A More Aggressive and Less Productive Apis mellifera Hybrid
Africanised honey bees are the result of crossbreeding between an African honey bee (Apis mellifera scutellata) and various European subspecies (such as Apis mellifera ligustica and Apis mellifera carnica). Scientists intended to breed a honey bee better adapted to tropical conditions, as the European honey bee colonies were poor honey producers and only survived under intensive management.
Due to their aggressive nature, Africanised bees pose a greater risk to human safety. Not because of a larger or more lethal dose of venom in their stings, rather due to their aggressive attacks, thus delivering a greater dose of bee venom (Sherman 1995). Deaths occur due to their tendency to sting in large numbers, with Franca & colleagues reporting the deaths of two men in Brazil from repeated stings (Franca et al., 1994)- one of the reasons for calling them “killer bees”.
This article explores the genetic differences between the Africanised honey bees and their European counterparts, examining how it leads to an ecological dominance from the invasive species.
Genetic Differences in Africanised Bees
Early studies found various, broad genetic variations between European and Africanised bees based on the sampling location (Hall, 1986). As technology progressed, these changes were narrowed down into the chromosomal and individual molecule level (“single nucleotide polymorphisms”).
Geography Influences the Rate of Africanisation
Africanised honey bees are hybrids of several lineages of the African honey bees, A. mellifera scutellata and other subspecies, lineages traditionally referred to as A (African), M (Western European), C (Eastern European), and O (Middle East).
A 2022 study from Zarate & colleagues found that Africanised bee ancestry decreased as latitude increased (venturing further north or south), and this trend was consistent in any sampling location (Zarate et al., 2022); these bees are unable survive in colder climates (Taylor & Spivak, 1984). The percentage of African genes increased in areas with hot and humid climates, such as southern Texas, an environment that is similar to their native African range.
Theoretically, the longer Africanised honey bees have been in a region, the higher the rate of African ancestry in bee populations1. However, in areas with large populations of European bees, this effect is reduced. Colonies in Mexico and San Diego had significantly less African ancestry compared to those in Colombia or Brazil (Zarate et al., 2022).
A combination of a relatively cooler climate, managed beekeeping practices (beekeepers would requeen a hive with desirable traits rather than having them mate naturally) and the relatively recent introduction of Africanised honey bees to these areas provides explanations for the reduced prevalence of African genes in the Mexican and southern Californian populations.
The Degree of Africanisation is Reduced at Higher Elevations
Africanised Honey bees found at higher altitudes had more European ancestry than those at lower elevations, possibly due to a cooler climate (Everitt et al., 2023). In their native range, African bees from Kenya had modifications in chromosomes number 7 and 9 that are essential in heat generation (Wallberg et al., 2017). These modifications were absent in the Africanised bees sampled by Everitt and colleagues, suggesting that the original, founding population (those African bees initially released in Brazil in 1957) did not possess these genes.
Africanised Bees have Enhanced Reproductive Capacity Genes
Genetic analysis by Everitt and coworkers showed that those Africansed bees at high altitudes had an increased expression of genes involved in honey bee development, like mTOR and MLF, as well as those and those involved in regulating drone sperm production (Everitt et al., 2023).
In areas with less favourable conditions (colder, wetter climates), queens performed fewer mating flights to avoid risks, and the same applied to the drone numbers in their congregation areas (Hernandez-Garcia et al., 2009). This combination results in a reduced mating capacity compared to colonies in dry and warm environments that favour Africanised honey bees.
Another study comparing two African bees found that those subspecies in mountainous forests (A.m. monticola from Malawi) had a lower mating frequency compared to those in the open savanna plains (A.m. scutellata; Franck et al., 2000).
European Drones at DCAs Displace Africanised Drones
Mortensen and Ellis propose to increase the number of managed European colonies in locations with high feral africanised bee populations (Mortsensen & Ellis 2016). As the European drones arrive at the drone congregation areas, they compete with their Africanised counterparts. Given that the queen mates with multiple drones, there is an increased likelihood of insemination by the European bees, reducing the overall “Africanised” profile.
However, the authors state that more work is required to determine how many European colonies are required in order to have an acceptable impact, especially since virgin queens go to more distant drone congregation areas than drones, presumably to reduce the chance of inbreeding (Jensen et al., 2005).
Ancestral Lineages A & M Drive Defensive Behaviour in Africanised Honey Bees
As the degree of African (A) genetics increase, the Eastern European lineage (C) decreases, whilst the Western European (M) ancestry maintains a significant amount in their genomic makeup. Both M and A ancestry contribute to defensive nest behaviours in the Africanised honey bees (Harpur et al., 2020). So it is not the A lineage alone, rather a combination of A & M that produces extreme defensive behaviours.
Africanised Honey Bees, Pests and Diseases
Africanised bees are themselves pests that are successfully established the Americas. An 11 year study by Pinto and colleagues showed that within 5 years, the wild European honey bees in south Texas were replaced with Africanised hybrids (Pinto et al., 2005).
In Colombia, Africanised Bees are Less Impacted by Varroa Mites
Tibata & colleagues found that Colombian honey bees were less impacted by Varroa mites and their associated diseases (Tibata et al., 2021). These colonies were untreated, and yet had a lower mite count compared to other Latin American colonies.
Further, the wild Africanised bees had lower Varroa mite infestation levels compared to European honey bees within the same region. They had no apparent symptoms of the honey bee diseases associated with Varroa infestations – Deformed Wing Virus, Black Queen Cell Virus & Israeli Acute Paralysis Virus. Although these pathogens are found at low levels in any hive, the presence of Varroa enhances their transmission and severity (Martin, 2001, Sumpter & Martin 2004).
The authors reported a low Varroa infestation level in wild, untreated Africanised colonies (a mite load of 4.5%, determined via alcohol washing). This count was lower than other colonies in Latin America, such as mite load of 6% in Mexico (Tapia-Gonzalez et al., 2007). In North America, the mite counts for stationary beekeeping operations were reported at 6%, slightly higher than the Mexican counts (Traynor et al., 2016).
In North America, Varroa is reported to cause over 40% of annual hive deaths (Kulhanek et al., 2017). Despite the use of miticides and other integrated pest management techniques, the losses are far greater than Colombia’s yearly colony losses of ~11% (Requier et al., 2018). Yes, commercial pollination and other migratory bee practices are contributing the colony losses in North America, however the authors note that 98.3% of Colombian bees are Africanised honey bees (Tibata et al., 2017).
Comparing Africanised and European Honey Bees for Disease Resistance
Ramos-Cuellar and coworkers found that Africanised honey bees were more resistant viral infections and Varroa infestations compared to European bees (Ramos-Cuellar et al., 2022). In the 365 colonies sampled from Mexico, the authors found varroosis (high Varroa mite infestations) to be most common, whereas Nosema disease was the least common.
Varroosis was significantly more prevalent in European honey bee colonies alongside a higher intensity of Deformed Wing Virus infections. The same is true of Black Queen Cell Virus infections, further suggesting that Africanised honey bees have a greater disease resistance than their European counterparts.
Factors that explain the higher relative resistance of Africanised bees to Varroa and other diseases include the high rates of colony swarming2 and hygienic, grooming behaviours, making Varroa less effective at a parasitising Africanised bee brood compared to their European counterparts (Guzman-Novoa et al., 2020).
Climate is not a Driving Factor of Disease Resistance in Africanised Species
Recall that the Africanised bees are better adapted to the climates of the Americas. Could this climate adaption enhance their disease resistance in comparison to European bees? Ramos-Ceullar and colleagues disagree, stating that it was the genotype, and not the environment, that imparted disease resistance (Ramos-Cuellar et al., 2022). No difference in brood or adult bee disease infestation rates were observed when both African and European genotypes were tested in subtropical and temperate climates.
The infection intensities were greater in European bees than Africanised bees, however the occurrence of infestations were not significantly different. Both bee genotypes were infected regardless of their climate, and it was the colonies’ genetic makeup that dictated their response to the infection.
Ecological Niches for Africanised Honey Bees
Ecologically, they are both honey bee species and thus cover the same ecological niche. The adaption of Africanised honey bees to tropical and subtropical conditions means they can outcompete European honey bees (as seen in Central and South America). Their highly aggressive nature protects them from predation and their frequent swarming ensures successful colony dispersal.
Where feral Africanised and European honey bee populations exist, there is a continuous trend of the European genotype to be replaced by the Africanised one. This suggests that the Africanised genotype has a stronger fitness and thus ecological advantage (except at higher latitudes, as mentioned above).
Geffre and colleagues showed that feral Africanised bee populations are more abundant in San Diego, and thrive despite carrying viral disease loads similar in to those in managed colonies. The research suggests that Africanised bees are better resisters of the negative viral affects at the levels a managed hive would require treatment (Geffre et al., 2021). Perhaps this is the next step in the evolution of honey bees?
Africanised Bee Identification
Visual identification of Africanised honey bees is difficult, for they are similar in size and colouration to their European counterparts. So we turn to their behavioural characteristics (which is still difficult – maybe it’s just a queenless hive or a the colony is having a bad day):
- Africanised bees are far more aggressive (“defensive”). They send worker bees more quickly to disturbances and will pursue a threat for over 500 meters (Zarate et al., 2023)
- They swarm more frequently than European bees (once every month or two versus once every year), and will establish nest in spots a European colony would not (Pinto et al., 2005).
- This is harder to discern for the non-expert. After all, how would you know if that spot behind the hot water meter boxes is a normal nesting?
- They are vigorous groomers and cleaners. Their hygienic behaviour makes them less susceptible to pests and diseases, (covered above).
If you live in more tropical climates and Africanised honey bees are established in your location, then a professional assessment from beekeepers and entomologists, or genetic testing will give you a certain confirmation. One article reported that they have differing wing veneration, but this is difficult to discern without expertise that 99.999999% do not have (Zarate et al., 2022).
If you live in cooler climates, then it is highly unlikely you’ve encountered an Africanised honey bee colony, as they do not withstand cold conditions – it could just be a poorly managed colony in need of a new queen.
History of Africanised Honey Bees
In the 1950s, Brazilian geneticists wanted to crossbreed European honey bees (A.m. ligustica & carnica) with African honey bees (A.m. scutellata) to boost local honey production. The current European species used in South America were less productive in the tropical conditions compared to their native environment, so a middle-ground solution was sought.
In 1956, A.m. scutellata queens were imported from Tanzania to commence crossbreeding experiments inside a controlled environment in São Paulo. One year later, 26 swarms of these hybrid bees escaped from quarantine, and the Africanised bees began to interbreed with the local European honey bee populations.
As the hybrid species emerged, it begun moving northwards at a rate of about 200-300 kilometres per year, reaching Central America in the 1970s, Mexico in the late 1980s, and southern United States in 1990. Their ability to outcompete with the local European bee populations and higher swarming frequency made them successful in invading new environments. They reached a natural stopping point in the colder, higher latitudes.
Today, Africanised bees are well-established in many parts of South and Central America and in the southern regions of North America. Efforts to control or eradicate them have been largely unsuccessful, and they remain a species that requires careful management. Beekeepers in Colombia have now learnt to work with these bees and continue the trade (Requier et al., 2017), whereas many others abandoned the profession due to the difficulties in managing these colonies.
European Bees can Benefit by Selective Breeding with African Bees
As the reader can conclude, theoretically a degree of European honey bee Africanisation may be beneficial for managed honey colonies if it’s done in a controlled, selective manner… hence the “theoretically”. Disease and parasite resistance would reduce the need for pesticides use and improve colony health. In the future, a careful selective breeding program aimed to achieving a balance between European and African genotypes is one solution to optimizing honey bee productivity in the Americas.
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Footnotes
- Note that the original European honey bee populations in Central America are far lower than California or Mexico. ↩︎
- A higher rate of swarming is also exhibited by Varroa’s natural host, Apis cerana, who maintain small colonies that swarm up to six times a year. ↩︎