Newly emerged bumblebees are highly susceptible to gut parasite infection

One factor that can affect infection susceptibility is host age, the effects of which vary in a range of ways. For example, susceptibility may increase with age, due to senescence or decrease with age as a result of maturation of the immune system. If certain ages are more susceptible to infection, populations with contrasting demographics, such as same-age cohorts versus a mixture of ages, will exhibit differing disease prevalence. We use the bumblebee, Bombus terrestris, and its interaction with the gut trypanosome Crithidia sp. as a model system to investigate age-related susceptibility in a social insect. Crithidia sp. are widespread and prevalent parasites of bumblebees that are spread between colonies via faeces on flowers when foraging, and within colonies via contact with infected bees and contaminated surfaces and resources. In the field, Bombus spp. live for approximately three weeks. Here, we inoculated bumblebees at 0, 7, 14 and 21 days of age and measured their infection after one week. We also measured the level of gene expression of two antimicrobial peptides important in the defence against Crithidia bombi in bumblebees. We found that younger bumblebees are more susceptible to infection by Crithidia sp. than their older siblings. Specifically, individuals inoculated on their first day of emergence had infection intensities seven days later that were four-fold higher than bees inoculated at 21 days of age. In contrast, the gene expression of two AMPs known to protect against the trypanosome, abaecin and defensin, did not significantly vary with age. These results suggest that age does affect susceptibility to Crithidia sp. infection in B. terrestris. The higher susceptibility of callows may have implications for the susceptibility of colonies at different stages of their lifecycle, due to the contrasting age demography of workers in the colony

Methods matter: the influence of method on infection estimates of the bumblebee parasite Crithidia bombi

The bumblebee gut parasite, Crithidia bombi, is widespread and prevalent in the field. Its interaction with Bombus spp. is a well-established epidemiological model. It is spread faecal-orally between colonies via the shared use of flowers when foraging. Accurately measuring the level of infection in bumblebees is important for assessing its distribution in the field, and also when conducting epidemiological experiments. Studies generally use 1 of 2 methods for measuring infection. One approach measures infection in faeces whereas the other method measures infection in guts. We tested whether the method of measuring infection affected the estimation of infection. Bumblebees were inoculated with a standardized inoculum and infection was measured 1 week later using either the faecal or gut method. We found that when the gut method was used to measure infection intensity estimates were significantly different to and approximately double those from the faecal method. These results have implications for the interpretation of previous study results and for the planning of future studies. Given the importance of bumblebees as pollinators, the impact of C. bombi on bumblebee health, and its use as an epidemiological model, we call on researchers to move towards consistent quantification of infections to enable future comparisons and meta-analyses of studies