How to protect bumblebee colonies safe from killer moths? Keep honeybee hives away from them

Since the pandemic, we are very aware of the power of social distancing to protect against infectious disease. But can social distancing be effective if the infectious agent isn’t a virus or bacterium, but an insect powered by a brain and wings, and with the instinct to seek out new hosts?

Now, a study published to Frontiers in Bee Science has shown that physical distance does play a leading role in protecting bumblebees against a flying insect parasite, the bumblebee wax moth Aphomia sociella. The source of the potentially lethal infection was another species, namely nearby hives of domestic honeybees.

“Here we show that infestation with bumblebee wax moths is much greater in bumblebee colonies that are closer to honeybee apiaries,” said Dr Indrikis Krams, a professor at Daugavpils University and the University of Riga in Latvia, and the corresponding author of the new study.

“Because the presence of the moths lowers the bumblebees’ immune response and output of new workers and queens, the health and productivity of bumblebee colonies is lower near apiaries. This puts further stress on populations of these important pollinators, which are already declining around the world due to habitat loss, pesticides, and global warming.”

Like moths to a hive

Bumblebee wax moths, native in Eurasia and North Africa, seek out nests of bumblebees, honeybees, or wasps to lay their eggs. Young larvae feast on wax cells, pollen and nectar stores, and the corpses and droppings of the hosts, while older larvae attack living host larvae for food. Heavy infestations may wipe out their hosts or drive them abandon the nest. Infestations are easy to spot, as the moth larvae spin dense silk layers to protect themselves.

Krams and colleagues set out to study the impact of nearby honeybee colonies on buff-tailed bumblebees, Bombus terrestris. In 2019 and 2024, they placed a total of 25 boxed bumblebee colonies at a distance between zero and 600 meters from one of six small apiaries near Krāslava, Latvia. They regularly checked each bumblebee colony for the presence of wax moths, and counted open and closed brood cells as well as recently eclosed bees.

The experiment ran from spring until early July, when young bumblebee queens and males disperse to mate.

To gauge the immune response of individual bumblebees, the researchers used a well-established test, the encapsulation response assay. This involved piercing the abdominal exoskeleton of 260 bumblebee workers and introducing a 3mm-long sterile nylon filament through the puncture, which was left inside for five hours and then removed. Arthropods respond to the presence of parasites in their ‘blood’ by recruiting immune cells called hemocytes, which produce the pigment melanin to immobilize and kill the foreign bodies. The shade of the melanin capsule around the retrieved filament was taken as a proxy for the strength of immunity, which tends to increase with the health of the colony.

The results showed that bumblebee colonies closer to an apiary were more likely to become infested with wax moths: whenever this distance increased by one further meter, the probability of infestation decreased by 0.6%. Colonies closer to apiaries were also more productive: for every additional meter apart, the number of bumblebee queen and worker cells increased by 0.14 and 0.44 in healthy colonies, respectively, and by 0.08 and 0.348 cells in infested colonies. Finally, the encapsulation response was 3.5% stronger for every extra meter from the apiary.

Keeping wax moths at a respectful distance

The researchers concluded that bumblebee colonies do worse in terms of immunity and productivity as they are closer to honeybee colonies. This is likely due to a two-punch effect: shorter distances make it more likely that wax moths will disperse to find and attack bumblebee colonies, while the competition over flowers intensifies between the two bee species.

“The compromised immune system of bumblebees in colonies infested with wax moths may in turn make them more susceptible to other parasites or pathogens. This is a concern, because their populations are already declining,” said Krams.

“Conservationists and beekeepers should avoid placing apiaries in areas where pollinator biodiversity is high, as well as in areas where bumblebee populations are in decline.  Plans for introducing honeybee apiaries should be devised to maximize pollination while minimizing the negative effects on bumblebee populations.”