Microbial Pressure and Social Immunity: Bumble Bees Increase Brood Hygiene After Exposure to a Bacillus...

To inhibit the spread of potential pathogens and parasites, social insects exhibit hygienic behaviour, the removal of dead or affected brood. Studies on bumble bees performing this behaviour are limited, compared to honey bees, termites, and ants. With changes in agricultural practices, bees are exposed more often to bioinsecticides, especially microorganisms, which they bring into their nests and might cause damage to brood and adult bees. In a field feeding study, we explored whether an exposure to a biopesticide, comprising the bacterium Bacillus thuringiensis ssp. aizawai (strain: ABTS-1857), affects colony development and hygienic behaviour of Bombus terrestris. Bumble bee colonies were studied in three groups: (1) repeated exposure (5 times) over 18 days to the plant protection product containing B. t. a. ABTS 1857, (2) artificial manipulation of the brood nest by wounding larvae to induce larvae removal, and (3) untreated control. Removed larvae were sampled daily and sorted into different weight and colour categories to analyse larval developmental stages. Over more than 3 weeks, we detected a much higher larvae removal in the B. thuringiensis treated colonies, which started 10 days after first exposure. Most of the removed larvae were light in weight (11-60 mg) and white coloured. Ejected larvae from the controls and brood manipulated colonies were all tested negative for B. thuringiensis, whereas biopesticide treated colonies were positive (PCR detection following selective plating). Regular colony assessments revealed no differences in colony development or production of new sexuals among all treatment groups, but heavier colonies removed more larvae. Further studies should focus on potential mechanisms, for instance the chemical ecology, behind the bioinsecticide induced hygienic behaviour.