New Study Sheds Light on How CO2 Affects Bee Reproduction

The team of researchers, led by an entomologist from Penn State's College of Agricultural Sciences, set out to figure out how carbon dioxide appears to bypass diapause, a phase similar to hibernation in which bees sleep during the winter, to trigger the reproductive process in bumblebee queens. Carbon dioxide, the researchers discovered, first caused a change in metabolism, which then caused secondary effects on reproduction.

The findings, published recently in Insect Biochemistry and Molecular Biology, contradicted previous hypotheses. "Previously, it was thought that CO2 directly affected reproduction, but this study provides some of the first evidence that this is unlikely," Etya Amsalem, associate professor of entomology, said. "We discovered that CO2 alters how macronutrients are stored and redistributed in the body. The fact that the reproductive process is initiated is simply a byproduct of these processes."

Carbon dioxide, according to researchers, is commonly used by beekeepers and researchers to sedate bees and other insects.  However, in addition to inducing a calming effect, carbon dioxide can also cause a variety of other physiological responses. While bumblebee queens typically enter diapause during the winter months before beginning a new colony in the spring, bee farmers and researchers may use carbon dioxide to trigger the queens' reproductive process earlier than naturally occurring.

Amsalem stated that she and the other researchers wanted to conduct the study for various reasons. First, because carbon dioxide is so widely used, she believes it is critical to understand its mode of action in insects, particularly bumblebees, where commercial apiaries use it to produce colonies for year-round pollination. The second goal was to understand better how using CO2 could influence research results.

"We know that carbon dioxide has a variety of effects, including behavioural effects," Amsalem explained. "So, if you're studying the effects of a specific manipulation on bee behaviour while also using CO2 as an anesthetic, are you studying the effect of the manipulation or the effect of the CO2?" The researchers conducted two phases of experiments for the study. The researchers first wanted better to understand carbon dioxide's physiological effects on bumblebee queens.

The researchers divided the bees into two groups: those that were not treated and those that were. The bees were then examined three times: immediately after the treatment, three days later, and ten days later. The researchers examined the ovaries and measured macronutrient concentrations in several tissues at each timepoint, which provided information about changes in metabolic function over time. "The CO2-treated queens had higher levels of ovarian activation than the untreated queens," Amsalem said.

"Treated queens also had a shift in macronutrient allocation, with fewer lipids in the 'fat body' and more glycogen [a stored form of glucose] and protein in their ovaries." The researchers determined how carbon dioxide affected metabolism and reproduction with this foundational knowledge. They used carbon dioxide to treat two more groups of queens: one with their ovaries removed and another with a juvenile hormone antagonist. According to the researchers, this antagonist reduces the levels of juvenile hormone in bumble bees, which regulates reproduction and accelerates metabolism. The scientists suspected that this hormone is important in how carbon dioxide affects the physiology of insects.

This time, they discovered that bees with their ovaries removed experienced a similar change in macronutrients to bees in the control group with intact ovaries. The researchers concluded that carbon dioxide affects metabolism first, because bees without reproductive organs experienced the same effect. Furthermore, bees treated with the juvenile hormone inhibitor did not exhibit these metabolic effects, confirming the role of this hormone in mediating the effects of carbon dioxide, according to the researchers.

According to Amsalem, the findings are critical for understanding how carbon dioxide affects bumblebees and all insects. "As scientists, we're trying to find an effect they all have in common so we can find the precise process or mechanism that is creating these effects," she explained. "For example, it promotes bee reproduction but may inhibit reproduction in other insects. It has an inconsistent effect on reproduction. However, we believe it is consistent in how it affects metabolism."

(Source: Pennsylvania State University)