Infernal cascade: Chronicle of an announced death
The collapse of a colony is most often multifactorial. Based on four cardinal factors (lack of food, toxins, cold stress, parasites), the colony is subjected to stress, resulting in a reduced immune response and the development of infectious diseases (viral infections and nosemosis).
The infectious loop is initiated (shown in grey in the diagram), but the colony still appears healthy. Sick bees leave the colony through “altruistic suicide.” As long as the colony remains populous, regulatory mechanisms allow the situation to be kept under control. When the population declines, maintaining brood temperature is no longer possible; brood development becomes abnormal or ceases altogether, leading to brood mortality and a further reduction in population through the vicious circle of the cooling loop (shown in blue in the diagram).
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Figure 1: Chronicle of a foretold death (Randy Oliver, 2010; adapted by S. Imboden, 2021); (https://scientificbeekeeping.com/sick-bees-part-2-a-model-of-colony-collapse/)
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Figure 2: The infection, cooling, and starvation loops do not necessarily occur sequentially but generally develop in parallel (S. Imboden, 2021). |
The beekeeper becomes alerted by the stagnation of colony development. A “snowball effect” further reduces the immune response and triggers a new infectious loop. The decline in the worker population accelerates task turnover toward precocious foraging through epigenetic recruitment. Two key factors are involved in bee ageing: vitellogenin levels and the inhibition of “ageing” by ethyl oleate (EO). Research has shown that this molecule—effectively a pheromone produced by foragers (the oldest bees)—plays a crucial role in the maturation and transition of younger bees. It acts as a chemical inhibitor that delays the onset of foraging. The initiation of foraging by younger bees depends on this pheromone and represents one of the key mechanisms of self-organization in response to colony needs. This pheromone emitted by older foragers inhibits the transformation of young bees into foragers in the following ways:
- During strong nectar flows and favorable weather, foragers are outside the hive working. Young bees remaining in the hive are therefore not exposed to ethyl oleate and transition more rapidly into foragers. This allows the colony to mobilize its workforce to exploit abundant nectar flows. The consequence is a temporary shortage of nurse bees in the hive, which stimulates the queen to increase egg laying.
- During poor weather conditions, by contrast, foragers remain confined within the hive and release ethyl oleate. Young bees are thus exposed to the pheromone and remain longer in the nurse stage. A large, confined population builds up in the hive, with a very high proportion of young nurse bees. This imbalance among bee castes frequently triggers swarming behavior, and with the return of good weather, swarming often occurs.
Bee “ageing” depends more on vitellogenin (Vg) levels and activity than on the chronological age of worker bees. Bees that maintain high Vg levels live longer, whereas those with low levels die relatively quickly.
When foragers become scarce, the starvation loop (shown in green in the diagram) is activated. This leads to a weakening of immune defenses due to the loss of the fat body (triggering a new infectious loop) and to metabolic stress, with poorly nourished bees unable to generate heat, thereby inducing a further cooling loop.
The beekeeper becomes concerned by the rapid decline in colony strength, with the bees no longer covering the brood frames. Eventually, only poorly nourished young bees remain in the hive, incapable of providing sufficient food and
