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Heated bees

This semester, I am a teaching assistant for Tufts University’s Experiments in Ecology (a.k.a. BIO 51). BIO 51 is a team taught class where undergraduate students learn about three different study systems, and design their own experiments. This semester’s study systems are honey bees, snails, and tea.

The honey bee unit is led by my adviser Phil Starks. For their honey bee experiments, students will be asked to focus on a particular worker bee task: heat shielding.

A honey bee hive is a living incubator for the developing young (a.k.a. brood). The photo below shows two different brood stages. The white grubs are honey bee larvae (younger brood), and the cells that are capped are honey bee pupae (older brood). No matter which stage the brood is in, adult worker bees have to keep the temperature just right at all times. Honey bee brood should not experience temperatures below 32 °C (89.6 °F) or above 36 °C. (96.8 °F). Too cold and the bees will struggle to fight off infections and find food as adults. Too hot and the bees will likely develop malformed wings, stingers, and/or proboscises (tongues).

When you think about the varying weather conditions honey bee hives are exposed to, 32—36 °C is a pretty tight range. How do they do it?

One way they do it is heat shielding.

Unlike brood, adult bees can withstand temperatures up to 50 °C (122 °F)! When the hive is subjected to localized heat stress (i.e. the summer sun beating down on the hive), adult bees will position themselves directly between the heat source and the brood. These heat shielding bees absorb the heat and thus, protect the brood.

If you put a heating pad on an observation hive for a few minutes and then remove it, you will see that bees have congregated in the heated area with their stomachs pressed up against the glass. The girls showing their bellies are heat shielding. If you have a classroom with an observation hive, this is a great activity to do with your students.

But, what happens after the heat stress has passed? The heat shielding adults have to cool off somehow. And they have to go somewhere—away from the vulnerable brood—to do it. A couple years ago, an undergraduate and I answered the “where?” using observation hives, a theater lamp, insulation, and a thermal imaging camera.

To create localized heat stress, we fit the insulation to each hive, cut out a small rectangle, and heated the cut-out area with a theater lamp. After 15 minutes of heating, we removed the heat source and took thermal images of the hive while it was cooling down.

Following localized heat stress, we found that the heated bees moved out to the edges of the hive. You can see this at 3 minutes in the table below; the red hot area in the experimental hive gets larger before it gets smaller. The hive cools to safe brood-rearing temperatures (no red) in about 12 minutes.

The control hive, which had everything the experimental hive had except adult bees, didn’t completely cool down to safe temperatures. Even after 18 minutes! Here, you can see the heated area gradually got smaller and smaller but didn't actually disappear.

So, the adult bees are actively moving the heat outward, away from the heated center. Remarkably, this is the same way our own cardiovascular system transfers heat!

Unfortunately, the BIO 51 students won’t be able to use a thermal imaging camera for their heat shielding experiments but I am excited to see what they think up. And what they might find!

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