Foraging strategies in the small skipper butterfly, Thymelicus flavus: when to switch?

Many insects foraging for nectar or pollen exhibit flower constancy, a learned fidelity to a particular species of plant that previously provided a reward. Constancy may persist even when alternative flowers are available that provide a greater or less variable reward. This strategy entails more travelling time than one of generalization (visiting all suitable flowers as they are encountered). The consensus at present is that this increase in travelling time is offset by decreases in handling time; switching between flower species incurs a cost in time spent learning to ‘handle’ the new flower species that is avoided by remaining constant. If this is so, then the optimal strategy should depend upon the density of flower species (and thus the travelling time), with switching occurring below a threshold density of the target flower species. This prediction is tested using the butterfly, Thymelicus flavus, by analysing foraging patterns under natural conditions. This species exhibited constancy: of 465 visits to flowers 85% were to the same species as last visited. As predicted switches between flower species occurred in response to low encounter rates of the flower species on which the individual had previously fed. However, butterflies ignored the vast majority of suitable flowers that they encountered, even when they were of the species to which they were constant. This casts doubt on explanations for flower constancy as an adaptive strategy that minimizes handling time and maximizes resource acquisition per unit time within learning constraints. ? 1997 The Association for the Study of Animal Behaviour The phenomenon of flower constancy, well documented in honey bees, Apis mellifera, seems to represent a sub-optimal pattern of foraging which remains to be explained convincingly (Woodward & Laverty 1992). Honey bees exhibit rapid sensory learning, and can use scent, colour, shape or a combination of all three to identify flower species that previously provided a reward (Koltermann 1969; Menzel & Erber 1978). This learning process takes just three to five consecutive rewards, and can persist for at least 2 weeks (Menzel 1967). Bees can even distinguish between flowers of different ages according to reward Correspondence: D. Goulson, Department of Biology, University of Southampton, Biomedical Sciences Building, Bassett Crescent East, Southampton SO16 7PX, U.K. (email: [email protected]. J. Ollerton is at the School of Environmental Sciences, Nene College, Park Campus, Northampton NN2 7AL, U.K. C. Sluman is at Blackbird Leys City Farm, Cowley, Oxford OX14 5SY, U.K. 0003–3472/97/051009+08 $25.00/0/ar960390 ? 19 100 (Giurfa & Núñez 1992). When foraging for nectar or pollen, individual honey bees generally exhibit a high degree of flower constancy, fidelity to one particular flower species that has previously provided a reward (93–98% of all visits in a single foraging bout, Grant 1950; Free 1963). Simultaneously other honey bees foraging in the same environment may exhibit constancy to different flower species. Other insect species behave in a similar way, including bumblebees and butterflies (Heinrich 1976; Lewis 1989; Goulson & Cory 1993). By adopting this strategy the insects are bypassing rewarding flowers. If they were not flower constant but visited with equal preference all flower species that provided a reward then they could reduce travelling time. This apparent inefficiency is even more striking when, in twochoice experiments, some bees remain constant to an artificial flower morph which provides a consistently lower reward than the alternative (Wells & Wells 1983, 1986; Wells et al. 1992). 97 The Association for the Study of Animal Behaviour