Written by – Brandon Hopkins, Washington State University, May 5, 2022
Cold temperatures keep bees from flying. This topic of bee flight and temperature comes up a lot. Almost everything I read says bumble bees and blue orchard bees will forage at lower temperatures than honey bees. Literature will say that bumble bees and blue orchard bees fly at temperatures as low as 45 F and that honey bees don’t forage until 55 F. However, I have seen honey bees with just a bit of sunlight on the hives flying at 45-50 F. Working with the beekeeper to place colonies in open areas with lots of sunshine can help get more bees out and foraging when temperatures are cooler. We have also worked on projects that involve investigation into the flight activity of different subspecies of honey bees. Honey bee subspecies adapted to cold mountainous areas (Alps, Caucasus and Tien Shan mountains) are black and likely use that to absorb energy for flight at lower temperatures compared to “yellow” bees of the tropic and Mediterranean climates. This also brings up the topic of: “what temperatures pollination of the flower is even possible”. This topic is covered well in a summary of a WSU session on cold weather effects on pollination. While there are some varieties of pollinizers effective at temperatures of 41 F; it seems a more common lower temperature range is between 47-50 F in apples. The temperature range seems similar in cherries and apricots with 41 F being the lowest temperature for pollen germination, but is ultimately not suitable for pollen germination. They found pollen germination at 41 F to be lower than 30% which is a minimum value in some of the cultivars tested. In all cultivars tested, temperatures of 50, 59, and 68 F were all well above the 30% germination threshold. The point being that even if bumble bees and blue orchard bees are more active at lower temperatures it might not make much of a difference if the biology of the flower prevents proper pollen germination and pollen tube growth at those lower temperatures.
There are a couple things growers can do to mitigate risk of poor pollination when we get cold temperatures during pollination periods
- Rent lots of strong colonies
Communication between growers and beekeepers is essential.
Colony size best predicts the ability for colony to thermoregulate. It is best for the grower and the beekeeper to have strong colonies during periods of cold weather like we just experienced here in Washington. Weaker colonies are not able to keep the brood area warm and can have some of the brood die from freeze kill when they do not have enough bees to keep the nest temperatures warm enough.
Beekeepers have to make decisions about dividing colonies in the days and weeks after almond pollination and before Washington tree fruit pollination. Often colonies come out of almond pollination very strong (16-18 frames of bees) and the colonies will swarm if the size and population isn’t managed. This often means that colonies need be divided or split to keep from losing swarms and to make up for the previous years’ losses. There are a great number of ways that these splits or divides are managed. Beekeepers can remove adult bees by shaking frames of bees and adding them to new hives with a new queen. They can take frames of brood and add them to weaker colonies (equalizing). They can take frames of bees and brood, add a new queen and create new hives (nucs). There are a great many more strategies for this process of making new colonies and managing the tendency to swarm in the spring. In any case, with great labor and expense, beekeepers can make colonies of a certain size or frame count when they arrive for pollination services. You might see that in some cases beekeepers and growers agree that 6-8 frames of bees per colony is a suitable average for colony size in a pollination setting. Under ideal conditions this is probably acceptable. You might also notice a lot of variability in recommendations for stocking rates. Anywhere from 1 to 8 hives per acre depending on the crop. Because of these two highly variable components (stocking rate and colony size), some systems such as almond pollination there is a push to move more toward a value such as number of frames of bees per acre rather than hives per acre. For example you could rent 10 hives an acre if they average 5 frames of bees or you could rent 5 hives an acre if they average 10 frames of bees. In the end what growers are needing is a certain number of foragers in the trees moving pollen between flowers in a defined window of time. With a very cold spring that window of time for bees to move pollen between flowers is even shorter. In this overly simplified equation, one potential way to balance the shortened time frame would be to increase the number of bees. That can be done by having stronger colonies and/or more colonies.
The stronger the colonies the more foragers they have to send out in mass during short windows of time when the temperatures are suitable for pollination. In a spring like we just had that might mean flight hours for pollination is measured in hours rather than days. The great thing about honey bee hives are that when you get these short windows of suitable temperature you get thousands of foragers bursting out of the boxes and they are very eager to bring pollen back to the hive.
- Diversity of pollinators (develop plantings and practices to have native bees in and around the orchard or buy solitary bees to supplement)
Diversity of pollinators will always be valuable. Studies show that diversity of pollinators can help move bees between tree rows and increase effectiveness of pollination. In some systems it has been shown to increase yields or fruit size. A diversity of pollinators can happen through intention management practices or the purchase of other commercially available bees like bumble bees, blue orchard bees or alfalfa leaf cutter bees. In almond orchards they have found that 1000 blue orchard bees per acre can be used to replace one honey bee hive per acre. I think supplementing with some native pollinators and practices that help establish resident populations are great. Pollinator plantings and floral edges around orchards continue to show mixed and promising results in published studies. Researchers and growers have found multiple benefits to this management practice and is tends to be good for honey bees and native pollinators. However, it is hard to image that with cold temperatures and a shortened pollination window with around 100,000 or more flowers per acre in cherries that anything other than honey bees would be up for the task.
Honey bee hives set for pollination in an apple orchard near Harrah, WA. Photo courtesy of Gib Earl.
Washington State University