Tsetse fly fertility damaged after just one heatwave, study finds

The effects of a single heatwave were even felt in the offspring of heat exposed parents, with more daughters being born than sons.

The study, published today in Proceedings of the Royal Society B, helps explain why tsetse are declining in some parts of their range in Africa and has important implications for the disease they spread, particularly sleeping sickness in humans and nagana in cattle.

Lead author Dr Hester Weaving from Bristol's School of Biological Sciences said: "A single heatwave damaged both male and female fertility of disease-spreading tsetse flies sending populations into decline.

"Ultimately, heatwaves can drive insect biodiversity loss through both direct death and fertility losses, which is concerning given that heatwaves are increasing in frequency and intensity due to ongoing climate change."

Scientists know that in a lot of animals, fertility is damaged at less extreme temperatures than those which kill them. In some cases, animals can become entirely sterile in response to heat, making them incapable of producing offspring. Generally, male fertility tends to be more temperature sensitive than female fertility, so the current results are surprising.

The team performed experiments in the lab using water baths at Bristol to mimic a heatwave event. To decipher if females or males were more sensitive to heatwave, they heat-exposed them separately and then paired them with unexposed members of the opposite sex. They measured how many offspring the flies produced and deaths over six weeks after the heatwave.

Dr Weaving said: "We looked at this in tsetse flies which spread the disease sleeping sickness in sub-Saharan Africa to humans, livestock and wild animals.

"They are fascinating insects as they develop a single egg at a time, feeding it as a larva in utero with a milk-like substance. The mother will then give birth to the larva which can be the same weight as themselves."

The researchers have shown that male fertility being more heat sensitive is not common to all insects.

Senior author on the study, Dr Sinead English, said: "Our study provides important insights to how climate change will affect disease-carrying insects. We can't assume that patterns in tsetse match those found in better-studied lab systems like seed beetles or fruit flies."

Now further insect species should be measured to see if this result is widespread among other insect species with important implications on their global distributions in the face of climate change.