Fecundity without nectar is insufficient for persistence of a blue butterfly

Organisms with complex life cycles undergo ecological transitions between life stages, often resulting in stage-specific resource use. The relative contribution of each stage-specific resource to vital rates influences population dynamics and subsequently whether habitats can support viable populations. In Lepidopterans, survival to reproduction requires sufficient resources for immature life stages, but the extent to which resources for adults are critical to population persistence is variable. We studied Boisduval's blue butterflies ( Icaricia icarioides ), in a greenhouse experiment, to quantify the effect of the adult diet, nectar, on vital rates. Butterflies fed ad libitum produced 3.4 times more eggs, on average, over their lifetime and lived 6 more days relative to those which only had access to water. We used these experimental data to parameterize a population model to test if vital rates with and without nectar result in viable population growth rates. We found that Boisduval’s blue butterfly populations will not persist without nectar resources (λ < 1). In this species, although host plant resources contributed to reproduction and allowed the butterfly to produce ~50 eggs without nectar, these resources consumed as larva did not compensate for adult malnutrition beyond a critical threshold. The relative abundance and quality of each stage-specific resource can therefore determine at what threshold other resource(s) are limiting the population. This study highlights the value of incorporating effects on vital rates across the life cycle to evaluate the effect on populations.