Conopid Flies: The Bumble Bee Body Snatchers

Natural threats among the flowers lurk. Dr. T'ai Roulston delves into the somewhat macabre world of bumble bee parasitism by conopid or thick-headed flies.

Bumble bees (Genus: Bombus) are of special conservation concern. At least five of North America’s 50 Bombus species are either vulnerable or in sharp decline. High losses are alarming because bumble bees are the most ecologically dominant and economically valuable group of wild pollinators. Human-induced drivers of bee declines include climate change, land degradation, and pesticide exposure. Natural threats also lurk and are reminiscent of a classic horror film – body snatching by parasites.

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).

Fig. 1: Conopid fly by T. Roulston

Fig. 2: A conopid larva inside the abdomen of its bumble bee host; Photo by T. Roulston

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).

Fig. 3: Graph showing varied degrees of conopid parasitism risk for three common bumble bee spp. (B. bimaculatus, B. grisecollis and B. impatiens)

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.

Fig. 4: Bumble bee B. grisecollis mounted a stronger melanization response than the other bee spp., increasing its ability to resist conopid infection

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.

References:
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.