Symptomatic Infection with Vairimorpha spp. Decreases Diapause Survival in a Wild Bumble Bee Species (Bombus griseocollis)

Abstract

Vairimorpha, a microsporidian parasite (previously classified as Nosema), has been implicated in the decline of wild bumble bee species in North America. Previous studies examining its influence on colony performance have displayed variable results, from extremely detrimental effects to no observable influence, and little is known about the effects it has on individuals during the winter diapause, a bottleneck for survival in many annual pollinators. Here, we examined the effect of Vairimorpha infection, body size, and mass on diapause survival in Bombus griseocollis gynes. We demonstrate that gyne survival length in diapause is negatively affected by symptomatic Vairimorpha infection of the maternal colony but does not correlate with individual pathogen load. Our findings further indicate that increased body mass offers a protective effect against mortality during diapause in infected, but not in healthy, gynes. This suggests that access to adequate nutritional resources prior to diapause might offset the harmful effect of Vairimorpha infection.

Keywords: Vairimorpha; bumblebee; diapause; nutrition; parasitism.

Figure 1

The prevalence of Vairimorpha sporonts in the gut smears of gynes from symptomatic and non-symptomatic colonies. (A) The average number of sporonts per field of view (average of 4 fields of view). Data are presented as boxplots displaying the median value as a line and 25–75 quartiles with dots above each box indicating outliers. (B) An image of a gut smear of a heavily infected gyne from a symptomatic colony. A representative sporont is indicated by a black circle. Scale bar = 10 µm. (C) An image of a gut smear of a gyne with no infection of Vairimorpha from a healthy colony.

Figure 2

Survival of gynes from colonies with and without symptoms of Vairimorpha bombi infection in cold storage. (A) Percentage of surviving gynes from symptomatic colonies (n = 16 gynes) and healthy colonies (n = 20 gynes). Gynes were monitored for survival weekly until the last gyne had died. (B) Length of survival in cold storage of gynes from symptomatic (n = 16 gynes) and healthy (n = 20 gynes) colonies as a function of the average sporont counts in gut smears. R² values are based on a linear fit.

Figure 3

The relationship between body size and mass and cold storage survival in gynes from symptomatic and healthy colonies. (A) Cold storage survival as a function of body mass (n = 36). (B) Cold storage survival as a function of head capsule width (n = 24). R² values are based on a linear fit.