroa mites. The beekeepers treat the colonies to minimize the number of mites going through the winter to harm the adult bees.  Also, some of the varroa mites tend to die in the winter months and by spring the varroa mite levels can be too low of a level to register when testing the bees for the mites. This is the cause for lower mite levels in the spring.

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