Drones
Introduction
I’ve been putting some content together for the Australian Bee Observation Network (ABON), a project at the ANU Bee Lab that aims to collect data on the impact of feral genetic diversity from Varroa as it spreads throughout Australia.
The goals of ABON are:
- collect genetic data from feral honeybees before, during and after the arrival of Varroa as it spreads throughout Australia,
- monitor the spread of Varroa in Australia,
- monitor the diversity and spread of Varroa-borne viruses, and,
- locate surviving feral honeybee populations and identify resistance mechanisms.
ABON depends heavily on the assistance of volunteers for fieldwork and collecting samples from feral colonies that provide the genetic data to support our research. This article – and others that I will write – is a form of “give back” to our supporters, many of whom are beekeepers, to increase their knowledge and skills as they deal with Varroa in their colonies.
The Honeybee Drone
Honeybees are often depicted in literature and poetry as symbols of perseverance, productivity, and plenty. Drones, in contrast, are metaphors for sloth and futility. Though they are often scorned as a drain on colony resources, drones are, in fact, a vital part of their colony.
First, some basic facts about drones. Of the three castes of bees in a colony, the queen and workers are female, and the drones are male. the figure below compares a drone (on the left) alongside two female workers on the right. The main visual differences are:
- drones are larger – nearly 2.5 times the size of the workers;
- the eyes of a drone are much larger than their sisters (about 3 times larger);
- a drone’s thorax is nearly twice as deep as a worker’s, housing powerful flight muscles to drive the larger wings that extend to the end of their abdomens in contrast to the smaller wings of the workers;
- drones often are quite different in their colouring and patterns when compared to their worker counterparts – the genetics of drones come only from the queen and consist of a mixture of the drone grandmother and grandfathers – quite distinct from the genetics of the current colony;
- drone abdomens are not tapered at the end like worker bees and the queen, but square and butt shaped; and
- drones don’t have stingers.
In summary, drones are larger, more powerful in flight, and vastly enhanced in terms of visual and pheromonal sensors compared to the other bees in a colony. As explained below, drones are supreme athletes who compete with each other to gain the prize of mating with a queen on her mating flights.
Other important facts about drones that are not visible in our photograph above include:
- the development cycle from an egg to the emergence of a drone is the longest within the hive at 25 days, whereas the workers are 21 days and 16 days for a queen),
- drone antennae are larger and more sensitive with 16,000 plate receptors; the worker antennae have 2,600 plate receptors,
- drones have smaller tongues and mandibles compared to their sisters and typically only feed in the hive; this is why drones do not forage,
- drone eggs are typically laid in early Spring until late Summer. Peak drone production is during swarm season,
- in most cases, 10-15% of the brood nest comprises of drone brood,
- drones reach sexual maturity around two weeks after they emerge from their cells and usually live for 3-5 weeks, and,
- if a drone is lucky to mate with a virgin queen, the act of inseminating the queen will mortally injure the drone.
Drone Congregation Areas
In most cases, a virgin queen requires the services of 15-20 drones or more during her mating flight to ensure that the colony she is to head is well-provisioned with diverse genetics and sufficient sperm for her lifetime of laying. A colony’s drone population must collectively mate with the queens of 15 or more foreign colonies to spread their genes and maintain their genetic presence in the ecosystem. Our figure below illustrates this concept.
For example, the queen of a particular colony at the left of the figure below will attempt to mate with as many unrelated drones as possible. Her drone prodigy will attempt to mate with as many queens in the ecosystem as they can achieve. If, by some chance, a queen mates with one of her drones, as illustrated in the bottom right corner of the diagram, then she will suffer from a small degree of inbreeding. The chances of this occurring are small in a diverse, random distribution of colonies in a natural ecosystem.
To achieve this genetic diversity efficiently, bees use Drone Congregation Areas (DCA)—specific locations where drones from various hives gather in large numbers to mate with virgin queens.
These areas tend to be consistent year after year despite changes in the surrounding environment. Drones that frequent these areas only live for several weeks, and there is no identified mechanism by which “memory” can be accumulated and passed from one generation of drones to the next.
Key characteristics of DCAs include:
- Consistent Locations: DCAs are often located in the same areas year after year, suggesting that drones and queens have an innate ability to find these locations.
- Height: DCAs are usually found at a certain height above ground, often ranging from 10 to 40 meters, depending on the terrain and vegetation.
- Environmental Cues: Drones are believed to use environmental landmarks and possibly other sensory cues to locate these areas.
- Large Numbers of Drones: Hundreds or even thousands of drones from various colonies gather in a DCA, which increases the likelihood of genetic diversity and reduces inbreeding.
- Mating Flights: Virgin queens fly to these areas to mate, typically mating with multiple drones of 10-20 or more during their flight to ensure genetic diversity within the hive.
- Timing: Mating flights usually occur at specific times of the day, often in the afternoon, when conditions with low wind are ideal for flight.
DCAs play a crucial role in honey bee populations’ reproductive success and genetic health, ensuring that queens can mate with drones from different colonies. Potential DCAs are located over open ground with a northerly aspect in the southern hemisphere, with protection from prevailing winds and easily recognisable features by which drones can navigate, such as roads, rivers or fence lines.
In any DCA with a calm day, most likely in the mid-to-late afternoon, at a temperature of about 18+ degrees Celsius, virgin queens are prepared for several mating flights of about 5-30 minutes per flight, reaching out several kilometres from the hive.
In contrast, drones emerge from their colonies for 3-4 flights per day of about 30-60 minutes each. Flying at about 20 metres altitude, drones locate queens by sensing their pheromones and then by sight at about 50 metres. Drones also release pheromones that help attract virgin queens and other drones. In ideal conditions, many thousands of drones form up to find and mate with virgin queens.
Colony Characteristics and Drones
Honeybees are members of a large order of insects called Hymenoptera. A characteristic of this order of insects is their haplodiploid sex-determination system. Males (drones) develop from unfertilised eggs. They are haploid, meaning they only have half their mother’s chromosomes. As mentioned earlier, the specific genetics carried by the drones are the genetics of the queen’s mother and father.
The queen and her other (female) offspring are diploid and develop from fertilised eggs with chromosomes from their parents (queen and drone fathers). Because of polyandry by the queen (polyandry simply means “many fathers”), the influence of drones’ genetics on the colony results in different sub-groups of related workers called patrilines.
This is most readily seen in the different colouration of workers in a colony. For example, our picture below shows a selection of worker foragers entering their colony. Note the varied patterns and colouration of some of these bees representing at least three patrilines in this image, yet all from the same colony.
Patrilines are important in colonies because they are representative of the genetics accumulated from successful colonies that are within flying distance of the queen when she undertook her mating flights. These patrilines collectively provide the “local” adaptation of a colony to its environment and often greatly influence the character of a colony in behaviours such as pollen hoarding, water gathering and mite defensive behaviours. The drones are the mechanism that transfers these genetics among the colonies in an ecosystem.
