As scientists learn more about honey bee reproductive biology, they are discovering that there are biological advantages to colonies when queens mate with multiple drones. When a queen mates with numerous drones (polyandry), the colony population is composed of a genetically diverse worker force. This genetic diversity is believed to increase colony fitness, by generating a more stable and resilient system of division of labor and reducing susceptibility to various pathogens and parasites. Mating with multiple drones also increases the likelihood of a sufficient supply of stored semen and lowers the probability of inbreeding (Oldroyd and Fewell 2007).

The number of colonies represented in a drone congregation area influences the relatedness between a queen and her mates and hence the inbreeding level of colonies; it also determines the relatedness between the mates of a queen, which affects genetic diversity within colonies. Using molecular genetic techniques, Baundry et al. (1998) studied a sample of 142 drones captured in a congregation area close to Oberursel Germany. A parentage test indicated that this sample contained one group of four brothers, six groups of three brothers, 20 groups of two brothers and 80 singletons. They concluded that colonies were apparently equally represented in the drone congregation and calculations showed that the congregation was comprised of males that originated from about 240 different colonies.

A colony population is made up of several patrilines or subfamilies of worker bees, each sired by a different drone. Workers within a subfamily are super sisters and have a coefficient of relatedness of 0.75 (share an average of 75% of their genes). However, workers from different subfamilies share no genes from a common father and have a coefficient of relatedness of only 0.25 (have about 25% of their genes in common) (Pageand Laidlaw 1988). Thus, there is a very high degree of genetic relatedness among bees within subfamilies but a high degree of genetic diversity between different subfamilies.

Queen and worker honey bees develop from fertilized eggs that contain 32 chromosomes (diploid), a set of 16 chromosomes from each parent. Normal drones develop from unfertilized eggs which contain only one set of 16 chromosomes from their mother (haploid). Since queens and workers have paired chromosomes, they carry two alleles for each gene, one on each member of the pair. If the alleles are of the same type (homozygous) at the sex locus (the physical region of the chromosome that determines sex), the embryo becomes a diploid male and is eaten by nurse bees shortly after hatching (Wyoke 1963). Individuals having different alleles (heterozygous) at the sex locus become female. Page (1980) suggested that multiple matings (polyandry) may compensate for the reduction in fitness resulting when a queen mates exclusively with a drone carrying one of the same sex alleles as herself. Such a mating results in 50% mortality of the diploid brood, producing a spotty brood pattern. Queens need to mate with unrelated drones to ensure viable offspring.

Virgin and mated queens differ dramatically in their pheromone profiles and these pheromones are important for regulating colony organization and worker behavior (Slessor et al. 1990; Plettner et al. 1997). Queens were instrumentally inseminated with semen from either a single drone (SDI) or multiple (n = 10) drones (MDI) and their interactions with workers were monitored in observation hives (Richard et al. 2007). Cage studies were used to monitor the attraction of workers to queen mandibular gland extracts (the main source of queen pheromone) from virgin, SDI and MDI queens. Richard et al. (2007) was able to demonstrate for the first time that insemination quantity significantly affects mandibular gland chemical profiles and queen-worker interactions. MDI queens elicited a stronger retinue than SDI queens in natural colony conditions, and their mandibular gland extracts were more attractive in preference assays with caged worker bees. Analysis of the mandibular gland chemical profiles revealed significant differences between SDI and MDI queens. These results suggest that insemination quantity can have profound effects on queen physiology and behavior.

Genetic differences among subfamilies within colonies are believed to increase colony fitness and productivity in several ways (Tarpy and Seeley 2006). First, genetic diversity may increase the behavioral diversity of the work force, which may enable colonies to extract resources from the environment more efficiently (Oldroyd et al. 1992) or resist fluctuations in the environment more effectively (Page et al. 1995; Jones et al. 2004). Second, genetic diversity may reduce the incidence of diploid male production as a consequence of the single-locus sex determination system. Third, genetic diversity may reduce the prevalence of parasites and pathogens among colony members (Sherman et al. 1988). It is assumed that there is heritable variation in susceptibility to parasites and pathogens, hence, genetically more variable colonies are less likely to suffer sweeping infections by disease-causing parasites. Multiple matings would likely produce genetically distinct subfamilies of workers with different alleles for disease resistance or behaviors associated with parasite removal.

Differences do occur in the behavior of members of different subfamilies (Calderone and Page 1991). Subfamilial differences within colonies of bees have been demonstrated for a broad range of honey bee activities (Kolmes et al. 1989). The high genetic similarity of workers within subfamilies, and correspondingly lower genetic similarity of workers from different subfamilies means that, for any behavior that has a genetic component, workers within a subfamily will behave more similarly than will workers of different subfamilies (Oldroyd and Fewell 2007).

Oldroyd et al. (1992) compared colonies of low genetic diversity with colonies of higher genetic diversity in regards to colony weight gain and size of brood area, which are important parameters associated with colony productivity. Colonies having varied genetic diversity were produced from five inbred lines. One inbred line was used as a queen mother of 62 experimental colonies. These queens were inseminated with various combinations of semen obtained from single colonies of the remaining four lines. In estimating colony performance, the seasonal weight gain and mean brood area of colonies comprising two or three subfamilies were compared with those of colonies comprising a single subfamily. Some specific combinations of subfamilies reduced colony performance, whereas others enhanced it.

Tarpy and Seeley (2006) studied the relationship between genetic diversity and disease susceptibility in honey bee colonies living under natural conditions. Each queen was artificially inseminated with sperm from either one or 10 drones. Of the 20 colonies studied, 80% showed at least one brood disease. They found strong differences between the two types of colonies in the infection intensity of chalkbrood and in the total intensity of all brood diseases (chalkbrood, sacbrood, American foulbrood and European foulbrood); both variables were lower for the colonies with higher genetic diversity. They also found significant differences in colony strength between the two types of colonies. Colonies headed by multiple-drone-inseminated queens had significantly more comb, more frames of brood and higher weight gains.

Additional research with honey bee colonies headed by queens who were instrumentally inseminated with either one or 10 drones further indicated that multiple matings improves a colony’s resistance to disease. These colonies were exposed to spores of Paenibacillus larvae, the bacterium that causes American foulbrood. On average, the colonies headed by multiple-drone inseminated queens had markedly lower disease intensity and higher colony strength at the end of the summer relative to colonies headed by single-drone inseminated queens (Seeley and Tarpy 2007).

Jones et al. (2004) showed that brood nest temperatures in genetically diverse colonies tend to be more stable than in genetically uniform ones. One explanation for this increased stability is that genetically determined diversity in workers’ temperature response thresholds modulates the hive ventilating behavior of individual workers, preventing excessive colony-level responses to temperature fluctuations. They concluded that genetic variance among patrilines within a honey bee colony is important in helping them to precisely maintain the optimal brood nest temperature over a broad range of ambient temperatures.

Matilla and Seeley (2007) showed that swarms issuing from genetically diverse colonies (15 patrilines per colony) established new colonies faster than swarms from genetically uniform colonies (one patriline per colony). To replicate the experience of feral colonies, swarms were created by forcing a queen and approximately 7700 of her worker offspring to cluster in a screened cage for three days, where they were fed sugar syrup ad libitum (in accordance with desire) to simulate preswarming engorgement on honey. Each swarm was subsequently relocated to a combless hive that was similar to what colonies naturally prefer (Seeley and Morse 1978). Accumulated differences in foraging rates, food storage and population growth led to impressive boosts in the fitness (i.e., drone production and Winter survival) of genetically diverse colonies. There were notable differences in the progress of genetically diverse and genetically uniform colonies during the early stages of colony founding. Colonies with genetically diverse worker populations built approximately 30% more comb than colonies with genetically uniform populations before construction leveled off after two weeks. Genetically diverse colonies maintained foraging levels that were 27 to 78% higher than genetically uniform colonies on three of the five mornings that they were observed.

Oldroyd and Fewell (2007) argued that there is an accumulating body of evidence to support the assertion that genetic diversity from multiple mating has a functional role in division of labor and in improving colony homeostasis. The links between genetic diversity, genetic task ‘specialization’ (genetically based tendency of workers of some subfamilies to perform some tasks more frequently than do others) and improved colony development are becoming clearer. Genetic diversity within the colony contributes to colony resiliency: the ability to respond appropriately to a dynamic environment. Each individual worker chooses which task she will engage in based on her own perception of the immediate environment and her innate interpretation of the stimuli around her. If each member of the hive had the same task stimulus threshold, colony structure would become extremely unstable. The homeostasis observed within a colony is the achievement of diversity in behavior-mediating genes which is maintained through the queen mating with multiple unrelated drones.