Whispering bee brood – a barrier against the dreaded Varroa mite

“With chemical signals, the brood can tell the adult bees ‘this is how old I am now’. The mites eavesdrop to know when it’s time to reproduce. What I’ve found is that brood in mite-resistant bee populations seem able to adjust these signals when mites are present in the hive,” says Nicholas Scaramella at the Department of Ecology.

Varroa mites carry viruses, and infestations often lead to colony collapse within three years. Wild honey bee populations have been severely affected, and beekeepers are forced to monitor and treat their colonies continuously – a costly process in which chemical treatments remain the most reliable option. These substances can leave residues in honey and wax, and may disrupt the bees’ interactions. They also contribute to resistance development in mites, making treatments less effective over time.

Studying bees that show natural resistance or tolerance to the mite is a promising path toward improved bee health. Such colonies have been found on Gotland, in Avignon (France), and Østlandet (Norway).

Nicholas Scaramella investigated these populations in his thesis. A key finding is that brood from the Gotland bees produced significantly lower levels of the chemical compounds that Varroa mites depend on for reproduction, compared to brood from non-resistant colonies.

“The brood was ‘whispering’ rather than ‘shouting’, which made it harder for the mites to pinpoint the right time to reproduce.”

Examining this was a painstaking process. Nicholas monitored the hives to record when the brood cells were sealed – a task performed by worker bees when the larvae enter the pupal stage. Just before the cells are sealed, the Varroa mite enters the cell and waits.

At specific intervals after cell sealing (0, 6, 12, 18, 24 and 36 hours), Nick collected the volatile chemical compounds from brood to capture the critical period when mite reproduction occurs. He then measured these chemical compounds released by the brood at these times.

“My thesis shows that the brood plays an important role in the colony’s defence against the Varroa mite.”

However, the brood only produced lower levels of some chemical signals when mites were present in the hive. This suggests that bees may not activate their defences unnecessarily – possibly because it requires a lot of energy or comes at other costs.

This research opens up new possibilities for developing more sustainable beekeeping – where the bees’ own communication takes centre stage.

Read the thesis

Phenotypic mechanisms of varroa resistant honey bees: from brood signalling to colony-level responses