Stingless bees remove dead brood to reduce disease transmission in colonies

A study shows that a defense mechanism known as hygienic behavior controls parasites and pathogens that attack these eusocial insects.

By Elton Alisson  |  Agência FAPESP – Ants, termites, bees and other social insects remove dead and infected young as a defense mechanism against the transmission of disease by parasites and pathogens within the colony.

This mechanism, which scientists call “hygienic behavior,” has been studied in detail in the true honey bee, Apis mellifera. Workers of this species use their mandibles to open brood cells containing dead or diseased larvae, then drag them out of the nest.

The hygienic behavior of three Brazilian stingless bee species – Tetragonisca angustula (jatái in Portuguese), Scaptotrigona depilis (mandaguari) and Melipona scutellaris (uruçu) – has now been studied by a group of researchers at the University of Sussex in the UK in collaboration with colleagues at the University of São Paulo’s Luiz de Queiroz College of Agriculture (ESALQ-USP) in Brazil through a project supported by FAPESP.

Their findings are described in an article published in the journal Biology Open.

“We evaluated hygienic behavior in these three species because they’re among the most widely used in Brazil for honey production and crop pollination,” Denise de Araujo Alves, a postdoctoral fellow at ESALQ-USP and one of the authors of the study, told Agência FAPESP.

The researchers collected brood combs from colonies of the three stingless bee species and froze the combs for two days to kill pupae and larvae, thus simulating the effects of a pathogenic agent.

After counting the number of brood cells with dead pupae and larvae in the frozen combs, the researchers reintroduced pieces of the combs into eight hives inhabited by each of the three species. These hives were then inspected every 24 hours for six days to count the number of cells that had been unsealed for removal of pupae and larvae.

The results of the experiment showed high levels of hygienic behavior for all three stingless bee species, which rapidly removed the freeze-killed brood.

M. scutellaris performed best, removing over 99% of dead pupae and larvae within 48 hours. S. depilis removed 80%, and T. angustula removed 62%.

“The hygienic behavior of these three stingless bee species is as efficient as that of A. mellifera,” Alves said.

Interestingly, the researchers observed that in the S. depilis colony with the slowest removal of the freeze-killed brood, 15% of the adult bees emerging from the cells had shriveled wings, thus indicating an as-yet-unidentified disease or disorder with symptoms similar to the effects of deformed wing virus in A. mellifera.

To test the ability of the more hygienic S. depilis colonies to detect and clean out cells with contaminated brood, the researchers performed a second experiment, introducing live brood from the colony that had produced workers with shriveled wings into all eight S. depilis study colonies and quantifying the cleaning out of cells for five days.

The results of this experiment showed that the colonies with higher levels of hygienic behavior in the first experiment were also more efficient in cleaning out live unhealthy brood, detecting and removing 12.5%, whereas colonies that displayed lower levels of hygienic behavior removed only 1%.

“This shows that there are variations within one and the same species. The more hygienic the colony, the faster it detects and removes unhealthy pupae and larvae,” Alves said.

“When we found a large number of workers with shriveled wings outside one of the colonies, we concluded that these bees exited the hive or were evicted by healthy adult workers. If the wing deformation is caused by a pathogen, it isn’t positive for them to remain in the colony.”

Disease management

According to previous research, hygienic behavior in bees is not learned but instead is an instinctive heritable trait of these social insects. In A. mellifera, this defense mechanism has been shown to control parasites and pathogens, such as the varroa mite and deformed wing virus. Colonies of this species that are fully hygienic (defined as removal of more than 95% of the freeze-killed brood within two days) can be obtained by selective queen rearing. 

Studies performed in the past ten years by researchers at the Sussex University’s Laboratory of Apiculture & Social Insects have shown reduced incidence of infection with the varroa mite and deformed wing virus as well as higher survival rates in honey bee colonies with hygienic queens obtained by selective breeding.

Moreover, these colonies produce at least as much honey as non-hygienic colonies, thus suggesting that workers in hygienic colonies do not remove healthy brood by mistake.

“Perhaps this procedure could also be used in the near future to obtain healthier stingless bee colonies for use in large-scale crop pollination,” Alves said. “Selection could occur during in vitro queen rearing to produce colonies with high levels of hygienic behavior for commercial use.”

Brazil is estimated to have about 250 native stingless bee species. Stingless bees (Meliponini) are found in the tropics worldwide and are increasingly used for honey production and crop pollination, but the researchers note that far less is known about their diseases and disease resistance compared with those of A. mellifera.

“Because they’re a highly diverse group and less studied than A. mellifera, stingless bees may well bear diseases that haven’t yet been identified,” Alves said. “However, the low levels of disease generally observed in stingless bees might be due to effective disease control mechanisms, so hygienic behavior could play an important role in the sustainability of stingless bee colonies.”

According to José Maurício Bento, a professor at ESALQ-USP and also a co-author of the paper, chemical communication is essential to the survival of social insects, but little is known about the chemical communication of stingless bees. 

“It’s possible that the chemical signals produced by pupae and larvae in colonies of stingless bees tell adult workers how healthy they are. This would facilitate the detection and removal of unhealthy brood. We’re now interested in the chemical composition of these volatiles, which opens up novel and interesting avenues for research,” Bento said.

The article “Hygienic behavior in Brazilian stingless bees” (doi: 10.1242/bio.018549), by Toufailia, Alves et al., can be read in Biology Open at bio.biologists.org/content/5/11/1712.