Despite a fair bit of rain over the last week it’s been warm with lots of bee activity. It’s hard to believe that the Himalayan balsam is still flowering by the river Nairn. But it is and the bees are still coming home coloured silvery white! The ivy is in full bloom near Connie’s garden and she reports it full of honey bees and others. This is a good time to check ivy for Asian hornet incursions since the glucose-packed nectar is highly attractive to this potentially threatening insect.
Connie and I spent Tuesday after school counting varroa, scraping propolis off dummy boards and blow torching woodwork. The colony treated with Apilife Var responded well. The bees did a good job of chewing up and clearing away the fragile strips and spreading the ingredients around the hive with a good varroa kill. Fortunately it has been warm enough for it to work well.
Connie got her first sting on Tuesday when a bee got stuck on her leggings. She coped really well but it was was very sore initially. Things quickly improved after an application of Bai Hua You (bottle on the right). A friend who keeps bees in China gave me some a few years ago and it does help with the pain. You can see I’ve used it a bit! It’s what the Chinese beekeepers always keep in their tool kit. It certainly masks the alarm pheromone, as does Tiger Balm which has the same effect. Connie cheered up when I told her that she was a real beekeeper now as all beekeepers get stung from time to time.
Gelda MacGregor’s zoom presentation for the local beekeeping association on bumble bees was very interesting and got me thinking again about thermoregulation and how it is that bumble bees can fly at incredibly low temperatures.
Did you know that there are 250 species of bee in the UK? These comprise 24 species of bumble bees, one honey bee and 225 solitary bees. Solitary bees have no worker caste to raise offspring and only drones and queen are produced. The queen gets mated, overwinters, finds a nesting place in spring and lays eggs. She leaves provisions for the larvae before heading out and leaving them to get on with it. The solitary queen works alone though her nest may be very close to others of the same species. They may even share an entrance.
Bumble bees are social insects because, like the honey bee, they have 2 castes of female; queen and worker and there are separate jobs to do once the spring nest is established. The mated queen overwinters in a sheltered place whilst the drones and workers die off. In spring the queen appears on early plants like willow. She needs to build her own reserves for egg laying so she collects pollen. She also needs a store of nectar for herself so she arranges little pots of honey in waxen cells around her nest. She works alone initially but soon the new worker bumble bees emerge and take on the foraging roles leaving the queen to lay and incubate eggs. The amazing thing about bumble bees is that the queen actually sits on the eggs like a broody hen and transfers heat from her abdomen by conduction to the eggs. I’ll tell you how she is able to keep her abdomen warm enough in a moment.
Bumble bee queens usually mate with only one drone, but some species mate with 2, unlike polygamous honey bee queens whose average number of mates is 7-17. Bumble bee sex takes place on the ground and not in the air like the honey bee. It can last a long time, up to several hours. The reason for this is that the drone produces a sticky substance which seals the queen’s vagina so she cannot mate with another, thus preserving and perpetuating his genes. Unlike the honey bee drone, he lives to see another day, and another mating.
Bumble Bee or Bumblebee?
I use two words because I’m following the entomology rule for insect common names. This rule states that if the insect is what the name implies you write two words. So, it’s ok to write blow fly or house fly because they are true flies from the diptera order. However, the same is not true for butterflies because they are are not flies therefore the names are run together.
Bumble bees originated in the northern hemisphere and are able to fly in the arctic. We hear them working long into some very cool spring evenings here in Nairnshire which is almost on the same latitude as Moscow. How is this possible you may wonder? Well they are very hairy insects with a thick covering of feather-like plumose hairs that trap heat. Like honey bees they can isometrically contract wing muscles to generate heat for which they also require the energy from nectar. But that’s not the full story.
Although the internal organs are pretty similar in honey bees and bumble bees, heat transfer is different. Because bumble bees are larger and hairier, less heat is lost by convection when flying. However, the main difference is in the construction of the circulatory system.
Both have open circulatory systems with long hearts with valves preventing backflow. So, haemolymph (they do not have red blood) is pushed forward to the head. The returning haemolymph sloshes around the body and returns to the heart via the abdomen where it picks up nutrients and gets rid of waste via the equivalent of our kidneys called Malpighian tubules.
Both insects require a thoracic temperature of 35 degrees Celsius to warm the flight muscles but bumble bees can easily overheat because they generate much more heat in their flight muscles than do smaller insects. If you look at the honey bee aorta in my drawing, you will see that as the aorta enters the thorax through the petiole, or waist, it takes on a convoluted cork screw appearance. This is where the aorta becomes a heat exchanger and helps the cold haemolymph moving forwards to the thorax to extract some heat from the hot haemolymph moving rear-wards. This convoluted heat exchanger is situated where it is, as it enters the thorax, so that the rear-wards moving haemolymph gives up heat just before leaving the thorax thus keeping the honey bee thorax warm enough to support flight.
Renowned entomologist and author Bernd Heinrich studied insect thermoregulation in the 1970’s, and we owe our present knowledge to him. Heinrich likens the bumble bee to a Lincoln Continental car that is so big it needs a cooling system of circulating fluid to disperse heat from the motor via a radiator. The honey bee is like the Volkswagon Beetle; much smaller and can be air cooled without any special mechanisms.
Bumble bees need to have a warmer thorax at certain times such as during flight, and at other times a they need a warmer abdomen for incubating eggs. But how is this achievable? If you look at the bumble bee heart and aorta you will notice that it makes a large loop under an insulating air sac (honey bees have these sacs too but I didn’t want to spend too long drawing all the parts in yesterday). These air sacs can insulate the thorax from losing heat to the abdomen. The loop brings cool haemolymph entering the thorax close to the warm haemolymph leaving the thorax having been heated by respiratory muscles. Heat exchange, with heat transferring from a warmer area to a cooler one, is possible with this system because only a few layers of cells separates the two channels. This is called a counter current system. Some vertebrates like seals employ this mechanism enabling them to swim by directing heat to their flippers in icy conditions.
If bumble bees didn’t have this counter current system, heat loss from the thorax would be much greater and there wouldn’t be a continuous flow of haemolymph to carry nutrients for fuelling the muscles. The muscles would easily cool causing problems during foraging. The average length of foraging trips is 1/2- 1.5 hours and, like honey bees, they can cover a three-mile radius which works out as a staggering 28.26 square miles. (3 miles squared =9 x π (Pi 3.14) =28.6 miles).
That’s heat conservation covered, but what about the long foraging trip on a hot day? Bumble bees are prone to overheating but amazingly they can bypass the counter current system and have cool haemolymph going up to the thorax with the warm coming back to abdomen alternately rather than simultaneously. With this situation there is no heat exchange through the cell walls and heat from the thorax is dumped in the abdomen. It is thought that the front part of the ventral diaphragm acts like an alternating switch allowing flow of haemolymph through the same channel but not at the same time.
You’ve probably seen film of kangaroos licking the insides of their front legs and lying in the shade with those parts exposed trying to cool a bit by evaporative heat loss? Well, bumble bees have a similar strategy of evaporative heat loss. They can lose 2-3 degrees C in flight by regurgitating nectar and holding it on the ends of their tongues which they wag about in the breeze.
For more info on bumble bees contact https://www.bumblebeeconservation.org/
Heinrich, B,. 1979, Bumblebee Economics, Harvard University Press, Cambridge.