Why is it so hard to breed better bees?
Those new to beekeeping frequently ask why we don’t just breed better bees. This is a logical question because breeding has long been the answer to many agricultural problems. When I say “breeding,” I don’t mean modern gene-insertion techniques that allow us to raise glow-in-the-dark cats, but the old-fashioned kind of breeding where you cross hand-selected individuals in order to amplify their best traits.
This traditional method has yielded bigger, fatter, disease-resistant, and higher-yielding plants and animals that are the backbone of modern agriculture. Over the years, it gave us more milk, bigger cherries, sweeter apples, blight-resistant tomatoes, pink daffodils, and hairless dogs.
Of course, what you do on purpose, you can also do by accident. Inadvertently, we’ve created a host of undesirable organisms using similar techniques. Methicillin-resistant Staphylococcus aureus (MRSA) and similar pathogens arose because we killed off most, but not all, of the individuals. Those that survived were the strongest, best adapted, and most able to persist in spite of antibiotics. We selected for the strongest ones by eliminating the weaker ones—the same selection principle operating in reverse. Closer to home, we’ve bred varroa mites that are resistant to nearly everything.
Why don’t we breed better bees?
So why don’t we just breed better bees? The answer is simple: we already have. Breeders have managed all kinds of marvels with honey bees. They have built bees that are gentle, bees that overwinter well, bees with increased honey production, and even bees that cope with varroa mites. Breeding isn’t the problem.
The problem with honey bees occurs after the queens leave the breeder. The traits bred into honey bee queens in carefully-controlled breeding programs soon disappear when the daughters of these queens are allowed to mate with open stock. Within a generation or two, the descendants of these super bees are right back to square one. The question is, “Why does this keep happening?”
The road blocks to maintaining better stocks
There are three main road blocks to maintaining well-bred populations—haplodiploidy, polyandry, and panmixia—plus a few other minor complications. And while those words may sound intimidating, don’t worry. Just fasten your seat belt, and you’ll be an expert on all three in no time. Get ready to impress your friends at the next dinner party.
Like all other members of the order Hymenoptera—including ants, wasps, and sawflies—honey bees are haplodiploid. Haplodiploidy means that some individuals are diploid, having two sets of chromosomes, while others are haploid, having only one set of chromosomes. If your knowledge of genetics is limited, suffice it to say that haplodiploidy doesn’t operate like the simple Mendelian genetics you learned in high school. I, for one, thought I was a genetic genius after I had the dominant/recessive pea grid worked out, but things in real life are not that simple.
Haplodiploidy makes breeding more challenging, and it has some surprising consequences. In honey bees, drones are produced from unfertilized eggs, which means each drone has only one set of chromosomes, while females—both workers and queens—have two sets. In most animals other than Hymenopterans, all individuals have two complete sets of chromosomes. Also, haplodiploidy results in inexplicable axioms, such as a drone has a grandfather but no father and can have grandsons but no sons.
But it gets even weirder. What they don’t teach you in Beekeeping 101 is that some fertilized eggs become diploid drones—that is, drones with two sets of chromosomes. This happens because the thing that actually determines sex is not the presence or absence of fertilization but the presence or absence of heterozygous alleles at the sex locus. Don’t click away yet—you can do this.
A piece of chocolate cake
You see, instead of having an entire chromosome that determines sex, like the X and Y chromosomes in humans, bees have one gene on one chromosome that determines sex. Specific places on chromosomes are called loci (the singular is locus), so the “sex locus” is just the place (think address) on the chromosome where the sex gene is found.
The European honey bee has about 18 different alleles of the sex gene. An allele is just a variation of a gene. All sex alleles do basically the same thing, but the genetic coding is a little different in each one. You can compare it to having 18 different recipes for chocolate cake—the end products are similar but the instructions for getting there vary.
So different bees are running around with different alleles (or instructions) for the sex gene. If an egg is not fertilized, there is only one set of instructions and the bee becomes a drone. If an egg is fertilized and has two different sets of instructions, the bee becomes a female. But—and here’s the kicker—if the egg is fertilized but receives two identical sets of instructions (two identical sex alleles) the bee becomes not a female but a diploid drone. Think of it like this: one set of instructions printed twice is not the same as two different sets of instructions.
The fate of diploid drones
These diploid drones do not survive. In colonies of social insects such as honey bees, the workers eat or destroy the diploid drones soon after the eggs hatch. Because they are destroyed early, having many diploid drones in a colony results in “shot brood” or “scattered brood”—brood combs that have lots of empties or brood of many different ages interspersed. In some solitary bees, the diploid male may die in the cell, or may emerge and mature but be sterile.
The table below shows what would happen when a honey bee queen (with two different alleles of the same gene) mates with five different drones, each with one allele. In this case, two of the drones have the B allele and the rest have different alleles.
Wherever you have homozygous alleles for the sex gene (two of the same alleles), you get a diploid drone. This table shows an extreme example because it has a small number of alleles and a small number of matings, but it illustrates how homozygous alleles happen. In this example, one queen with two different alleles for the sex gene mates with a series of five drones, resulting in only 70% viability of the fertilized offspring.
For the average beekeeper, this property doesn’t make much difference. Since honey bees have about 18 alleles for the sex gene, and a queen may mate twelve or more times, there is little likelihood of diploid males. But for breeders who are trying to control the gene pool of both the queens and the drones, diploid drones can become a real concern. Inbreeding decreases the number of alleles in a population and thereby increases the occurrence of diploid drones. Large numbers of diploid drones weaken a colony because the nurse bees waste resources trying to raise these bees only to kill them later.
|Drones||Queen #1 Allele A||Queen #2 Allele B|
|Drone #1 Allele A||AA||BA|
|Drone #2 Allele B||AB||BB|
|Drone #3 Allele C||AC||BC|
|Drone #4 Allele B||AB||BB|
|Drone #5 Allele D||AD||BD|
Honey bees are also polyandrous. Polyandry means “many men” and refers to the fact that a queen bee mates many times. Having many mates is not unusual by itself, but a queen bee stores all the sperm in her body for the rest of her egg-laying life. So when she lays her eggs, the eggs are fertilized by an assortment of males. Each of these different couplings represents a different sub-family in the brood nest.
Workers within any single sub-family have the same mother and father and are called “super sisters” because they share about 75% of their genes.1 Workers belonging to different sub-families have the same mother but different fathers. They are known as half-sisters, and share about 25% of their genes. When people ask, “Why are my bees all different colors?” that’s often the answer: they represent different sub-families in the same nest. It’s most apparent if a queen mates with both Italian (yellow) and Carniolan (black) drones.
But color is not the only difference. Because subfamilies are the offspring of different drones, they will vary in many ways such as overwintering ability, disease resistance, temperament, or foraging strength. Even more important, subfamilies tend to stabilize a colony. If the queen had a bad mating—or even two—the offspring of the bad mating are only a small proportion of the entire colony, meaning the colony can survive even though some individuals do not.
In addition to haplodiploidy and polyandry, honey bees are famous for a trait called panmixia. Panmixia refers to totally random mating, resulting in a thorough mixing of genes throughout a population.2 Drone congregation areas—locations where, just as the name implies, large numbers of drones collect and hang out until a virgin queen flies by—were designed with panmixia in mind. They assure excellent mixing of genes, because drones from large geographical areas meet and compete for the right to mate.
While polyandry by itself assures multiple matings, panmixia assures the mates are not all boys from the local ’hood. Instead, the bees in a drone congregation area represent a diverse population. Panmixia has a stabilizing influence on a population, providing a check against genetic drift and inbreeding depression, a fancy term that indicates a reduction in fitness due to sharing too many of the same genes. It provides a buffer against genetic tragedy unless, of course, you are a breeder trying to select for certain traits—then it’s more like an albatross.
Additional Genetic Problems
Honey bees in North America have had a restricted gene pool for a long time. This is partly due to the Honey Bee Act of 1922 which prohibited the importation of honey bees into the United States. The idea was to prevent honey bee disease organisms, specifically tracheal mites, from entering the country. But the unintended consequence was to cut off the flow of genes from the honey bee’s native lands.
Although I hear this argument from time to time, I personally don’t give it much credence. After all, there were no restrictions during the 300 years from the early 1600s to 1922. In addition, honey bees proliferated across the continent in those years, having plenty of opportunity for natural genetic variation that occurs in any large population. Even so, in recent years genetic materials via drone sperm have been imported into the US in order to supplement the gene pool.
On a different front, recent research has shown us that the honey bee genome is extremely limited in the number of genes that allow for detoxification, a trait common among social insects.3 Scientists speculate that instead of evolving quickly to protect itself from environmental contaminants, the honey bee traditionally used other mechanisms of defense. Traits such as hygienic behavior, propolis collection, and sacrifice of the individual for the good of the colony are tools used to protect a colony from such danger.
However well this worked in the natural world, it is problematic in an environment filled with man-made hazards including insecticides. While other invertebrates including mosquitoes, cockroaches, and even varroa mites can develop resistance during the evening news, the honey bee is more genetically stable and less able to respond quickly to environmental toxins. This characteristic has been called “ecological naïveté.”2 It appears that nature prepared the honey bee for a world that was simpler and more innocent than the complex one in which it now resides. As a consequence, it takes longer—perhaps too long—to develop natural resistance.
A lesson from conservation biology
Island biogeography is the study of how species form and why they go extinct. Both speciation and extinction happen faster on islands because there is a restricted flow of genes into a protected area. The mantra of island biogeography is simple, “Isolation plus time equals divergence.”2 In other words, to change a population you must isolate it and keep it separate over time.
Breeding for a specific trait is not the same as speciation, but it works in much the same way. In order to breed for a specific trait, you must restrict the flow of genes into your breeding population over a number of generations. This increases the probability of the desired gene showing up in any single individual. Unfortunately, it also increases the incidence of unwanted genes—something breeders must prepare for.
Although it would be helpful to own an island or remote tract of land for breeding purposes, an “island” can be formed from other types of isolation. A freeway, a mountain range, a lake, a city, or a dearth of floral resources are all things that can restrict the flow of genes into an area. A famous example is Central Park in New York City. From the sky it is a huge green park surrounded by immense buildings and endless pavement. Central Park would not be an island to honey bees because they can fly great distances, but it is an island to certain native bees with foraging ranges of several hundred yards at most.
Those beekeepers who are successful at raising varroa-resistant bees have generally spent many years in an area that is somewhat protected from genetic contamination. These two factors, time and separation, mean they’ve had a lot of influence on the bee populations in their local area. In other words, they have been able to flood their area with “good” genes, so there is a higher probability that their queens will mate with drones who also have varroa-resistant traits. In essence, they shift the gene pool within their “island.”
Beekeeping in the real world
In the beginning of this article, I said that breeding wasn’t the problem. Now, thinking about the traits I just mentioned, let’s go back and look at the trouble. In the real world, a carefully bred and inseminated queen will work as advertised. Say you buy a queen bred and mated for hygienic behavior. Her offspring will most likely show the desired trait and the varroa mites will be dispatched. But at some point, the colony swarms and your queen is lost to the wilds.
If you do not intervene, one of her daughters will become the new queen. She carries the desired trait from both her mother and father, but when she mates, she mates with the local stock of drones. Perhaps some of those drones have the hygienic gene, especially if some other local beekeepers bought bees from the same breeder. But most of the drones have no such trait. So whether your colony shows resistance or not is a numbers game.
Nearly all varroa-resistant genes are recessive,4 so if your new queen mates 16 times, and only one or two of those matings are with a drone with varroa-resistant genes, you will have, at most, just two subfamilies in your colony exhibiting the trait. Assuming all subfamilies are represented equally, that would be about 1/8 of the bees or 12.5%—perhaps too low to do much good. The more varroa-resistant drones in the area, the better chance you have of seeing some resistance.
Bees by the numbers
But imagine for a moment that the local bee club in your town just purchased 250 packages from a producer in the south. Assume for a moment that they all survived. You would then have, theoretically at least, 250 queens each laying 1,000 eggs per day for the months of April, May, and June. If you assume 15% of those bees are drones, then you have (250 x 1000 x 90) x 15% or 3,375,000 drones in your area during that three-month period. And those are just the drones from the new packages, not those from established and feral colonies. Worse, each and every one of them is eager to mate with the offspring of your expensive hygienic queen.
Sure these are ballpark numbers, but the message is clear. If you bring a varroa-resistant queen into an area where there are lots of bees, but little varroa resistance, the trait will soon disappear.
In summary, a persistent beekeeper with a lot of colonies can significantly shift the gene pool in his favor. But if an area is constantly bombarded with random bees from other places, it is extremely difficult to develop a resistant line. It will require time, effort, and significant planning.
I’m not saying we shouldn’t try to breed better bees. But we need to understand why it’s a long row to hoe, why it may be difficult to imitate the success of others, and why the resistant traits are hard to maintain. Because of the unique biology of honey bees, it can take a long time to see substantial change.
Honey Bee Suite
- Breed MD, Moore J. 2015. Animal Behavior. Elsevier Inc. Academic Press 71-107
- Quammen D. 2004. The Song of the Dodo: Island Biogeography in an Age of Extinctions. New York. Scribner.
- Evans JD, Aronstein K, Chen YP, Hetru C, Imler JL, Jiang H, Kanost M, Thompson GJ, Zou Z, Hultmark D. 2006. Immune pathways and defence mechanisms in honey bees Apis mellifera. Insect Mol Biol. 15(5):645-56.
- Kefuss J, Vanpoucke J, Bolt M, Kefuss C. 2016. Selection for resistance to Varroa destructor under commercial beekeeping conditions, Journal of Apicultural Research: 54(5) 563-576, DOI: 10.1080/00218839.2016.1160709
This artcile was reprinted in An Beachaire, The Irish Beekeeper. Vol. 73 No. 11, November 2018.