Fuel use in flight and its dependence on nectar feeding in the hawkmoth Amphion floridensis

Fuel use varies widely among insects; however, the potential determinants of variation in fuel use have not been explored experimentally. This study examines whether fuel use during tethered flight depends upon feeding status in the nectarivorous hawkmoth Amphion floridensis. Fuel use in this study is characterized by the respiratory quotient, measured at intervals during a flight using modified closed-chamber respirometry. Moths were either fed twice daily to satiation with 30 % sucrose or unfed, and their fuel use was measured during flights on the first, third and fifth day after eclosion. Flights lasted up to 30 min, with measurements taken at their onset and at 10 min intervals thereafter. Nectar feeding greatly affected fuel use in A. floridensis: fed moths relied primarily on carbohydrate, whereas unfed moths relied almost exclusively on fat reserves. Fuel use did not change during a flight, even when flights lasted 30 min or more. Males were initially more extreme than females in their response to feeding treatment: they burned more carbohydrate when fed and more fat when unfed. By the third day after eclosion, however, fuel use in males and females became identical. Rates of oxygen consumption were uncorrelated with respiratory quotient, were higher in fed moths and declined during a flight. These data indicate that fuel use in this nectarivorous hawkmoth is flexible, that carbohydrate is important as a primary flight fuel and that an understanding of ecological factors, particularly foraging habit, is critical to understanding fuel use in insects.