Assessing risk of conopid parasitism in foraging bumblebees using radio frequency technology
Bumblebees (Bombus spp.) are social insects that play a vital role in the pollination of commercial and wild plants worldwide. Bumblebees demand high amounts of pollen and nectar to maintain and grow their colonies, and colony size is closely related to survival and reproduction. Trips to return required food resources expose foragers to various risks outside of the colony. One such risk is parasitism by endoparasitoid conopid flies (Dipera: Conopidae) that use bumblebees as host organisms. Conopid parasitism can impact individual foraging activity, reduce worker lifespan, and may limit colony success by reducing the size of a colony’s worker force. This study, conducted at Blandy Experimental Farm (Boyce, VA), examined how cumulative time spent away from the colony influences the risk of parasitism experienced by individual bumblebee workers during mid-summer when conopid flies are seasonally abundant locally. This was achieved by using a radio-frequency identification (RFID) system to monitor the activity of radio-tagged individuals in B. impatiens colonies over two 8-day periods (June 16-23 and June 29-July 6). We assessed the number of flights each tagged bee took and its cumulative flight time during an observation period. Each bee was dissected after the observation period to determine its parasitism status.
We adhered transponders to 384 bees and recovered 275 of them. Of the recovered bees that were recorded as leaving the colony, 57.4% were parasitized. Our results demonstrate that bumblebees experienced a substantially greater risk of parasitism per unit of time spent away from the colony in early July than in mid-June. In mid-June, bumblebees incurred a 50% chance of becoming parasitized after ~30 hours of flight time, whereas in early July this probability was reached after only ~17 hours of flight. Making assumptions about the average time it takes to return a pollen load and the amount of pollen required to provision one bee larva, we estimated that during peak parasitism in July, nearly every forager would be infected during the time required to fully provision a single offspring. Because bigger colonies are more likely to reproduce, it seems that conopid fly parasitism could have a significant impact on colony success; if a colony cannot achieve a healthy size, it may fail to reproduce. Further research is needed to investigate how conopids might influence the success of bumblebee colonies and the role that colony size might play in surviving seasonally frequent attacks by this parasitoid.