Empty Hive: Why They Left and Where They Went
Colony loss is a process rather than an event, and it often goes unnoticed. An empty hive can be the first indication of a problem. Honey bees are masters of adaptation and extremely resilient. They readily adjust to changes in their environment and move forward with their work as conditions deteriorate, making it difficult for the beekeeper to predict that the hive will soon be empty. Certain characteristics of colony development are integral in the rapid depopulation of a hive. Becoming familiar with them may help detect an underlying problem before it’s too late.
1. Lifespan is determined by the age at which a honey bee becomes a forager. Social role, not chronological age, regulates honey bee senescence (Munch 2010). A lack of behavioral development correlates with a near lack of biological aging. Once a worker starts foraging, however, mortality rate soars and remaining life expectancy plummets to about a week or so (Visscher 1997).
2. Transition to forager is regulated by “social inhibition”. Foragers produce an inhibitor pheromone that keeps middle-aged bees working inside the hive (Leoncini 2004). When the forager population declines, those bees that no longer receive adequate amounts of pheromone switch to the task of foraging (other factors include hormone and protein levels; Amdam 2002).
3. Bees with a reduced lifespan make an early transition to forager. Because the onset of foraging determines lifespan, bees with a reduced life expectancy experience accelerated behavioral development and begin foraging earlier in life (Woyciechowski 2009).
4. The needs of the colony override individual age in the division of labor. Although a clear correlation exists between age and task assignment, the needs of the colony trump age in the division of labor. A forager may regress to nursing, as a nurse may advance to forager (Guzman-Novoa 2004).
5. Gathering of food is the colony’s highest priority. One of the defining characteristics of a colony in the throes of late-stage collapse is the abandonment of brood. Regardless of age structure or colony size, the need to forage takes priority over all other jobs in the hive (Khoury 2011).
It’s clear that foraging plays a fundamental role in the population dynamics of a colony. It’s the final job, normally performed by the oldest bees in the hive. Therefore, any condition that reduces a bee’s lifespan also reduces the number of foragers. Consider the above concepts in the context of a heavy mite infestation, in which bees that were parasitized in the brood cell emerge with a normal appearance and display no signs of disease but experience a 30% or greater reduction in lifespan (Amdam 2002). This reduces the time available for affected bees to contribute to colony growth and forces younger bees to forage prematurely to compensate for the losses. With the proportion of mites infesting brood hovering above 50%, an overwhelming number of new bees may be subject to the same fate, creating an abnormally high demand for new foragers. This has the devastating effect of drawing more and even younger bees into the caste (Robinson 1996).
The devastation arises from the inadequacy of increasingly younger bees to effectively perform the duties of a forager. Working outside the hive entails an extreme level of both risk and metabolic strain. Bees that begin foraging earlier in life are heavier and have underdeveloped flight muscles, resulting in significant loss of flight efficiency (Vance 2009). The lower the age (development) of the forager the fewer trips that are made, and each trip is of longer duration (Perry 2015). Precocious foragers are at high risk of death from the first flight out. Survival in the field is heavily dependent on experience. One out of every three bees that forage prematurely are lost within the first 30 minutes of flight activity. The same body of research documented workers foraging as young as four days old. Such bees were shown to have only a 20% chance of survival over the course of a 30-minute orientation flight. The inherent risk of foraging, its extreme metabolic costs and the incomplete development of flight capacity in a young bee increases the likelihood she’ll fly out for the first time and never return (Perry 2015).
Mechanisms that regulate division of labor and provide stability in colony social structure are the same that accelerate population decline under a chronically elevated rate of forager loss. The continued recruitment of increasingly younger bees into the foraging role soon becomes an aggressive cancer and eventually causes depopulation of the brood nest (Khoury 2011). The shortage of nurse bees results in malnourished and unincubated brood, leading to a decline in birth rate. It’s a total death spiral.
In the case of mite-induced collapse, a viral epidemic has also gripped the colony. Sick bees voluntarily leave the hive and infested foragers drift to neighboring colonies (Goodwin 2006). Mite collapse is trademarked by the occurrence of dead bees half-emerged from the brood cell. The young adult breaks through the cell capping, only to find an abandoned nursery that has quickly grown too cold and too empty to support eclosion. The remaining mite population is concentrated on a declining number of bees huddled in the brood nest, reducing their life expectancy and accelerating their development. Most or all soon find themselves outside the hive in a foraging role they are unable to fulfill, finally taking off on a one-way flight to nowhere. The wonder is not that colonies collapse so fast, but that it could ever take longer than a week or two. At any time during this process, wasps or other bees may discover the distressed hive and further hasten its demise.
From the outside it would appear the bees suddenly abandoned the hive and fled in an organized fashion, but that does not sync with the chaotic disaster unfolding on the inside. Also, consider that when bees swarm, the queen barely reaches the nearest tree before setting down. If colonies were regularly absconding in the fall, we would find them hanging from a nearby branch like we do during swarm season, but they’re nowhere to be found because they’re dead. In some cases, the remnants may find their way into a nearby hive but most likely do not live long enough for that opportunity. They disperse and die, succumbing to starvation, infection, exhaustion, exposure or predation. Maybe the queen remains with a handful of mite-infested bees, or maybe she was taken by wasps or fled on her own as the roof caved in. Open the hive at the height of depopulation, and you may find her frantically scurrying across the hive wall with no apparent direction (personal observation). As with so many aspects of beekeeping, the same thing can happen many different ways.
Even as the colony fell apart, activity at the entrance may have appeared normal due to the constant replenishment of foragers. Inside there may have been an abundance of brood and honey on the last inspection, but such qualifiers are slow to react (Perry 2015). Detecting collapse early when there is still time to intervene is difficult for any beekeeper. Check for proper age distribution in the brood nest. Plenty of young bees should be there. Take notice of the bee-to-brood ratio. Springtime at the height of the nectar flow is the only time there should be more brood than nurse bees. Look for young bees outside the hive where they don’t belong. Check for adequate defense at the entrance. A colony under severe stress is not hospitable. Overly defensive bees may be all the indication needed to determine that things are not well on the inside. Lastly, beware your strongest hive. A large, productive colony should sound an internal alarm every time. High populations of bees produce high populations of mites. The trouble begins when that colony starts to contract in late summer and fall. The best advice is to be proactive concerning mites and not to rely on snapshot assessments of colony health. Always keep in mind that with your most productive queen comes both a benefit and a burden.
by Peter Somers
The curious case of aging plasticity in honey bees, Münch 2010
Survivorship of foraging honey bees, Visscher 1997
The Regulatory Anatomy of Honey Bee Lifespan, Amdam 2002
Regulation of behavioral maturation by a primer pheromone in honey bees, Leoncini 2004
Life expectancy and onset of foraging in the honeybee, Woyciechowski 2009
Behavioral and life-history components of division of labor in honey bees, Guzman-Novoa 2004
Quantitative Model of Honey Bee Colony Population Dynamics, Khoury 2011
Regulation of honey bee division of labor by colony age demography, Robinson 1996
The effects of age and behavioral development on honey bee flight performance, Vance 2009
Rapid behavioral maturation accelerates failure of stressed honey bee colonies, Perry 2015
Drift of Varroa destructor-infested worker honey bees to neighbouring colonies, Goodwin 2006