Butterflies selected one of two contrasting locations for 10 visits without feeding followed by 16 visits with feeding. For each visit a location provided either 35% sucrose for 20 s versus 70% sucrose for 30 s or 35% sucrose for 20 s versus 70% sucrose for 20 s. These pairings tested between choices based on rates of net energy gain, net gains per visit, and/or net gains per meal. The first pairing produced choice based on maximizing net gain per meal. Choice for the second pairing was based either on maximizing net energy gain rate or net gain per visit. The change in foraging rule was associated with a change in volume per visit for 70% sucrose Understanding choices and the evolution of nectar rewards requires detailed information on alternatives, and interpretations of foraging behavior for nectar feeders should not be based solely on a criterion of maximizing net energy gain rate during foraging.

Volume and energy ingestion rates, meal sizes (intakes to satiation) and meal frequencies were measured for previously unfed adult painted lady butterflies (Vanessa cardui L.) fed sucrose solutions or nectar from Lantana camera flowers in the laboratory. Volume and energy rates of crop emptying, assimilation efficiencies and mature egg production over 1 week were measured for V. cardui fed on sucrose solutions to assess mechanisms for and consequences of maximizing net meal energy. Viscosity reduced volume ingestion rates as sugar concentration increased, and 35–52.5 % (w/v) sucrose produced a maximum rate of energy gain from sucrose solutions. Ingestion rates were lower from Lantana flowers. Increasing Lantana nectar concentration from 33 to 70 % sucrose would produce about the same rate of energy gain for a meal. Virtually all ingested sugars were assimilated. Energy processing rates of 30 μl meals did not vary with sex, varied little with concentration and were 12–30 times the rate of energy use for maintenance. For females this may be due to the linear dependence of mature egg production on the amount of sugar ingested. Average meal timing compensated for variations in food concentration. Meals may be initiated before complete crop emptying, and this would increase the overall rates of energy processing, particularly for small meals. If Vanessa are not time-constrained while foraging, selecting concentrated nectars would decrease foraging frequency and increase the number of mature eggs produced after a meal.