Death and diminishment: parasitoid flies (Diptera: Conopidae) reduce foraging efficiency before killing their bumblebee host

Host-parasitoid interactions typically result in either a dead parasitoid or a dead host. Understanding the effects of parasitoid success on a host can be estimated primarily as how much an early death curtails host reproduction. When parasitoids attack the nonreproductive caste of social insects, however, the effects are not the reduced reproduction of the host but rather the sum reduction in host contributions to its colony. In addition to the loss of host workdays due to premature death, there is potential for additional cost through reduction in foraging efficiency as the infection develops. To better understand these pre-lethal effects, we allowed conopid parasitoid flies (Conopidae) to infect workers from a colony of the bumblebee Bombus impatiens (Apidae) in the lab and then moved the colony to an outdoor location. Bumblebee foragers were monitored using RFID technology and an automated analytical balance positioned between the colony and the outside environment. We found that infected bumblebees foraged similarly to uninfected workers halfway through their fatal infections. Starting at day 6-7, however, infected bees took fewer trips per day, which resulted in a significant reduction in resources returned to the colony over the last 3 days of the experiment. Both infected and uninfected bees were likely to remain out of the colony at night after their fourth day foraging, but infected bees started staying out sooner. These pre-lethal effects of a developing parasitoid add to the negative effects of a shortened lifespan on host contribution to its colony.