Foraging specialization in bumblebees

A foraging eastern bumblebee, Bombus impatiens. Photo by Michael Motto. 

One of the advantages of living in a social group is division of labor, or task specialization. An animal living on its own must collect all of the resources necessary for its survival, as well as find shelter, find a mate, and (in some cases) rear its young. An animal living in a group can specialize on one or a few of these tasks while other group members perform the other necessary tasks. The eusocial bees, such as honeybees and bumblebees, are an extreme example of social living. Accordingly, it has been proposed that there is extensive task specialization in bees, even just within foraging. Bumblebees are believed to specialize on either foraging for nectar or foraging for pollen, and within each foraging type they may specialize on particular plant species or patch location. Specialization is thought to be particularly advantageous because learning how to forage for nectar or pollen, and learning different flower types, takes time. Specializing, therefore, makes bees more efficient foragers. It is unknown, however, over what kinds of timescales this specialization occurs. Additionally, there are aspects of bee physiology that may influence how bees specialize. The sensitivity of the sensory systems of honeybees determines what they specialize on when foraging. Individual bees with more sensitive olfactory systems for pollen, and bees with less sensitive olfactory systems forage for nectar. Bumblebee workers vary broadly in size. Larger bees are more efficient foragers, and also have larger olfactory sensory systems, but it is unknown how these factors influence bumblebee specialization.

This week's paper addresses the timescale over which bumblebees specialize, and the role of physiology in specialization by bumblebees. It is in Scientific Reports, lead authored by Avery Russell at the University of Arizona. Russell and colleagues examined the nectar and pollen foraging behavior of individual foragers within a colony over their lifespan. To do this they attached a colony to two foraging chambers through a branched tunnel. Individual bees leaving the colony could therefore make a choice in the branched tunnel to go to the foraging chamber that always had pollen or the chamber that always had nectar. To record the choices of individual bees there was an radio frequency identification (RFID) reader at the entrance to each foraging chamber, and a 1.5 × 1.0 × 0.5 mm RFID tag was glued to the back of each foraging bee. The RFID system then conducted automated data collection of which bees entered and exited which foraging arena over their lifespans. Once a bee died, Russell and colleagues measured a wide range of morphological characteristics of that bee including body size and the number of sensory receptors on the antennae. 

The authors found that most bees were generalists over their lifetime (foraging for both pollen and nectar) but over the course of a day or two, many bees (about 50%) would specialize on one food type. Additionally, the work of foraging was performed predominantly by a minority of bees. 17% of bees performed half of the foraging bouts on a daily basis. Bees that more olfactory sensory receptors foraged more for pollen over their lifespan, but neither olfactory receptors nor body size influenced the extent of specialization demonstrated by bees. 

Specialization on a day to day basis but not over a lifetime could be ideal if the needs of the colony changes over time. As brood size increases there is more need for pollen to feed developing larvae, whereas as colonies age there is less brood and thereby need for pollen. Additionally, the resources brought back by other foragers may influence colony needs. Specializing for a day or two may enable a bee to forage efficiently but also update foraging behavior with colony needs. This is such a cool experimental design to study the long term foraging choices of individual bees, with a lot of potential to address a wide range of questions!