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Robbing Stress

Posted November 22, 2024

Introduction

Honey bees forage for kilometers across the landscape, and they have sophisticated adaptations that allow them to take advantage of sparse, ephemeral floral resources. During extreme resource scarcity, they can deploy a comparably extreme foraging tactic known as robbing. Robbing is a high risk, high reward, and a tactic whereby workers attack and often kill neighboring colonies to steal honey. However, robbing is relatively rare under natural conditions because it is a high-risk tactic, in which foragers attack, fight, and kill members of the ‘victim’ colony. This places unique demands on the robbing colony, which must mount an offense, but also collect and store an unusually large quantity of food.

Aggression Stress

If colonies use floral resource abundance or other seasonal cues to anticipate robbing threats, they may proactively shift their defensiveness over the season. Food scarcity and defensive behaviors are correlated and peak in the fall when robbing is common, even without evidence of an acute robbing event. Interestingly, brain concentrations of aggression-related biogenic amines, serotonin and dopamine, peak during August and September when robbing peaks, but it is not yet determined whether this variation arises from abiotic (external) cues or robbing experience. Simultaneous expression of two high energy behaviors, aggression and foraging activity, is difficult to sustain, suggesting robbing has unknown colony level energetic consequences that may manifest over time. For individual robbers, it is unclear how foragers accommodate an increase in aggression, as these two phenotypes are typically mutually exclusive, including at the level of neural regulation.

In a social species like the honey bee, changes in foraging strategy require shifts in several groups of specialized workers that are involved in collecting, storing, and processing food. In cases of extreme food shortage, honey bee colonies can switch to a high-risk, high-reward foraging tactic known as robbing, which normally involves stealing mature honey from other colonies but can also be triggered by syrup feeder competition. Colonies engaged in robbing show a corresponding increase in defensive behaviors displayed by specialist guard bees, presumably because the conditions that provoke robbing also increase the risk of colony invasion. Previous studies suggest aggressive behaviors displayed by robbing forager nestmates modulate guard defensiveness and provoked heightened defensiveness from guard bees at their home colony.

The strongest pattern that was observed was that guard bees were more defensive towards returning forager nestmates when there was a high level of bee traffic at the food source the foragers visited. This result, consistent whether assessing colony-level patterns or defensive behaviors directed towards individually foragers, indicates that conflict at a food source gives rise to changes in forager aggression and guard response, possibly explaining the behavioral patterns associated with robbing and an impact of open feeding.

Worker Stress

Honey bees possess an extreme foraging tactic that they employ under conditions of resource scarcity. This tactic, robbing, requires coordinated changes among worker bees to accommodate enhanced food collection, processing, and storing, as well as nest defense. It’s been demonstrated that robbing foragers show unusually high aggression, and that this shift may trigger greater defensiveness from nestmate guards once foragers return home. Looking at the cues that coordinate the change in defensive effort from guards show that forager conflict at the food resource is a strong predictor of guard defensiveness. These findings suggest that guards use behavioral cues from their own foragers to estimate their risk of attack and increase their defensiveness accordingly.

Honey bee aggression, typically performed in the context of nest defense, is sensitive to social and ecological information experienced throughout an individual worker bee’s life. This plasticity presumably allows honey bees to optimize their investment in costly nest defense activities, and to defend the colony collectively. Because nest defense involves a specialized subset of worker bees, these individuals are thought to be most sensitive to aggression modulating information. Also robbing studies suggest that non-defensive specialists (foragers) also adjust their aggression in response to environmental information. These studies emphasize first that there are contexts beyond nest defense that may require plasticity in aggression, and second, that individuals other than defensive specialists adjust their aggression in response to personal experience.

It is interesting to note that studies suggest that foragers may show elevated aggression even after interacting with their own nestmates at a food source. This raises an important unexplored role of context in nestmate recognition and response. Extensive studies show that guard bees use ecological context and personal/colony experience to modulate their defensiveness towards non-nestmates, being more permissive to entry when robbing threats and experiences are reduced. In these cases, guards presumably identify but ignore non-nestmates. However, foragers visiting heavily trafficked resources may be unable to correctly identify nestmates using typical olfactory cues, or they may respond to competitor presence using visual or tactile cues, which supersede or override any olfactory information that identifies a nestmate. It’s been observed that the presence of fighting bees at a colony entrance is sufficient to induce casting, a robbing forager fight behavior, suggesting foragers respond to visual cues at a distance. This suggests that foragers can modify their behavior without physically contacting another bee. Perhaps in addition to transitioning to a robber-typical entry behavior, foragers, upon observing a contested resource, become more aggressive in preparation for fighting, a response that may be agnostic to nestmate identity.

Agricultural and Weather Stress

Colony management, presently a requirement for successful pollination of many cash crops worldwide, is itself a significant source of honey bee stress. For example, outside of the obvious stress to colonies of working hives, during crop bloom in intensive agricultural landscapes, honey bees are densely packed into small areas, leading to nutritional stress from competition and lack of floral diversity. The world-wide transport of Apis mellifera has also introduced novel pathogens and invasive parasites, most notably the Varroa mite, which has decimated colony survivorship and productivity in most regions where it has taken hold. In addition, agricultural land use generally has large effects on bee nutrition. Large monoculture crop bloom results in boom-and-bust periods of resource availability, and honey bee colony health strongly tracks with these feast-famine cycles.

In the U.S.A., a general period of nectar dearth arises annually in the mid-late summer, in part from the loss of native flowering prairie lands that naturally bloom at this time as well as normal warmer and dryer weather. Weather variations can also greatly impact late summer nectar abundance and availability if regions experience unseasonable drying or drought conditions. Two weather patterns that can cause unusual weather fluctuations, opposing and breaking the important Pacific Ocean trade winds driving our weather, and resulting in unusual temperature and moisture fluctuations are called El Niño and La Niña events. These events occur every two to seven years, on average, but they don’t occur on a regular schedule. Generally, El Niño occurs more frequently than La Niña. Episodes of El Niño and La Niña typically last nine to 12 months but can sometimes last for years. The U.S.A. is expected to see a La Niña event over 2024-2025 that will usher in warmer winter temperatures and dryer conditions which are expected to increase honey consumption in the winter and dryer summers leading to limited Fall foraging.

Open Feeding and Exposed Frame Stress

There is little evidence that food type influences forager interactions with guards. Rather, conflict at a feeder or food source is the best predictor of increased aggressive interactions, even when accounting for the effects of seasonal change. Thus, intraspecific conflict at the food resource during robbing may drive shifts in individual forager aggression, activating guard defensiveness as one component of a syndrome of colony-level changes required to accommodate the robbing foraging. Intraspecific competition is when members of the same species compete for limited resources. This leads to a reduction in fitness for both individuals…nestmate against nestmate or non-nestmate.

While honey stores at victim colonies draw in large numbers of foragers during a robbing event, it was found that honey is not sufficient to either attract large numbers of foragers or provoke guard defensiveness once those foragers return home. In contrast, one in four colonies showed evidence that foraging on sucrose, rather than honey, elevated guard defensiveness. Higher attraction of bees to the sucrose feeder may be partially responsible for the treatment difference in colony defensiveness. Because bees from other colonies were also more likely to visit the feeder at this time of year (indicated by increased feeder bee traffic without a concomitant increase in colony foraging traffic), another possibility is that competing with non-nestmates at the feeder has a greater impact on forager aggression than competing with nestmates. Also foraging preferences could explain the elevated attraction to sucrose: it could indicate water seeking behavior, which may be particularly relevant during drought conditions. While honey has a higher overall sugar concentration (~82%), it also has much more glucose and fructose compared to the sucrose feeder (~40% glucose, 35% fructose, 5% sucrose, and 20% water by weight). Higher water sources by weight are likely being preferred in dryer times such as late summer.

Extraction Stress

Honey harvesting can also be a time of increased robbing stress. Exposed frames, open facilities, time in the hives all compound to trigger robbing, defensiveness, and aggression responses. These events can also unnaturally increase the number of bees in a particular area introducing conflict amongst nest and non-nestmates for resources. To address this issues some beekeepers choose to not have any bees on their property where they extract their honey helping reduce robbing behavior but without taking precautionary steps weaker colonies can become targets and wide open entrances locations for defensive aggression.

Queen Stress

The queen, the mother of all bees in the hive, is also deeply affected by robbing. A queen under normal conditions can lay up to 2000 eggs per day. However, the stress of a robbing event and the resulting resource scarcity can significantly decrease her egg-laying rate, slowing the growth of the colony. In severe cases, the queen may even be superseded – replaced by a new, more vigorous queen.

The queen bee does more than just lay eggs – she produces pheromones, chemical signals that maintain the unity and morale of the colony. A strong, healthy queen sends out abundant, potent pheromones, ensuring the worker bees remain diligent in their tasks, including defending the hive. This strong ‘queen presence’ is a natural deterrent for robber bees, who are less likely to risk an attack on a robust, well-organized colony.

Conversely, a weak or failing queen produces fewer and less potent pheromones. The unity of the hive can start to crumble without this ‘pheromone glue’ holding it together, leading to a lack of coordination among worker bees. Without a strong defensive force, the hive can become vulnerable to robbing bees looking for easy targets.

Nest Stress

Nest defense in the honey bee (Apis mellifera) is a complex collective behavior modulated by various interacting social, environmental, and genetic factors. There are marked differences in nest defense behavior amongst the more than two dozen recognized subspecies of the western honey bee. These subspecies fall into several broader biogeographic clades: Africa (A), Western Europe (M), Eastern Europe (C), Middle East (O), and Arabian Peninsula and Eastern Africa (Y). Defensiveness varies greatly within and across groups. However, genetic analysis of honey bees used commercially in the U.S.A. show them to be hybrids whose genomes are made up nearly entirely of a mixture of C and M lineages. In addition, honey bees from the Western European (M) and Middle Eastern (O) lineages have also been introduced to the New World, and often hybridize with honey bees in both feral and managed populations.

Initially, during nectar dearth, individual guards accepted 80% of introduced nest mates and 25% of non-nest mates. As nectar conditions improved, both the intensity of robbing and guarding and the cost of non-nest-mate acceptance declined. In response, individual guards became more permissive to nest mates and non-nest mates until eventually an “accept-all” threshold occurred—all nest mates and non-nest mates were accepted. This data is consistent with a shifting acceptance threshold. A simple linear relationship occurred between the number of guards and the number of fights suggesting that guarding may be regulated by intruder intensity or otherwise regulated in an adaptive manner.

Reduce Robbing Stress

Beekeepers who notice robbing behavior at their colonies should take necessary action to reduce or stop the behavior before it escalates into a frenzy.

Eliminate the stimulus. Bees are attracted to sticky, sugary substances. Remove easy-to-access honey, sugar syrup, refuse combs, etc.
Close or reassemble any colonies being worked while robbing is occurring.
Clean or cover exposed honey or syrup.
Cover or plug any openings in vulnerable hive with tape, grass, or other materials.
Reduce the entrance of the colony being robbed.
Smoking bees in not an effective way to stop robbing behavior.
Some beekeepers report being able to calm the robbing behavior by running overhead sprinkler irrigation on the colonies being robbed. This discourages the robbing bees, who return to their own colonies.
If these steps do not reduce robing behavior, the victim colonies should be removed from the apiary and located elsewhere until they are strong enough to protect themselves.
Work colonies carefully but quickly during nectar dearth.
Do not work more than one colony at a time.
Remove any excess honey that the bees in a colony are unable to protect.
Avoid dripping syrup, nectar or honey on the outside surfaces of or the ground around hives.
Do not leave wax, honey debris, or combs exposed in the apiary.

Conclusion

In essence, robbing is a shock to the biological and social colony system that can unravel the intricate tapestry of roles and responsibilities within the hive. The magnitude of the impact that robbing can have on bees has an impact that extends far beyond just the honey stores. Understanding these dynamics is a crucial part of helping our bees thrive in the hives we provide.

Referenced Materials

Harbo Assay, Varroa Sensitive Hygiene Testing

Mold, should I be concerned?

Spreading Varroa Resistant Traits

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START BEE-TENDING.

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