By Alison McAfee and Emily Huxter

Originally published in Hivelights.

In June of 2021, Western Canada and the Pacific Northwest experienced a heatwave unlike any other. Temperatures broke records, with the hottest clocking in at 49.6 °C in Lytton, BC, shortly before a wildfire devastated the village. But humans are not the only ones who suffered in the heat-wave; honey bees, too, struggled to keep cool.

Although honey bees can cool down their hives by bringing home water and fanning their wings to speed up evaporation, the extreme temperatures in BC’s interior were overwhelming. Some beekeepers reported death of approximately half of the nucs they produced shortly before or during the heatwave, while others observed queen loss and an abundance of dead drones in their apiaries.

What can we, as beekeepers, do to help keep nucs cool? During another round of hot weather that was forecasted for July and August, 2021, we tested two different methods for heat protection – insulating lids and feeding light syrup – and evaluated their efficacy over a twelve-day period. We expected that insulating the lids with a polystyrene foam cover would prevent intense, top-down solar radiation from heating up the nucs, whereas feeding light syrup may facilitate cooling by providing a convenient water source and encouraging evaporative cooling.


We placed temperature loggers under the lids of eighteen five-frame, wooden nucs and recorded temperatures every ten minutes from July 28th to August 9th, 2021. Six of the nucs were covered with a sheet of 2” thick polystyrene foam, six nucs received a 2 L feeder pail of 1:1 sugar syrup, and six nucs were left untreated (see photographs). All nucs were spaced 1.5” apart, had the same entrance size (which was about half the width of the bottom board – the other half was occupied by empty boardman feeders), and loggers were placed off-centre toward the back of the lid to avoid obstructing the top feeding hole. Half way through the experiment (August 3rd) feeder pails were refilled. Nucs alternated in orientation, facing either North or South, and ambient temperatures were recorded with another temperature logger placed inside a solar shield fastened to the side of a nearby house.

Experimental nucs received either a polystyrene foam cover (back row) or were fed light syrup (front row). Photos by Emily Huxter.


Over the observation period, the lowest and highest recorded ambient temperatures were 9.5 °C and 35.5 °C respectively. In the control hives, internal temperatures ranged from 14.5 °C to 41.5 °C, syrup-fed hives ranged from 16 °C to 40.5 °C and polystyrene-covered hives ranged from 25.5 °C to 39 °C (Figure 1). The polystyrene cover clearly had a temperature-stabilizing effect, reducing the maximum temperatures while increasing the nightly low temperatures. The temperature fluctuations in the syrup-fed hives, however, followed similar patterns as the untreated controls, although slightly less extreme. There was no effect of north- versus south-facing entrances.

Figure 1. Ambient and internal temperature data for control (a), syrup (b), and polystyrene foam (c) nuc treatments. Day 1 corresponds to July 28th, 2021.

Since we were most concerned about temperature extremes, we calculated the average daily high and low temperatures experienced by each colony. We also extracted the absolute maximum and minimum temperatures recorded during the monitoring period. These data show that feeding syrup reduced the average daily high temperature by 1.1 °C, but this effect was marginally not significant (Figure 2a). The polystyrene foam cover, however, reduced the average daily high temperature by 3.8 °C and this effect was highly significant. The foam also significantly increased the average daily low temperatures by 4.6 °C. The absolute maximum temperatures experienced by these colonies mirrors these trends (Figure 2b).

Figure 2. Temperature extremes. Average daily high and low temperatures (a) and absolute maximum and minimum temperatures (b) experienced by the colonies are shown. Statistics were performed using a one-way ANOVA for high and low temperatures separately.


Radiant heat can be a significant source of stress for honey bees, especially for nucs and mating nucs, whose small size reduces their ability to thermoregulate. Although the ambient temperature did not exceed 35.5 °C during our monitoring period, the temperature under the lids of control nucs climbed up to 41.5 °C, highlighting that the ambient temperature is not a good indicator of the conditions inside a hive. Care should be taken when managing hives at high temperatures, especially when introducing new queens, which are often placed near the top bars where the hive is the hottest.

Our data clearly show that an external, 2” polystyrene foam cover can significantly reduce the temperature under the lid during hot weather. In this experiment, the simple insulation method reduced the average daily high temperature inside the nuc by 3.8 °C, which likely reduces the risk of temperature stress for the queen and developing brood, which are both sensitive to temperature fluctuations.

Such a cover is easy to apply; if all hives are leveled at the same angle, a single sheet can rest across multiple hives, avoiding the need to cut the foam to size. We recommend using foam covers or insulated lids throughout the year, since it protects against the heat during the summer, while retaining heat and preventing condensation during the winter.

The syrup treatment tended to lower the temperature under the lid, and although the difference was not quite significant, it might still be a good idea to feed light syrup during a heatwave. We did not measure core nest temperatures in this experiment, which might benefit more from syrup evaporation than lid insulation. This is because the bees can deposit water or syrup on comb and into cells throughout the nest, which then evaporates, to target areas that may need more cooling than others.

Exceptionally extreme conditions, like those experienced during the June 2021 heatwave, may require more elaborate methods to protect hives from heat. Additional factors remain to be tested, such as combinations of polystyrene foam, syrup feeding, shade nets, the size of the entrance, and effects on internal versus peripheral nest temperatures. However, this work already shows that the simple, inexpensive approach of lid insulation offers some protection.