Host manipulation of bumble bee queens bySphaerularianematodes indirectly affects foraging of non-host workers

Sphaerularia bombi Dufour is a major parasite of bumble bee queens that manipulates its host's behavior: parasitized queens do not breed and found nests but continue to fly into the early summer months. We examined the indirect consequences of this host manipulation on non-host workers in central Hokkaido Island, Japan. In this area, parasitism of Bombus terrestris by S. bombi is common but does not affect every queen; therefore, as summer begins, B. terrestris queens continue to dominate some flower patches and disappear from others. At sites dominated by parasitized queens, we found that the nectar standing crop of red clover was smaller, B. terrestris workers carried out fewer legitimate visits to red clover and more nectar robberies, and the workers were smaller than at other sites. Removing queens from a site increased the nectar standing crop of red clover, the frequency of worker visits to red clover, and the size of the workers. These results suggest that host manipulation by S. bombi increased competition for flower resources among host queens and non-host workers and altered the interaction between plants and non-host flower visitors.