Thermal constraints on activity patterns in nectar‐feeding insects

All the insects visiting Tilia and Heracleum flowers showed diel patterns of activity, which could best be explained in terms of weather. Of the recorded parameters, solar radiation gave the best correlations with activity. Correlations between activity and radiation depended on the size and colour of particular groups of insects; small bright species visited fully insolated flowers, ( r high), while large dark forms visited early and late in the day ( r low or negative). Consequently mean reflectance and mean weight of foragers showed consistent trends through a day and predictable relations with radiation for both flowers, suggesting that thermal costs are a critical determinant of feeding times. By contrast, the nectar reward (as sugar amounts) in flowers was poorly correlated with timing of insect visits, indicating that caloric reward is not limiting; although nectar concentration (only measurable for Tilia ) did correlate reasonably well with total visitation. The exception to this pattern was Bombus. This endothermic species visited early in the day when nectar was abundant. Thus visits correlated well with reward and negatively with radiation, showing a dependence on high rewards to offset thermoregulatory metabolic costs. For most insects foraging appeared to be largely determined by the visitors thermal requirements and the restrictions imposed by avoidance of overheating. Nectar rewards were not critical, and nectar concentration, itself dependent upon climatic parameters, was probably only incidentally correlated with diel patterning for all but the largest (endothermic) insects.