Trait Mediated Interactions Affect Honey Bee Foraging

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0075841

Tan, K., Hu, Z., Chen, W., Wang, Z., Wang, Y., & Nieh, J. C.  2013.  Fearful Foragers: Honey Bees Tune Colony and Individual Foraging to Multi-Predator Presence and Food Quality.  Public Library of Science One 8(9): 1-9. 

Trait mediated interactions are indirect predator and prey relationships supplementary to traditional predation.  The fear response of the prey to predators can often cause more widespread effects within an ecosystem.  Tan et al. 2013 explore the role of the presence of predators and food quality on foraging habits of honey bee workers.  Because of the pivotal role of honey bees as pollinators in an ecosystem, predators can cause a cascade effect by decreasing primary producer output indirectly.  Tan et al. 2013 hypothesize that honey bees are deterred from visiting predator occupied flowers and the plants have a decrease in pollination leading to a decrease in seed production.  In order to fully understand the trophic dynamics of ecosystem relationships, trait mediated interactions, not only direct predation, must be measured.

In their 2013 study, Tan et al. studied the proliferation of honey bee pollination during peak hornet season in July to December 2012.  Hornets are known to predate honey bees under natural circumstances, so their indirect effects on honey bees are a viable focal subject for a trophic ecosystem study.  The two hornet species which actively hunt honey bees during this season are V. velutina and V. tropica.  Tan et al. 2013 observed three bee colonies and measured multiple variables in order to determine how the hornets affected pollination habits.  The honey bee workers were collected at the hive entrance and were trained by being taken to an unscented sucrose solution bee feeder.  Honey bees would return to the hive and recruit more workers to collect the solution.  The researchers placed three feeders 30 centimeters apart and treated each feeder with a different treatment:  control, tethered butterfly, and tethered hornet.  Honey bee workers were subsequently observed by their preference of feeder.  In addition to the choice experiments, Tan et al. 2013 also measured the heat-balling predator response for both species of hornets.  Heat-balling is an aggressive defensive response by honey bees to kill predators near the hive by congregating around the offender.

In the heat-balling experiments, honey bees attacked both hornet species, but allocated significantly more mass to killing the larger V. tropica species.  However, in the predation experiments, V. velutina committed significantly more aggressive acts towards pollinating honey bee.  In the colony allocation experiments, significantly more workers were allocated to safe locations (butterfly and control) compared to predator locations (hornet).  Additionally, as predicted, individual bees chose safer foraging sites significantly more than dangerous sites regardless of the richness of fructose.  Lastly, the worker bees spent the most time at safe foraging sites compared to time spent at dangerous sites.  This study conclusively demonstrated the fear response of bees caused a significant effect on foraging strategies.  Despite the predatory hornet being restrained, honey bees were not willing to approach more dangerous feeders.  Predators can exhibit a top down effect on an ecosystem solely by intimidating their prey and triggering avoidance behaviors.