Bumblebee Parasites

Nosema bombi 
(Microsporidia:Fungi) is an obligate intracellular parasite that can cause systemic infections in bumble bees. Because this organism is not predictably common, the fitness effects of Nosema infections have been somewhat difficult to establish. Studies have shown that Nosema can (i) inhibit colony founding, (ii) inhibit colony growth, and (iii) reduce worker survival.

Further, Dr. Sydney Cameron (University of Illinois) and colleagues published a landmark study in 2011 that examined several bumble bee species across the U.S. over three years and found that Nosema infections tended to be found more frequently in individuals of species experiencing range contractions/declines, and that infections tended to more intense in these declining species when compared with species that have stable populations (Cameron et al. 2011). My own survey study (Malfi & Roulson, 2014), which sampled bumblebees from wild populations in northern Virginia, found Nosema infections to be most prevalent and severe in two locally rare species that have been recently been identified as undergoing range contractions (Colla et al. 2012).

Photo: Nosema bombi spores at 40x using phase contrast microscopy. Credit: Rosemary Malfi.

Photo Credit: E. Wherli
Crithidia bombi is a Protozoan trypanosome that reproduces in the gut of the bumble bee. This organism is a single-celled flagellate, and is easily seen under the microscope because of its motility (see video below). Infections of this parasite are relatively common, and its fitness effects vary widely across studies, which may have to do with context-dependent virulence (see Brown et al. 2003). Infections of this parasite have been shown to (i) alter foraging behavior and impair learning in worker bees, (ii) inhibit colony founding, and (iii) increase worker mortality. Recently, a cryptic species of Crithidia was isolated from bumblebees (C. expoeki) in Alaska and Switzerland and described (Schmid-Hempel & Tognazzo, 2010); it is not yet clear how widespread this species may be in North America.

Photo: Electron microscope photograph of a single-celled Crithidia bombi organism.  Credit: E. Wherli.

Live Crithidia bombi in gut tissue smear @ 40x using phase contrast microscopy.
Video Credit: Rosemary Malfi

Conopid Flies

Conopids are a family of parasitoid flies (Conopidae, Diptera).  A parasitoid is an organism that requires a host in order to complete its own life cycle, but unlike a true parasite, will inevitably kill the host (often, but not always, through consumption). If you have seen the movie Alien, you are already familiar with a fictional parasitoid! Conopid flies, as a family, utilize an array of host organisms, with bees and wasps being the most commonly used. At least four genera of conopid flies utilize bumblebees as hosts across a wide geographic range (North America, Europe, Asia). 

How do conopid flies and bumblebees interact? An adult female conopid fly will assail a foraging bumblebee in flight, and oviposit a single egg into the abdominal cavity of the bee. The egg hatches an endoparasitic larva, which feeds on the host bee's hemolymph (analogous to blood) in early instars. In later instars, the larva consumes the gut tissue of the bee, resulting in the host bee's death about 10-12 days after the egg was oviposited.  Not long after the bee's death, the larva will pupate within the body of its host. The fly overwinters as a pupa and emerges as an adult the following spring.  
Conopid Fly Life Cycle. 
Left to Right: (1) Late instar larva inside dissected bee abdomen, 
(2) Conopid pupa extracted from abdomen, (3) Adult conopid fly (Conops spp.).

Photo Credits (Left to Right): R. Malfi, R.Malfi, Jaco Visser.

Although the life cycle of conopid flies has been generally described, little is understood about the relationship between conopids and bumblebees, and the ecological impact these parasitoids may have on bumblebee populations. More is known about conopid fly species that attack European bumblebee species; less information has been gathered on the species identities, geographical distributions, and ecology of conopid flies in North America. To see the ongoing research my collaborators and I have done on conopid flies, see the Research section of this website.