2045 Individual insects can learn and remember an impressive number of cues or motor patterns more or less at the same time (Gould, 1987; Srinivasan et al., 1998). For instance, honeybees can learn to approach a petal in one orientation to get a reward at one time of day and a petal in a different orientation at a different time of day (Gould, 1987). However, insects also behave in ways suggestive of constraints on learning or memory. For example, pollen or nectar foraging insects often show fidelity to flowers of one or a few plant species even though flowers of other species are equally or even more rewarding, a phenomenon known as flower constancy (Waser, 1986). Because constancy increases travel time between flowers, it is potentially costly, and is often explained as a product of learning and memory constraints that limit the number of flower types an individual can efficiently learn and handle (Proctor and Yeo, 1973; Waser, 1983, 1986; Lewis, 1986, 1989; Woodward and Laverty, 1992). Some limited evidence supports this explanation; in bees and butterflies, learning to forage on a novel floral type degrades performance on a previously learned type (Lewis, 1986; Chittka and Thomson, 1997). Errors in a previously learned task caused by learning of new associations with similar stimuli, termed retroactive interference (RI), is thought to be a major cause of forgetting in humans and other animals (Keppel, 1968; Bouton, 1993; Anderson, 2003; Wixted, 2004). For this paper we consider forgetting as either a problem of permanent decay of information or a more temporal problem of failure to retrieve the appropriate information at a given time. Chittka and Thomson (1997) found RI, or mistakes in the first learned task, in bumblebees trained separately on two tasks in successive blocks. If constancy to a floral type can be interpreted in terms of learning and memory constraints, perhaps so too can the tendency that insects have for partitioning their activities in time or among individuals. Many solitary insects, for example, segregate activities, such as feeding and egg-laying, into extended bouts so that different activities are not mingled in time. Social insects are further characterized by a division of labor, in which a given individual within the colony engages in a limited set of activities, such as foraging or nest maintenance, at any one time in its life. In honeybees, foragers additionally specialize in either pollen or nectar collection, although some mixing of these tasks occurs (Page and Fondrk, 1995). In general, division of labor between individuals in social insect colonies can be considered to limit the number of tasks an individual must learn, possibly in part as a response The Journal of Experimental Biology 208, 2045-2053 Published by The Company of Biologists 2005 doi:10.1242/jeb.01582 We examined the effect of learning a new task on the performance of a previously learned task with the same set of visual cues in bumblebees, Bombus impatiens. Previous studies have shown that given a binary choice at each task, bumblebees do not show retroactive interference, or mistakes in the first task, if the two tasks are in different contexts, feeding and nest location. Here we tested whether adding a third, unrewarded choice to each task affects the performance of bees learning in two contexts. In addition, we examined whether workers differ in their expression of interference and learning ability based on size. Performance of workers at a feeder task was degraded by the introduction of training to a second task at the nest entrance. Mistakes at the feeder were biased toward the color cue that was not rewarding in both tasks; suggesting that irrelevant or background stimuli are more prone to decay or forgetting during interference. With respect to interference, we did not find an effect of body size on the amount of interference; however, size was related to how quickly interference occurred. Among individuals showing retroactive interference, larger bees showed interference earlier in phase 2 than did smaller bees. Overall, larger workers learned each task more rapidly than smaller workers. We conclude that the timing of interference is a tradeoff between acquisition of the new task and performance at a previously learned task. Given that foragers in nature tend to be larger than nest workers, we suggest that size- related learning differences be considered as a factor in division of labor between large and small bumblebees. Key words: interference, learning, bumblebee, Bombus impatiens, body size, contexts, foraging. Summary Introduction Learning in two contexts: the effects of interference and body size in bumblebees Bradley D. Worden*, Ana K. Skemp and Daniel R. Papaj Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85710, USA *Author for correspondence (e-mail: bworden@email.arizona.edu) Accepted 10 March 2005 THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY