Clara Stuligross, 2013

Earlham College

Plight of the Bumblebee: Assessing Risk of Parasitism in Foragers

Bumblebees play a vital role in the pollination of commercial and wild plants worldwide. Many species have experienced population declines in recent decades; among these, species foraging late into summer are disproportionately represented. Bumblebees demand high amounts of pollen and nectar to maintain and grow their colonies, and colony size is closely related to survival and reproduction. Foraging to return these critical resources is key to success, but the perils of a forager may be great. One potential pressure on worker force size is parasitism by endoparasitoid conopid flies (Diptera: Conopidae). This research, conducted at Blandy Experimental Farm in the northern Shenandoah Valley of Virginia, examined how cumulative foraging time influences risk of parasitism during mid-summer. Using radio-frequency identification (RFID) technology to monitor the activity of individuals in Bombus impatiens colonies, we found that parasitism increased during mid-summer; bumblebees incurred a 50% chance of becoming parasitized after foraging for approximately 30 hours in mid-June, whereas in early July a bee flew for only about 17 hours before achieving the same probability of parasitism. Given the average amount of pollen collected by foraging bumblebees in a given trip, and the duration of those trips, we estimate that nearly every forager during peak parasitism will be infected during the time it takes to provision a single offspring. Parasitized individuals were also significantly more likely to spend the night away from the colony. Because bigger colonies are more likely to reproduce, the pressure of conopid parasitism, which reduces the lifespan of parasitized workers and, potentially, the size of the worker force and the ability for bees to return critical resources to increase their colony size, can have significant impact on colony survival and success.

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