Winter 2024 Hive Deaths

A Case Study

Dad noted two of our hives were less active this winter. Towards the end of the season, there was no activity from either of them. Upon inspection, both had a few dozen workers remaining, not many dead bees and plenty of sealed brood and larvae. Adult small hive beetles were present, and a modest number of mites on the bottom board. A few months prior, both hives completed flumethrin treatment for Varroa. When the dead were tested for American foulbrood, the results were negative, and there was no obvious sign of other diseases.

It’s hard to know what killed them. I concluded that one absconded (solid base hive) and the other had something similar to colony collapse disorder (the queen and a few nurse bees left behind, majority of workers absent). Odd, considering both had plenty of honey and workers in late autumn. Sydney has mild climates with the coldest daytime temperatures around 10-15°C. What went wrong?

Part two on absconding and lead up behaviours can be found here. This article will supplement a video autopsy, to be released… soonish?

Autumn Colony Assessment

The bees were kept in a single wooden, 10-frame full depth Langstroth box. Both had migratory lids, and were distinguished by the base: one solid, the other having a meshed (“screened” or “ventilated”) bottom board filled with oil. Towards the end of autumn, the supers were removed, leaving a single box to assists bees with insulation. I deemed the colonies reasonably healthy, with a low Varroa mite count (<5 mites/ 300 bees). Two frames were fully capped and centre frames had a strong laying pattern. On paper, the hive could overwinter without a worry.

Winter Weather – Mild and Inconsequential

East coast Australia winters are mild – looking at the Bureau of Meteorology’s weather data (Winter 2024 report unavailable as of writing), Sydney had:

Bottom line: these are warm winters with no snowfall. Bees continue brood rearing and foraging- flowering plants are available year-round in suburban areas.

So the bees are healthy and the weather is mild. Perhaps the fault is in Yours Truly’s beekeeping approach?

Hive Base Composition – Solid & Screened

When using a mesh base, I slide a bottom board underneath containing vegetable oil to kill any mite or pest larvae that falls through and deter scavengers (ants, roaches, slugs…) from accessing the hive wastes. If the bottom board was empty, the pests could climb out and return to the hive, or into the soil to pupate and continue their life cycle. This is my preference. Other beekeepers use no bottom board, or leave it empty. In the solid base hive, no such luxury exists. Whatever falls from the frames stays at the bottom until its removed by the workers.

Bottom Board Health Risks vs Insulation

When the bottom board is filled with oil, water that falls from the rain or debris will remain on top, forming two layers and contributes to hive humidity. Great in summer, not so much in winter – the extra moisture and lack of external heat means bees spend more energy maintaining the 35°C and 50% humidity in the brood chamber.

Over time, the hive debris-water combo gives rise to moulds, whose spores can rise with the water vapour and into the hive. In a healthy colony, the mould is suppressed, and work goes on. Under repeated stress in winter clusters? It becomes another stressor on the bees.  

An empty tray provides a layer against escaping heat and assists with insulation. Note that it’s not as potent as a solid base, since air escapes around the edges, unless a seal is made around the board’s borders (Mathialagan et al., 2019). From a temperature control perspective, the ability to manipulate the bottom board means beekeepers can increase or decrease insulation/ventilation according to the season, by adding or removing the board as necessary.

This problem is irrelevant to solid base hives, as water can only via the hive entrance; other avenues are sealed off. Should water enter, it would be in small amounts that could be absorbed by the wood or dried by the bees.

Oil trays: my downfall?

Based on the above, there is a high chance that the oil tray contents led to the downfall of my screened base colony. With each rainy day, small amounts of water were added to the board. Over time and combining it with the hive droppings, the water-mold combo would have stressed out this otherwise-healthy colony. By late winter, the colony could no longer hold, and collapsed.

Hive Positioning – Insufficient Sunlight?

The solid base hive was kept under a mulberry tree. This tree is deciduous, and under ideal conditions provides shade in the summer, and sun in the winter. Due to the arrangement of my garden, I could only place the hives on a west-facing garden bed. I think this lack of sunlight greatly impacted hive health. The colonies could only receive afternoon sun, leaving them cold in the mornings. A double blow, since it reduced the duration of winter sunlight, and increased the intensity of the summer rays.

Lack of morning sun means colonies maintained their evening cluster for longer periods in the morning, delaying the onset of their forage. Additionally, more resources are required to clear out the overnight moisture – a problem more apparent in the mesh board hive… Verdict: lack of morning sun contributed to the hives’ downfall.

Relevance of Pest and Disease in Winter

Pests like small hive beetles and wax moths are of lesser importance, since the low temperatures result in less activity and subsequent presence in healthy colonies. Varroa itself is in a similar situation, as reduced brood rearing means less mite populations. However, should the infestation be high in summer and autumn, Varroa-associated diseases like deformed wing virus begin manifesting.

Modelling work by Martin found that DWV initially had little effects on colony populations (Martin 2001).  When mite populations peaked in late summer, the effects were noticeable on the autumn bees, resulting in a mixed-health population entering winter. This imbalance in the diseased young bees and already-old bees could collapse colonies during winter or early spring. Less mites were required for a colony to die from DWV than acute paralysis virus, who had a higher mortality rate- a larger mite population (10,000+) was required to cause colony collapse from APV*.

Nosema disease impacts colonies more in cold climates as bees are unable to defecate outside. In regions where bees form a winter cluster, overcrowding inside the hive increases the risk of a Nosema epidemic. This can be managed by overwintering hives indoors and taking them outside on warmer days for the bees to perform cleansing flights. However, due to the time and money requirements, indoor overwintering isn’t feasible for the average beekeeper. I saw no signs of “bee diarrhoea” in and around the hive, so I dismiss Nosema as a cause of death.

A matchstick test was done for AFB and the results were negative. Nor where there signs of the discoloured, C-shaped larvae typical of EFB. No chalkbrood mummies were apparent, nor other deformities of the brood nest. Even the cell capping did not have perforations common to Varroa abandoning the now-dead pupa. My verdict: this was NOT directly related to pests and disease.

*This is the error of models – it a colony did have over 10,000 mites in winter, then its problems are far greater than acute paralysis virus; it would more likely collapse before reaching the 10k mite mark. Still, it serves as a useful thought experiment.

Varroa Populations in the Meshed Base Hive

A few days after the death of the meshed base hive, mites were seen on the bottom boards. Did they come from the brood cells? Maybe, however I did not see perforations in the capped brood indicative of Varroa leaving the dead pupa. If these mites fell off the dead bees, then that suggests a high mite population. However, there weren’t many dead bees in the hive at the time of autopsy, so these mites could have come from the brood cells.

The alcohol wash test was conducted late Autumn showing <5 mites per sample. Given that mite populations do not increase in winter, it could be that the number of mites I observed was that of the entire colony, not just a sample of 300 bees. No matter, I consider that Varroa may have had a role to play in this colony’s downfall.

Would Death Have Been Avoided if They Were Overwintered Indoors?

It’s unlikely that overwintering indoors would have spared the colonies, for Australian winters are mild. One protocol for overwintering is to keep bees in a room at ∼5 °C, with air circulation and ventilation systems that exchange heat and carbon dioxide produced by colonies with fresh outdoor air (Referenced in Williams et al., 2010). Such a setup is beyond my abilities. Even if it were available, it’s for beekeepers with hundreds of hives and a truck, not a hobbyist.

Why Didn’t the Empty Hives get Robbed?

I don’t know why the neighbouring colonies didn’t rob. If I had to guess, it’s too cold for the bees to focus on honey and nectar foraging– any foragers would have been collecting fresh pollen for their bees. The hives had been dead for a few weeks; plenty of time for robber bees to commence.

Upon initial inspection, I found a modest amount of ants and adults small hive beetles. No signs of chewed wax comb or dead-by-combat bees was evident. This supports my suggestion that winter is not the time for robbing, since the goal is to survive, not thrive.

If Eggs & Larvae Were Present, Why Didn’t They Make a New Queen?

The answer to this would be the same as to why they weren’t robbed: winter is not the season for rearing queens, even if was relatively mild. Suppose the queen had died due to natural causes. If the colony had reared an emergency queen cell – what drones would mate her? Not many, which increases the chances of poor or no mating. Come spring, she will emerge as a drone layer, and will need replacing anyway.

I Found the Queen in the Meshed Base Hive

A sad discovery. Her Majesty was found upright, dead on the combs within the meshed base hive. Did she starve or freeze to death? I do not know. Based on this finding and the presence of eggs in the cells, I wager that this colony did not abscond, rather the number of workers suddenly decreased causing rapid collapse of the hive.

Verdict: Solid Base Absconded, Meshed Base… Collapsed?

Rambling aside, I believe the solid base hive absconded in winter, contrary to normal bee biology. I could not locate the queen nor her eggs – only mature larvae and capped brood. That hive was weak to begin with, and the colder weather pushed it to leave the wooden box. Unless they found a pre-existing hive, it’s likely they died shortly after leaving. For weak colonies in the future, I would:

• House in plastic Langstroth boxes which provide superior insulation relative to wood.
• Provide supplemental feeding – candied sugar and pollen patties with added micronutrients
• Last resort: winter them indoors/somewhere warmer.

The mesh base hive is a puzzle. The disappearance of worker bees, leaving behind brood, eggs, and plenty of food stores is akin to colony collapse disorder. Pesticides are not an issue in my area, and the Varroa populations were relatively low. Does this mean the mites had transmitted a highly effective viral disease to this colony?

I have one hive remaining, and that one is currently battling a Nosema outbreak. Will she survive the winter? Stay tuned.

A common factor to all three hives is poor location. Their west-facing positioning means they mostly receive afternoon sun and are sheltered from the elements. This poor location (for lack of space in my garden) negatively impacted my colonies and probably made everything worse. As they say in real estate Location, Location, Location.