What is colony depopulation, and why does it have multiple causes?

Understanding colony depopulation

Colony depopulation is a decline in the adult bee population. Colony Collapse Disorder (CCD) is a more specific syndrome involving rapid loss of adult workers, often leaving the queen, brood and food stores with too few bees to sustain the colony. Depopulation can involve interacting pressures such as Varroa destructor, associated viruses, Nosema infections, acute and sublethal pesticide exposure, hornet predation, heat and nutritional stress. No single causal agent consistently explained the CCD cases in the study by vanEngelsdorp et al. (2009).

Management practices such as brood interruption or late splits can also change colony demography. Poor timing can add pressure when the colony is preparing for winter. Some combinations of stressors can act synergistically and increase risk disproportionately. The effect depends on the stressors and conditions. [vanEngelsdorp & Meixner 2010] [De Miranda & Genersch 2010] [Goulson et al. 2015]

Nosema biology, effects and interactions

Nosema ceranae, now common in many regions, and N. apis are microsporidia that infect the midgut epithelium of adult bees. Infection can alter nutrient use and metabolism and shorten lifespan. It can also affect nurse physiology and the ability to support brood. Changes in orientation, drift or a decline in forager numbers may occur, although these observations alone do not establish infection. Additional stressors can worsen outcomes.

Varroa and associated viruses can burden host defences. Some sublethal pesticide exposures have increased susceptibility to Nosema or spore counts in experiments. Prolonged heat and protein shortage can add physiological pressure, with outcomes varying by conditions. Nosema can contribute to depopulation when colonies face other stresses. [Higes et al. 2008] [Mayack & Naug 2009] [Goblirsch et al. 2013] [Doublet et al. 2015] [Pettis et al. 2012] [Martín‑Hernández et al. 2009]

Varroa amplifies risk, and timing matters

Mite pressure can rise in late summer as the replacement of adult bees slows, increasing the risk of collapse if infestation is left unmanaged

Varroa destructor damages developing and adult bees by feeding on host tissues, particularly the fat body in adult bees. It also transmits viruses, especially deformed wing virus (DWV). Parasitism and viral infection can impair the physiology and longevity of winter bees, whose fat-body reserves and vitellogenin help support overwintering. Timing matters because Varroa should be brought below the locally appropriate intervention threshold before winter bees are reared. In the conditions described here, that means planning and acting from mid-July so the nurses that will rear winter bees develop under lower parasite pressure. The timing should follow local brood dynamics and measured infestation. Late treatment can reduce mites, but it cannot reverse physiological damage already sustained by developing winter bees. [Ramsey et al. 2019] [van Dooremalen et al. 2012] [Rosenkranz et al. 2010] [Di Prisco et al. 2013] [Medrzycki et al. 2010] [Mattila & Otis 2007]

Pesticides and predation reduce bee numbers and disrupt colony activity

Acute poisoning can be accompanied by large numbers of dead bees around the hive. Sublethal effects can be harder to detect. Depending on the substance and exposure, they can affect orientation, feeding or the microbiota. In combination with other pressures, they can reduce forager survival. Hornet predation can suppress departures, increase entrance defence and alter the colony’s energy balance. Small colonies, recently divided colonies and those undergoing brood interruption may be especially vulnerable when the workforce is needed to rear winter bees. Coordination with growers, suitable entrances and physical protection where effective can reduce exposure or give bees more opportunities to forage. [Godfray et al. 2014] [Pettis et al. 2012] [Monceau et al. 2014]

Extreme heat and hive microclimate

During heatwaves more bees may fan or collect water, leaving fewer to forage. Brood at the edges of the nest may face dehydration. Prolonged heat can add physiological stress, and the effects on gut health and infection risk depend on conditions. In small colonies, nuclei or colonies undergoing brood interruption, diverting workers to cooling can slow late-summer recovery and increase the risk of depopulation. Shade, suitable insulation, ventilation adapted to the hive and nearby shaded water can help manage the microclimate. [Human et al. 2006] [Martín‑Hernández et al. 2009]

How interacting pressures can start a downward spiral

One possible sequence starts when brood interruption or a late split creates a gap in worker replacement. Heat can divert bees to ventilation and water collection and reduce pollen foraging. Residual Varroa and viral pressure can shorten adult lifespan, while Nosema infection may further impair workers already under stress.

Hornet predation and possible sublethal pesticide exposure can further reduce flight activity or impair orientation. Even with a queen and stores, a colony may then struggle to cover and rear its brood. As worker numbers decline, recovery becomes harder. Studies of multiple stressors show that their combined effects can be nonlinear under some conditions. [Goulson et al. 2015] [vanEngelsdorp & Meixner 2010]

Winter bees and why preparation can begin in July

Winter bees have greater fat-body reserves, more vitellogenin and a longer lifespan. They maintain thermoregulation and help restart brood rearing near the end of winter. Their development depends partly on the nurses that rear them. Parasitism, viruses, Nosema, heat and inadequate protein can undermine this workforce and the developing cohort. In many temperate areas winter bees are raised from late summer into autumn. The planning window should precede that transition. In the conditions discussed here, mid-July is a practical starting point for monitoring parasite pressure, protein supply and hive microclimate. Care for the workers that will rear winter bees is part of protecting the winter cohort itself. [Amdam et al. 2003] [Seehuus et al. 2006] [Mattila & Otis 2007] [Genersch 2010] [Medrzycki et al. 2010]

Management measures

Use regular, standardised Varroa counts before and after intervention to establish whether infestation has fallen below the relevant threshold in time. Brood interruption can improve the effectiveness of appropriately selected treatments against exposed mites when capped brood is absent. Plan the intervention around colony strength, season and the authorised product instructions. Provide appropriate protein and, if needed, suitable low-HMF syrup as an energy source. Arrange access to shaded water and adequate ventilation during hot weeks.

A sudden forager decline, uneven brood or unusual food consumption has several possible causes. Consider diagnostic testing for Nosema. Hygiene measures can include replacing old comb as appropriate, managing moisture and limiting robbing and drift. Inactivated yeasts, beta-glucans, buffered organic acids and plant-derived products may be marketed for nutritional support. Evidence varies by formulation, and their use should follow local rules and label instructions, including restrictions during honey production.

When pollen is scarce, a suitable protein supplement with a balanced amino acid profile can support nurses and brood. Coordinate pesticide application times and methods with growers, and provide a reliable water source to limit exposure. Against hornets, match the entrance to colony strength and consider entrance protection where it has worked in comparable conditions.

Finally, match population management to the season. Late splits without sufficient stores or parasite control can make colonies more vulnerable. When planning the end of brood interruption, allow enough time for one or more complete brood cycles before winter, according to local conditions and colony strength. [Rosenkranz et al. 2010] [Di Pasquale et al. 2013] [Fries 2010] [Pettis et al. 2012] [Monceau et al. 2014]

A working schedule from mid-July to September

From the second half of July in the conditions described, prioritise reducing Varroa below the local threshold before the winter cohort is reared. Winter workers typically emerge from late summer into autumn in temperate climates. They differ physiologically from summer workers in their fat reserves, vitellogenin levels and lifespan. They maintain the colony through the cold months and support the first spring brood.

Plan a brood interruption or a suitable alternative, and arrange shade, water and nutrition to cover pollen shortages. In August, complete planned interventions, check their effect by counting mites and manage the hive microclimate during heatwaves. Adapt entrances or use physical protection as needed where hornets interfere with flight. In September, check for reinfestation, address weak nuclei and prepare for winter by adjusting insulation, ventilation and stores.

This sequence aims to protect the nurses that rear winter bees and leave colonies strong enough to overwinter and resume development. Adapt dates to local forage, climate and colony condition. [Rosenkranz et al. 2010] [Human et al. 2006] [Fries 2010] [Monceau et al. 2014] [Mattila & Otis 2007]

Economic implications for beekeeping businesses

Protecting the generation that rears winter bees can reduce spring replacement costs. Fewer nuclei may need to be purchased or made, fewer queens replaced and fewer hours spent dealing with losses. Stronger colonies may also be better positioned to use early nectar flows and take on pollination work.

For example, an improvement of 10–15% in overwinter survival could materially affect an annual margin. This is an illustrative scenario. The actual outcome depends on baseline losses, costs, production and prices. Healthier, more uniform colonies can also support more stable revenue and commercial reliability. [vanEngelsdorp & Meixner 2010]

Conclusion

To reduce the risk of depopulation, include the bees that rear winter workers in the management plan. In the conditions described here, the second half of July is an opportunity to act. Bring Varroa below the locally relevant threshold, support protein nutrition, manage heat and hygiene, and reduce pesticide exposure and hornet pressure. An integrated plan timed to colony development can support overwinter survival and production.

Practical note: Follow applicable regulations and product labels. Discuss health and treatment decisions with the responsible veterinarian or local bee health adviser. Interactions among stressors vary with climate, forage and management, so use local observations to plan interventions.

Main references

Genersch 2010; vanEngelsdorp & Meixner 2010; Rosenkranz et al. 2010; Higes et al. 2008; Mayack & Naug 2009; Goblirsch et al. 2013; Doublet et al. 2015; Pettis et al. 2012; Martín‑Hernández et al. 2009; Human et al. 2006; Di Prisco et al. 2013; Medrzycki et al. 2010; Mattila & Otis 2007; Seehuus et al. 2006; De Miranda & Genersch 2010; Goulson et al. 2015, Science. https://doi.org/10.1126/science.1255957

Author: Davide Simeone. Published July 2026