Determinants of foraging profitability in two nectarivorous butterflies

ABSTRACT.

• 1 I studied flower selection and foraging energetics of Agraulis vanillae L. (Nymphalidae) and Phoebis sennae (Pieridae), two butterfly species common to north central Florida. I identified the major nectar resources exploited by several populations of these butterflies and, for each plant species, measured available nectar volumes and concentrations, corolla lengths, and density. I quantified foraging behaviour of each butterfly species at each nectar source (flower visitation rate and percentage of foraging time in flight), and used these data to estimate the net rate of energy intake of each butterfly species at each nectar source.
• 2 Estimated mean energy contents of individual flowers of the eleven exploited plant species spanned three orders of magnitude, ranging between 0.015 and 9.27 joules. Mean energy content of individual flowers was strongly correlated with mean foraging profit of both butterfly species.
• 3 Mean nectar volume strongly influenced energy content and varied widely within and among species, ranging from 0.0076 to 1.853 μ1. Nectar concentration varied between 17.1% and 40.4% sucrose-equivalents. Nectar volume was the best single predictor of foraging profitability (correlation coefficients of 0.994 and 0.984 for Phoebis and Agraulis respectively). Corolla length also strongly affected foraging profitability for both butterfly species; flower species with longer corollas were generally more profitable.
• 4 Flower density and nectar concentration showed weak or nonsignificant associations with foraging profitability.
• 5 The usefulness and limitations of these floral characteristics as bases for foraging selectivity, and the selective pressures foraging butterflies might place on the visited plants are discussed.