How Robbing Occurs in Weak Colonies
I started 2024 with a swarm. I walked in the mid-morning sun, hearing the buzzes of the garden. I….Wait… why are getting louder? and why are so many bees flying around aimlessly? Turns out a clump of bees landed on the bottlebrush tree Melaleuca citrina – it was time to deal with a honey bee swarm.
Fortunately, it was a low branch. There are many stories of swarms in trees at inconvenient heights. I shook the swarm into a Langstroth hive and waited a few hours for the bees to settle in. At night, the hive entrance was sealed and the box transported to the apiary. All went well for the next few days until I noticed aggressive flight patterns and erratic arrivals at the landing board – these were not forager bees, rather robber bees.
Why do bees rob? How do they know when to rob weaker hives? Are there environmental triggers for robbing bees, and how is robbing controlled? There is plenty of information on how to stop robbing, and little on the reasons for it.
How to Stop Robbing When it Occurs
Ideally, rehouse the bees away from your apiary for a few months so that the queen lays eggs and the colony stores accumulate. This isn’t easy for the urban beekeepers, where space is a premium. So what are the alternatives?
Add More Bees: Combines and Brood Frames
Depending on your resource availability, the colony population can be boosted by placing sealed brood from other hives (be sure to shake off the donor colony’s bees beforehand!) or combining colonies. The hatching brood provides a population boost, whereas the combine will introduce the population (and resources) of the weak hive to a stronger one. The latter will leave you with one less hive and one stronger colony.
For my captured swarm, I added two frames of capped brood from the main hive. I will judge the success in a few weeks’ time. **Few weeks update: They absconded. Perhaps a combination of robbing stress and colony weakness?
Entrance Reducers Make Hives Easier to Defend
I used a plastic entrance reducer to modify the hive entrance into a space three bees wide; only three bees can enter or exit the hive at any given time. The entrance size can be extended or reduced depending on the colony needs, or sealed up entirely for transport.
A homemade (and more labour intensive) alternative involves layers of gaffer (heavy cotton cloth) tape to seal or reduce the entrance size. Multiple layers are used and the bees’ mandibles are unable to chew through the adhesive. This is more ideal for moving hives.
Installing Robber Screens at the Hive Entrance
A robber screen is a physical, mesh barrier placed at the hive entrance that allows resident bees to fly in and out and deter robbers. It works by altering the entrance, confusing the robbers as they cannot gain access to the hive entrance. The resident bees create their own egress points by utilising the openings created by the screen1.
The screen covers the hive entrance, and is secured using screws or wire. After installation, the resident bees take some time (hours or a day) to adjust to the entrance, and then resume flying as usual. The would-be robbing bees are unable to find a way in and are successfully deterred.
What Makes Bees Rob Weak Colonies?
In the natural beehive, robbing is a rare and costly adaption to manage the nectar dearths of late summer. As the flowering plants die out, bees can turn to robbing as a way to feed their colony. Invading bees attack weaker neighbouring colonies and raid their honey stores. The successful bee draws the honey up into her stomach and transfers it to her nestmates upon return.
As a result of human interventions, bees will perform waggle dances to advise their nestmates of this profitable foraging site (Nieh, 2010). Such an adaption can trick foragers into thinking that this is also a syrup feeder, as they are also a dense source of nutrition. So the bee conducts her dance and recruits additional nestmates to raid the colony.
Extreme robbing can lead to colony collapse when honey stores are depleted, or adds additional stressors to a colony already impacted by pests and disease.
“Bald Bees” as a Sign of Robbing
A telltale sign of robber is a bald bee. The golden, fuzzy hairs on her body are either removed or patchy, exposing the black cuticle (‘skin’) colouration underneath. If a colony gets robbed, beekeepers will suddenly see a lot of these bald bees. Some alive, others dead, along with the other signs of robbing such as chewed combs, and an absence of honey & nectar.
The issue arises when multiple bald bees are spotted. One or two bald bees could mean an older forager, a returning robber (who got her hairs attacked during fighting) or a foreign bee that was accepted by the colony (guard bees tend to scrub down the unknown bees who drift and want to beg their way into the hive). In more extreme cases, the bee could be infected with chronic bee paralysis virus, as the diseased bees have their abdomen hair hacked off by nestmates (Rinderer & Rothenbuhler, 1975)
Note that if you still saw chewed chewed comb and missing honey, but all the dead bees remain fuzzy, then it suggests the colony died first, and the robbing occurred post-mortem – there were no bees alive to fight off the robbers.
Disease, Varroa and Poor Hive Management Produce Weaker Colonies
Is a colony weak because it is affected by disease, or is it the weak colony that gets affected by disease? Both are true, depending on the disease and the hive life history. Honey bee diseases are not equal in severity. Some, like chalkbrood disease and Nosema are manageable and colonies make a full recovery. Others like American foulbrood or varroosis are usually lethal. It is in the latter cases that a colony becomes a robbing target.
As the robbing bees loot the hive, they can ingest AFB spores from contaminated honey, or have a mated Varroa mite drop onto their abdomen. She returns with this affliction, and the previously-healthy colony is now infected, which can cause its own downfall.
What of poor management? Beekeepers with poor technique increase the stress on colonies, making them more susceptible to infection and disease. Poor management includes:
- Inadequate water and nutrition and failing to provide supplementary feeds during times of nectar dearth.
- Paradoxically, this supplementary feeding could be a driver of robbing behaviour.
- Poor hygienic practices, using contaminated equipment and failing to clean tools/replace old comb.
- Failing to monitor for pests and diseases.
Avoid these pitfalls and you’re doing a pretty good job as a beekeeper.
When do Bees Rob?
The period with the highest risk of robbing is in the late summer, when there are nectar dearths. This can be assessed by monitoring the number of sugar feeder visitations. As floral resources dwindle, the number of foragers at the feeder increases. Researchers furthered this by testing feeders with progressively lower sugar concentrations, and found high visitations at the lowest concentrations in late summer2 (Garbuzov et al., 2020). By contrast, the high-concentration solutions were more frequently visited in the spring and early-mid summer.
Another factor driving robbing is that bee populations are at their peak – spring and summer are the optimal brood rearing seasons, and so there is pressure to stockpile resources for the coming winter. For a predatory species like the Asian giant hornets, they can invade honey bee colonies and consume their resources. For a much gentler honey bee, they conduct aggressive robbing.
Robbing in Other Honey Bee Species
Most robbing occurs in apiaries, where colonies are managed. Other honey bees in the Apis genus partake in robbing, with some species robbing from each other. This is because all honey bee hives are fundamentally resource-rich nestings.
The dwarf honey bee, Apis florea robs from A. mellifera colonies. Despite their smaller size (7-10 mm compared to 13-16 mm in A. mellifera), their workers can overwhelm A. mellifera guard bees and enter hives (Chahal et al., 1986). A. florea also robs from A. cerana colonies, and vice versa.
The giant honey bee Apis dorsata robs from A. cerana, whereas both A. cerana and A. koshevikovi (another cavity nesting species) rob from A. dorsata (Koeniger et al., 2002). Considering that A. dorsata nests in clusters (dozens of nests can be found on the same tree), one can expect a high degree of intra-species robbing. However, Paar and colleagues determined that around 1% of workers in an A. dorsata colony were from a foreign colony, and no robbing or aggressive behaviours were noted (Paar et al., 2002). Such a small rate was attributed to natural worker drift. If colonies are a metre apart, it is easy for a returning forager to land onto a new one and continue working as usual.
Within the Asian honey bees (A. florea, A. andreniformis, A. dorsata and A. cerana), Breed and colleagues found no aggressive responses to foreign bees who attempted to enter hives under normal conditions (Breed et al., 2007). The authors hypothesise that this docility could be due to the lower quantities of stored food in Asian honey bee hives and the absence of robbing behaviour. Note that A. cerana did exhibit guarding behaviour, but only under robbing conditions within managed apiaries.
This contrasts the A. mellifera, who are highly aggressive to foreign foragers.
Regulating Robbing Behaviour in Apis mellifera
The waggle dance that bees use to signal a food source is a driver of robbing. Since foragers dance more intensely at a richer food source (sugar feeders), it is likely that the dancing associated with robbing is also more vigorous (Nieh, 2010). Should the defending colony successfully repel the robbing bees, then the defeated foragers return and change dances. The foragers shift from waggle to “tremble” dancing, which stops forager recruitment at the target location and causes them to start receiving/processing the current resources food source (Seely, 1992)3.
More related to robbing are the brief vibratory pulses that serve as stop signals (Nieh, 2010). A returning forager butts her head against the waggle dancer and delivers a brief (~ 150 millisecond) vibration to stop her dancing. This is a negative feedback loop, as foragers will stop dancing and reduce the visits of foraging bees to the target location. Useful in signalling the depletion or unsuitability of a resource, enabling bees to focus on finding a new source.
Nieh noted that stop signals are also conducted by foragers returning from crowded sugar feeders. Overcrowding leads to bees fighting bees from other colonies, and the defeated ones return to their hive to signal the end of forager recruitment to that sugar feeder (Nieh, 2010).
Characteristics of a Robber Bee
Foragers bees specialise on resin, nectar and/or pollen or water gathering, based on the colony needs. There is no fifth adaptation that specialises in “robbing”. Kuszewska & Woyciechowski found that robber bees tend to be shorter lived and risk-taking (Kuszewska & Woyciechowski 2014). Normal foraging bees are more likely to embark in these risk-taking behaviours at the later stages of their life.
Another 2009 study found that workers with a shorter life expectancy undertook riskier tasks more often than those longer-lived ones (Woyciechowski and Moroń, 2009). Basically, the older the bee, the riskier the job, with risk being defined as:
- Foraging farther from the nest,
- Foraging at higher/lower temperatures,
- Foraging in presence of competitors (example: an artificial sugar feeder),
- Robbing.
So the individual robber bee is expected to be older and at the later stages of her life. Brain gene expression in robbing bees showed an increase in genes associate with aggression, which is an adaption that can help a robber combat the guard bees (Grume et al., 2021). This same aggression however cause the robber’s own hive guards to deter her from entering her own hive. What does the outcast do?
How Robbing Spreads in an Apiary
Grume and colleagues showed that the robbing hive also increased its own defensiveness by having more guard bees at the hive entrance (Grume et al., 2021). Increasing guard bee recruitment makes sense in the context of the bee yard: If the majority of the hive population is out robbing, then the hive itself becomes more prone to robbing. This leads to a domino effect that starts from a single weak hive, causing a robbing frenzy and dead bees in the whole apiary.
Their study showed that robbing comes at a cost in forager losses (Grume et al., 2021). The more aggressive foragers are rejected by their own guards, and if they attempt to drift to other hives, they will also elicit aggressive guarding behaviour from that hive’s bees. Even if that bee were to enter, she may have been exposed to pests or diseases in the victim hive, thus spreading the condition to her own hive.
Is Robbing Behaviour Advantageous?
Robbing is a double-edged (triple-edged?) and costly tactic:
- Losing foragers from the robbing colony, as the guard bees react to the aggressive robber and prevent their returning nestmates from entering.
- Putting the robbing colony at risk of being robbed, since most of the foragers are out attacking the weaker hive.
- For those robbers that successfully steal and return to their hive, what else do they bring back with them? Diseases? Mites? Pesticide-laden honey?
I would argue that robbing is not advantageous on the basis of disease transmission and making the robbing colony prone to robbing.. Considering that this behaviour takes place in managed apiaries, beekeepers can intervene to control and prevent robbing from spreading by protecting the weaker hives and offering supplemental feeding.
Read the whole post and found puns, typos mistakes? Voice your displeasure with a strongly-worded message to admin@tgbee.com.au, or by filling this form.
Footnotes
- The resident bees are in their own home, and so have the time to learn the new entrance and exits – after all, this is their colony. The robber bees aim to fly in and fly out with no attention to detail, making them unable to overcome the obstruction and thus fly away. ↩︎
- For those interested, the lower ranges were 0.1 and 0.25 molar [34.2 g/L & 85.5 g/L] sucrose, with 1.5 & 2.0 molar [513 g/L & 684 g/L] sucrose for the upper ranges. Their setup is shown below. ↩︎
- The same is true under normal conditions. If there are periods with rich forage, the tremble dance recruits more workers to prioritise nectar processing from the returning workers (Thom, 2003) ↩︎