What’s killing the bees? Varroa, and other problems

Randy Oliver Beekeeper, Scientist, Writer
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You only occasionally actually see them in a hive — the pinhead-sized crablike Varroa destructor mites that have in the past seventy years become the nemesis of beekeepers in every continent save Australia. Varroa is indisputably the number one problem affecting bee health today.

The main problem is called the “varroa-virus complex.” Honey bees are host to about 20 viruses, most of which were formerly considered relatively benign. Varroa changed all that — not only does the mite vector viruses from one bee to another (similar to how mosquitoes vector malaria), but it also injects an immune suppressor into the bee, which then activates “latent” viruses into bee-killing machines (similar as to how HIV induces infections to explode in AIDS patients).

The second problem started as beekeepers worldwide desperately sought out miticides that could be safely applied to bee hives in order to kill this “bug on a bug.” And they worked — at first. But in a few short years varroa developed resistance to the first two miticides, resulting in a wave of colony losses of 50-90 percent beginning around 2004. An unforeseen consequence of chemical control of varroa is that residues of those miticides now constitute the most prevalent pesticides currently found in bee hives worldwide. These residues have changed the baseline toxin load with which bees have long had to deal — the natural plant toxins frequently found in nectar and pollen, industrial pollutants, as well as agricultural pesticides.

Varroa isn’t the only recently-introduced parasite to be affecting bee health. At the same time that varroa really started to be an issue, the bee populations of both North America and Europe were invaded by the intestinal pathogen Nosema ceranae. United States Department of Agriculture (USDA) data indicates that its rapid increase in prevalence in the early 2000’s closely tracked the increase in colony mortality. I’ve spent the past few years intensely following nosema in my own bee operation, and can’t help but feel that Nosema is also a contributor to our recent problems.

I don’t typically see nosema as being the main problem — that distinction still belongs to varroa. The most recent USDA survey of hives across the country found that the average varroa infestation rate in the United States in autumn is above the danger level for virus epidemics. In any year that I hear reports of beekeepers having a hard time with controlling varroa, winter losses go up.

I feel that it is safe to say that the consensus of bee researchers is that today’s elevated colony loss rate is due to several factors:

  1. The evolving varroa/virus complex, and poor mite control by beekeepers.

  2. The added stress due to widespread prevalence of Nosema ceranae.

  3. The recent extensive loss of good bee forage land due to changes in agricultural practices.

  4. The forcing of bees onto cropland frequently sprayed with pesticides, coupled with the new toxin baseline due to the miticides that beekeepers now use to control the varroa mite (as I wrote on Monday, the neonicotinoid seed treatments generally do not appear to be a problem).

The latter points are a main topic of discussion by the commercial beekeepers who manage over 95 percent of all bee hives in the United States. The changing face of agriculture is reducing the amount of good forage lands available to bees. Today’s farms have largely eliminated livestock, hedgerows, and pasture land, which formerly supplied bees with abundant forage. Many agricultural areas are now “green deserts” as far as bees are concerned, there being precious little for bees to find to eat in the weed-free monocultures of corn, soy, and wheat stretching from horizon to horizon.

We beekeepers are in a difficult transition time. We happen to be caught in the middle of both the biological evolutionary process of our bees adapting to varroa, viruses, and nosema, as well as seeing bee-friendly pastureland disappear at an alarming rate. We’ve been inadvertently adding to the problem with in-hive miticides. And today’s high commodity prices are encouraging farmers to unnecessarily apply pesticides as preventative “risk management.”

We’ll work things out. Unfortunately, it’s not going to be easy. A number of large commercial beekeeping operations are failing. Bees and beekeepers can use the public’s support in encouraging the preservation of wildlife habitat on farm lands, and the planting of bee-friendly forage. And we need to transition to more eco-friendly agricultural practices, which include more attention to realistic assessments of the impact of all pesticides upon pollinators.

Randy Oliver is a beekeeper and biologist. He writes at scientificbeekeeping.com.

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Randy Oliver