It’s getting to that time of year – when robbing bees become far more common. You surely haven’t had time in a ‘robby’ apiary to divert your mind to why, and how, this commotion of bees is occurring. Luckily, resident science writer Dave Black has given it some thought.
By Dave Black
Humans have known for a long time that stealing honey from bee’s nests is a pretty risky, if potentially rewarding, foraging tactic. Bees know it too, and yet, it’s surprisingly unusual – except in the European honey bee Apis mellifera. It might be true that domesticated bees are being monitored more carefully, but there is relatively very little evidence of wild bees robbing. It’s not crazy to suggest that, far from being a natural behaviour, the amount of robbing we see amongst managed European honey bees is an aberration, at least in part a consequence of our management of them and their environment. Have we somehow changed the risk/reward calculation?[i]
Rob or ‘Aggressively Compete’
It’s not at all clear that open nesting bees are more likely to be robbed than cavity nesters, or whether large stores of food are more or less vulnerable than small ones. In nature the open-nesting Dwarf honey bees (A. florea, A.andreniformis) are not known for being aggressive or robbing, although tiny A. florea bees have been seen behaving aggressively towards A. mellifera at feeders, and on the rare occasions they were seen robbing, were far more successful at stinging A. mellifera while escaping the same fate[ii].
The Asian honey bee (A. cerana) and A. mellifera are known to rob each other (mellifera generally, but not always, wins). Giant honey bees (A. dorsata), despite their size, are not known for avarice, but will sometimes rob Asian honey bee nests. In turn much smaller Asian and Koschevnikov’s honey bees (A. koschevnikov) have been spotted pilfering Giant honey bee nests. Stingless bees too are known to ‘rob’, or at least ‘aggressively compete’ for a food resource being sequestered by other nests or other species.
In general however, while robbing happens, evolution seems to have crafted a careful détente among bees; warfare is rare. The bees must assess the robbing risks and benefits that present and abandon situations where overly aggressive foraging becomes too risky. Observation shows that colonies preferentially target “winnable” weak and perhaps diseased colonies and quickly call off attacks when strong colonies mount an effective defence.
Opportunistic Conflict
The direct challenge during robbing is between the robbers and the guards from the opposing colonies. We can see that foragers specialise in providing particular ‘services’, collecting resin, nectar, pollen, or water for example, and in bees at least, there are no specialised ‘soldier-type’ individuals likely dedicated to seasonal, opportunistic conflict, either offensive or defensive.
The ‘guard’ bees are young-ish bees (two to three weeks old) on their way to becoming outside workers. The health and immunocompetence of guard bees is on the wane, but not lost[iii]. While their exposure to disease risk increases (they check any and every bee that enters) they are not guards for long (a day or two prior to foraging full-time) and when they are seriously challenged their job is to recruit reinforcements from the unemployed, older foragers who harass and sting the intruder.
What seems to be characteristic of ‘robber’ bees however is that they have little of their life-span left[iv]. They are the oldest bees at the end point of what is a progressive loss of immunocompetence as bees transition from in-hive tasks to outside tasks. In short, they are a small and expendable suicide-squad.
The Logistics of Conflict
Their opportunism presents their colony with a problem though. The decision to rob requires both an increase in forager numbers to simultaneously fight bees from the victim colony and sustain the sudden increase in foraging activity, foraging which at least in part now involves something sticky, viscous, and slower to handle than any floral nectar.
The colony can’t abruptly over-commit and possibly sacrifice all its foragers if it wants to survive. In addition the robber’s colony has to rapidly find the labour to vet the arrivals and to receive the bounty. Many bees will have to stop what they were doing and take up guarding or receiver bee and food-storing roles when all of a sudden high numbers of aggressive bees are found on the threshold with a large quantity of nectar and honey. Are they friend or foe, coming or going?
Guards from robbing colonies must also elevate their defensiveness, even towards their own foragers, just in case. If the conditions are right for robbing, will another colony look to rob them while everyone is busy or away? The dilemma is, how does a colony assess whether the risk is worth the reward? The fact that it requires so much coordination and ‘risk-mitigation’ suggests it is not just opportunist after all.
All in it Together
In the set of dances known to regulate foraging two have been observed to have some relevance in regulating the workers that rob. When receiver bees can’t process the food arriving, rather than performing recruitment ‘waggle’ dances, the foragers switch to ‘tremble’ dancing to recruit more receiver bees instead. Tremble dancing is understood to trigger several different kinds of alternative behaviours, an instruction the equivalent of, “Stop that and go and make yourself useful”. A forager may also produce a ‘stop’ signal. This is an alarm often directed at a dancer recruiting (‘waggling’) for a destination that turned out to be ‘dangerous’ (as a result of wasp or spiders attack for instance), but can be co-opted to discourage recruitment when the level of competition or aggression is too high.
Robbing is a decision that requires consensus from the whole colony, even when it’s making the best of a bad situation. From the colony’s point of view robbing needn’t have the adversarial and exceptional connotation we apply, it is merely one point on a spectrum of foraging behaviours. If robbery can be managed in the normal course of events it will be, if it can’t, it won’t be. So-called ‘silent’ robbing even occurs in the midst normal foraging and, behaviourally, cannot be distinguished from other foraging. Beekeepers hardly ever notice it.
In broad terms the causes of this ‘aberrant’ foraging are reasonably predictable. Bees take the risk because there is very little or no food that is easier to harvest. It’s feast, or famine. A seasonal floral resource shortage will eventually force them to both exploit any stored food they discover and to be more defensive of their own stores. Sometimes that can be pinned on us. Nutritional stress from competition with each other and lack of floral diversity is significantly greater when large colonies of honey bees are densely packed into small areas, or where crop and grazing land bloom results in boom-and-bust periods of resource availability, and becomes even more pressing if temperatures drop and nectar processing becomes more energetically expensive.
Most beekeepers know the signs.
Dave Black is a commercial-beekeeper-turned-hobbyist, now retired. He is a regular science writer providing commentary on “what the books don't tell you”, via his Substack Beyond Bee Books, to which you can subscribe here.
References
[i]Rittschof, C.C., Nieh, J.C., 2021. Honey robbing: could human changes to the environment transform a rare foraging tactic into a maladaptive behavior? Current Opinion in Insect Science 45, 84–90. https://doi.org/10.1016/j.cois.2021.02.005
[ii]Chahal, B.S., Brar, H.S., Gatoria, G.S., Jhajj, H.S., 1986. Aggressive Behaviour of Apis Florea towards Apis Mellifera in Hive Robbing and in Foraging. Journal of Apicultural Research 25, 134–138. https://doi.org/10.1080/00218839.1986.11100706
[iii]Cappa, F., Petrocelli, I., Cini, A., Pepiciello, I., Giovannini, M., Lazzeri, A., Perito, B., Turillazzi, S., Cervo, R., 2020. Immunity of honeybee guards reflects their transition from house bees to foragers. Ethology Ecology & Evolution 32, 289–295. https://doi.org/10.1080/03949370.2019.1695228
[iv]Kuszewska, K., Woyciechowski, M., 2014. Risky robbing is a job for short-lived and infected worker honeybees. Apidologie 45, 537–544. https://doi.org/10.1007/s13592-014-0267-4
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