Surviving Varroa by Tom Seeley

Introduction

This week is warmer after nearly a fortnight of overcast days and very cold nights. The OSR crop nearby has been in full flower since then, but, if this weather continues as predicted, there will still be time for the bees to make a decent crop of honey. I’ve just jarred up my last bucket of stored honey so I hope things go well. I’ve kept the insulation on under each hive roof, and a quick inspection reassures me that there is no chalkbrood, or signs of chilling given the extra space that empty supers impose on a colony. The floors are clean and the entrances busy with foragers coming and going.
A few weeks ago a friend brought round a small colony of bees that he had cut out from the roof of a church near Lochness where bees have been in residence for over forty years. These bees are small, dark and hairy and I’m waiting for drone pupae to develop to the purple-eyed stage before I can send samples of 15 drone back legs to BeeBytes in Edinburgh for genetic analysis https://beebytes.org/about/. I’m counting the daily varroa drop and monitoring this small colony closely for signs of hygienic behaviour —needless to say I am very excited about the project and more about that in a future blog. Thank you, Mac.

This week, Emeritus Professor, Tom Seeley shares his knowledge and experience of a population of wild honey bee colonies that is surviving with varroa. Once again, thank you Tom, for another excellent and inspiring guest blog.

First Swarm

When I was 10 years old, I watched a swarm of honey bees move into a cavity in a black walnut tree near my parent’s house, and over the rest of the summer I enjoyed observing the bees at the knothole doorway to their home.  Ever since then, I have been fascinated by how honey bees live in nature.  It is a fascination that I have pursued over several decades by conducting studies of the wild colonies of honey bees that live in the forests near Ithaca, New York, the home of Cornell University.  This small city lies at the southern end of Cayuga Lake, a narrow, glacially-deepened lake that runs north for nearly 65 kilometers (Fig. 1).  It is one of the eleven Finger Lakes that spread south to north across the middle of New York State, like the fingers on a pair of outstretched hands.  The landscape between these lakes is one of rolling, open farmland with deep rich soils that lie atop a bedrock of limestone.  But south of these lakes the terrain is hilly and heavily wooded (Fig. 2).  It is poor farming country, but the vast deciduous forests covering these hills provide prime habitat for wildlife, including black bears, beavers, bobcats, coyotes, foxes, and ravens.  Also, wild colonies of honey bees.

My studies of the honey bees living in these forests began in the 1970s, therefore some twenty years before Varroa destructor arrived in North America.  One of my early studies looked at the patterns of survival of the wild colonies (Seeley 1978).  I wanted to see how long these colonies live, i.e., how long a site is continuously occupied.  To do so, I found 42 wild colonies living in trees and buildings—cabins, churches, and old farmhouses—and then I checked their 42 nest sites for signs of life in May, July, and September (Fig. 3 A and B).  If I spied bees bearing pollen loads flying into a nest entrance, then I knew that the colony at this site was still alive (Fig. 4).  This monitoring of wild-colony nest sites went on from May in 1974 to May in 1977. 

In addition, from June 1974 to September 1976, I maintained an apiary of 15 simulated wild colonies (SWCs) at my parent’s home a few miles east of Ithaca.  These colonies were established using swarms captured the previous summer in bait hives that I set out in the hill country south of Ithaca.  I labelled each colony’s queen with a paint mark on her thorax.  Each colony lived without management in a single-story, 10-frame Langstroth hive (Fig. 5). I used such small hives for my SWCs because I wanted them to occupy nest cavities whose size matched the mean value for the wild colonies’ homes in the Ithaca area:  about 40 liters (Seeley and Morse 1976).  Unlike the true wild colonies that I was monitoring, my SWCs could be inspected closely, and I did so in May, July, and September.  During each inspection, I located the colony’s queen and checked whether she had a paint mark.  If not, then I knew there had been a queen turnover in the colony, probably from swarming, and I labeled the new queen so I could detect future queen turnovers.  

What did I learn?  First, I learned that a “founder colony”— one that has recently been founded by a swarm moving into a tree or building—has a rather low probability of surviving its first year.  Specifically, I found that only about 20% of founder colonies started in the spring or summer of a given year were still alive when I checked them the following May.  Most died over winter, probably from starvation.  I also found that if a colony manages to survive its risky first year, and thereby becomes an “established colony”, then it has a high probability of surviving each year thereafter.  Approximately 80% of established colonies survived each year.  And lastly, I observed that an unmanaged colony that occupies a nest cavity the size of one deep hive body is likely to swarm every year.  No surprise there.

These studies from the 1970s showed that, at the time, most wild colonies died during their first winter, but that if a wild colony managed to survive its first winter, then it lived, on average, for another five years (the calculation of average colony lifespan is explained in Seeley 1978).  These early studies also revealed that colonies that survived their first winter needed to produce, on average, just one swarm each summer thereafter to maintain the population of wild colonies in a region.  Lastly, these studies showed that one swarm per colony each summer was a realistic average rate of colony reproduction.  

Varroa Arrives

Then, in the early-1990s, Varroa destructor made its way to New York State.  My first man-to-mite encounter occurred in June 1994.  I was happily labeling young worker bees with paint marks for an experiment when, to my dismay, I spied a reddish-brown Varroa mite scuttling over a bee that I was about to label with a dot of paint.  Obviously, this was not good news, but I hoped for the best.  Sadly, I experienced almost the worst; over the winter of 1994-95, some 80 percent of my colonies perished.  So, like most beekeepers, I quickly learned how to treat my colonies with miticides to keep them alive.  These experiences led me to suppose that all the wild colonies living in the woods around Ithaca would soon be dead, for certainly nobody was treating them with the life-saving miticides. 

I was thrilled, therefore, when I discovered in August 2001 that there were still wild colonies of honey bees living in the Arnot Forest, a 24-km² research forest that is owned by Cornell and is not far from Ithaca (Seeley 2007) (see Fig. 1).  It was even more exciting to discover that these wild colonies were thriving despite being infested with Varroa!  This was not a complete surprise, however, because I had expected that there would be strong natural selection for mite-resistant bees (and maybe also for low-virulence mites) in populations of honey bee colonies that are not given mite-control treatments.   

Mite Resistance

My expectation of strong natural selection for mite-resistant bees has been confirmed by a genetic analysis of two sets of worker bees that I collected from wild colonies living in the forests south of Ithaca (Mikheyev et al. 2015).  One set of bees was collected in 1977 (well BEFORE the arrival of V. destructor) and the other set was collected in 2011 (well AFTER the arrival of V. destructor).  To compare the genetics of the 1977 and 2011 bees, my co-workers performed whole-genome sequencing of 32 bees from each group.  This revealed that 232 nuclear genes scattered throughout the genomes of these bees had undergone strong selection between 1977 and 2011.  This genetic analysis also revealed that the population of wild colonies living in the forests south of Ithaca experienced a collapse (a “genetic bottleneck”) between 1977 and 2011.  It is likely that this period of intense colony mortality, during which ca. 90% of the wild colonies died, occurred shortly after the arrival of V. destructor in the Ithaca area back in the early 1990s.  But then this population of wild colonies recovered.   The density of wild colonies in the Arnot Forest is the same now (with Varroa) as it was in 1978 (without Varroa):  approximately 1 colony per square kilometer.  Another indication of the renewed abundance of wild colonies in this region is that I have no trouble establishing bee lines to wild colonies whenever I go bee hunting in the woods around Ithaca.

These studies provide evidence that the honey bees living in the Arnot Forest now possess a suite of powerful behavioral defenses against Varroa.  I have not completed my studies of what these behavioral defenses are, but there are strong signs that they include uncapping (and recapping) brood cells that contain immature Varroa, and killing adult Varroa. These behavioral studies, together with the colony-survival studies mentioned here, are revealing a natural, non-treatment solution to the problem of Varroa destructor.

References

Mikheyev, A.S., M.M.Y. Tin, J. Arora, and T.D. Seeley. 2015.  Museum samples reveal rapid       evolution by wild honey bees exposed to a novel parasite.  Nature Communications 6, 7991.        Doi10.1038/nomms8991.

Seeley, T.D. 1978.  Life history strategy of the honey bee, Apis mellifera.  Oecologia 32, 109-      118.

Seeley, T.D.  2007.  Honey bees of the Arnot Forest:  a population of feral colonies persisting       with Varroa destructor in the northeastern United States.  Apidologie 38, 19-29.

Seeley, T.D, and R.A. Morse.  1976.  The natural nest of the honey bee (Apis mellifera).        Insectes Sociaux 23, 495-512.

Figure legends

Fig. 1.  Aerial photo showing the vast forests south of the small city of Ithaca, New York (yellow dot) and the Arnot Forest (yellow square).  For a sense of scale, note that Cayuga Lake extends north of Ithaca for nearly 65 kilometers.

Fig. 2.  View of the forest-covered hills south of Ithaca.  Photo taken in early October, close to the peak time of the autumnal colors.

Fig. 3.  A.  One of the bee trees that I am monitoring.  B.  Nest entrance of a wild colony.  It is typical in its size (about 20 cm²) and in having a coating of propolis on the tree’s bark.

Fig. 4.  Bee flying into a bee tree bearing a load of pollen.  This showed me that a live colony was inside this tree.

Fig. 5.  One of the hives that I used for housing my simulated wild colonies.