Bumble bees host a variety of parasites that reduce individual and colony fitness to varying degrees. Two well-known culprits are the trypanosome Crithidia bombi and the widespread fungal pathogen Nosema bombi. Less studied are impacts of a large group of parasites called parasitoids. Ongoing research by T’ai Roulston investigates the evolutionary ecology of interactions between bumble bees and a particular family of parasitoids known as conopids or thick-headed flies (Order: Diptera, Family: Conopidae).
Faced with high infection rates of a deadly parasitoid, how do bumble bees persist? And what types of resistance have bumble bees developed over time? Dr. Roulston and colleagues found that some species, like Bombus impatiens, escape pressures of parasitism by having a reproduction cycle later in the year after peaks in risk of conopid attack have passed. As shown in Fig. 3, B. grisecollis’ cycle overlaps most closely with that of conopids so are most at risk. Therefore B. grisecollis likely has other lines of defense.
How do conopid flies parasitize their bumble bee hosts? Dr. Roulston described the process. Conopids aggressively intercept foraging bumble bees and insert their eggs inside the bees’ abdomen. The larval conopid develops inside the bee abdomen, consuming the abdomen and thorax until hollow (Fig. 2). Astonishingly, during a 12 day period of being internally consumed, infected bees remain active. This bizarre nightmare continues. In an effort to elude scavengers, the conopid fly turns its bee host into a shovel, forcing it to dig its own grave (Müller 1994). The conopid spends a year as a pupa before crawling out of the bee abdomen as an adult fly ready to repeat the cycle. How frequently does conopid parasitism occur? Parasitism rates can be high. Across six species in Northern Virginia the range is 25 - 80% (Malfi & Roulston, 2015).
Dr. Roulston found that bumble bees have internal immune defense against foreign objects. Further, immune strength differs among species. Despite equal levels of parasitism, Bombus griseocollis exhibited the strongest immune response against the invading conopid larvae by covering it with melanin (Fig. 4). Melanin marks conopid larvae as a target then attacks the invader with specialized immune cells. Melanization killed ~ 30% of the conopid larvae found in Bombus griseocollis (Davis, Malfi and Roulston, 2015). Bumble bees have thus evolved various tactics against being consumed alive.
Dr. Roulston’s research provides grisly natural history details while elucidating complex ecological interactions. His work on parasite prevalence and dynamics between conopid flies and their bumble bee hosts helps us better understand how key drivers of bee declines are interconnected. The broader significance of this work can contribute to pollinator conservation efforts. Click here to learn more about his exciting research.
Davis, S. E., R. L. Malfi, and T. H. Roulston. 2015. Species differences in bumblebee immune response predict developmental success of a parasitoid fly. Oecologia. 178: 1017–1032.
Malfi, R. L., S. E. Davis, and T. H. Roulston. 2014. Parasitoid fly induces manipulative grave-digging behaviour differentially across its bumblebee hosts. Anim. Behav. 92: 213–220.
Müller, C. B. 1994. Parasitoid induced digging behaviour in bumblebee workers. Anim. Behav. 48: 961–966.
About the authors:
Hanna Kahl is a master’s student at University of Maryland in Cerruti Hooks’ lab researching the effects of red clover living mulch on arthropod pests and pollinators.
Lisa Kuder is a PhD student from the vanEngelsdorp bee lab. Her research focuses on road ecology, specifically improving highway rights-of-way for pollinators.