Watch Your Weight!
How well your bees go into the winter is of paramount importance to the colony’s success next season and its potential to produce a good crop. All the preparations for winter should have been done by now. Assuming your colonies are queen right, the other two most important things are efficient Varroa treatment and ensuring adequate stores are available.
So how much stores do your bees need over the winter?
The answer to this question is complex because it depends on many factors such as colony size, what kind of winter it will be, how well insulated and ventilated the hive is, to name but a few obvious ones. It goes without saying that where in the world you are plays a significant role. In the northern hemisphere, whether you are at the Northern Exposures or Mediterranean will make quite a difference to the amount of stores your bees will consume through the winter. This can range from 8kg for the Mediterranean to over 40kg in northern USA and Canada. Based on local experience one can estimate and ensure that the provisions are on the high side.
In this bulletin we look into how remote monitoring provides useful insight into colony health and status at this important time of the year. Starting with weight of the hive, it is possible to see how much stores the bees have and just as importantly how quickly they are consuming them. Is the weight of this colony within the expected range, and is the weight changing at the expected rate for a healthy productive colony in this place, at this time? Just as telling is the brood temperature of the colony. We will explore how the variation of these parameters alone and in relation to others can help you manage winter preparations and ensure the bees are in the best possible condition.
So let’s look at some examples of hive weights and its significance.
Graph 1 shows a colony in Italy that was still actively rearing brood in October, as identified by a steady brood temperature of about 35°C. Following a late September ivy flow which brought in minimal nectar due to poor weather, the hive weight shows a steady decline in stores. While there is some nectar brought into the hive, namely from Calamintha nepeta (catmint) and Dittrichia viscosa (Inula or sticky fleabane), daily net loss is 200g or less. Knowing the geography I would be confident that this colony is well provisioned for the winter.
Graph 2 provides an example of a steady state in terms of colony metabolism. There is a very small daily net variation in the weight which indicates that the bees are bringing in as much forage as they are consuming. As in the above example this colony is well provisioned of the winter and also shows levels of activity that indicate strong numbers.
Below is an example where hive weight seems to vary transiently on a few occasions more significantly than one would expect. Overlaying rainfall data explains the anomaly immediately. Wooden hives absorb water and gain weight during rainfall which in turn is lost upon evaporation.
If for whatever reason your bees are not provisioned with adequate stores for the winter then there will be a need for supplemental feeding. To quote a 100 year old authoritative source (C.C. Miller, A Thousand Answers to Beekeeping Questions, 1917, Cornell University Library)
Q. When is the best time to feed the bees?
A. The best thing is never to feed them, but let them gather their own stores. But if the season is a failure, as it is some years in most places, then you must feed. The best time for that is just as soon as you know they will need feeding for winter; say in August or September. October does very well, however, and even if you haven’t fed until December, better feed then than to let the bees starve.
While the advice may need to be seen in the context of the accelerated environmental changes of our times it still remains very relevant. A hive scale is an invaluable tool for monitoring weight and managing a feeding regime through autumn and winter. Graph 4 below shows hive weight of a colony which was low on stores going in to August following weak nectar flows in June and July. The colony is fed regularly with sugar syrup from mid-August through to early October, as can be seen from the sharp step increases in weight.
You can also see weight decreases after each addition of syrup, this will be partly due to the bees evaporating the water so it can be capped and stored. However the colony may also be consuming food, which will also contribute to weight loss. However the hive scale data enables the beekeeper to carefully track weight and know when and how much syrup to add to ensure that overall store levels are increased. This hive shows a net weight increase of about 30kg.
Autumn sugar syrup is typically made at a ratio of 2 parts sugar to 1 part water, so it will contain about 33% water. However the bees need to reduce this to about 17% water before it can be capped. The bees will fan to evaporate all this excess water. It is possible to identify this behaviour with the Arnia system. Graph 5 shows a colony being fed sugar syrup between 18 September and 28 September with the associated step change in hive weight.
When fanning activity is added to the graph (red line, graph 6 below) you can clearly see the bees fanning franticly during the period of feeding. As the weather gets colder the sugar syrup gets cooler and the colder air can’t hold as much moisture so it becomes harder and harder for the bees to evaporate the water; also there will be fewer bees to do all the work. They will eventually give up when the syrup temperature drops to about 10°C and it will be left alone in the feeder.
Hive scales are also extremely useful for spotting late nectar flows that could bring in some unexpected last minute honey before winter. Graph 7 shows hive weight from 2 two colonies that were fed in early September, but there was a late nectar flow for about 1 week from 8 October.
Focussing on the weight of one of these colonies and adding the ambient temperature (light green) and flight activity (red line) to the graph it shows that there was a spell of mild weather with daytime temperatures in the mid-teens and strong daily foraging activity.
These late flows will only bring limited nectar to the hive. In the above example there is a small net increase in stores, but mostly it will just help to minimise consumption. Feeding during a late nectar flow will mean that the syrup is mixed with the honey to help build up stores more quickly. This can also make honey from flowers such as ivy less crystallised and maybe easier for the bees to consume.
Weight of the hive is intricately linked to the temperature of the hive and cluster in particular. Unlike other insects, honey bees cluster to retain the heat produced by their metabolism thus allowing them to remain active through the cold season too. Generally speaking the colder the weather, the greater the energy required to maintain the temperature of the cluster, but only to a point, specifically 10°C. At this temperature bees still maintain a loose cluster which is the structure most efficient for thermoregulation. As the temperature rises and the cluster breaks up energy requirements start to increase again as bees become more active and also may start raising brood. Another important factor to consider is that smaller colonies are less efficient at thermoregulating, i.e. they consume more energy per bee to maintain a temperature than do larger colonies.
Monitoring brood temperature provides a valuable indicator of queen and brood status at this time of year. Graph 9 compares the brood temperature of two colonies in the same apiary.
We can see that the brood temperature on Colony 1 (yellow line) is dropping and displaying more variability. However Colony 2 is maintaining a tighter brood temperature (green line) and is likely to have a bigger brood area compared to its neighbour. However is this colony better provisioned for winter? Overlaying the hive weights it becomes apparent that the colonies are comparable (Graph 10 below). Colony 2 starts off with slightly less stores (red line) but it actually consumes less over the period than colony 1. This could be due to a couple of reasons; either the bees are more efficient at thermoregulating the brood or at collecting stores. Either way it indicates a fitter colony.
Lastly, a word of warning, if you find that your bees are gaining weight unseasonably, they could be robbing a dying or collapsed colony. If so, there is a chance that the robbed colony is Varroa ridden and your bees are getting infested. It is beyond the scope of this article to explain the necessity to keep bees responsibly but the Varroa bombs, as they are increasingly being referred to in literature, are a reality and only through education and dissemination can we have a hope of tackling the problem. This situation can occur at any time of the year but October and November is probably when there is most risk as it is the time when unmanaged colonies are most likely to succumb to Varroa associated problems.
Honey bee colony survival during winter depends on worker bee health and on the quantity and quality of stored food. Hive monitoring provides incredibly useful data for beekeepers at this critical time of year. The ability to overlay data from different hive sensors such as hive scales, brood temperature, colony acoustics and the weather provides insight not just into store levels but also queen and brood status and the health and activity of the colony as a whole.