The scaling of free flight energetics of the mango stem borer beetle, Batocera rufomaculata

Abstract

Adult body size in insects can be influenced by environmental conditions during larval growth. The effect of such intraspecific variation in body mass on flight performance is poorly understood. In Batocera rufomaculata, a large tree boring beetle, often engaging in dispersal flight, adults emerging from larvae that developed in a dying host tree under nutrient-deprived diet conditions, are smaller than individuals who developed in nutrient rich environments. This leads to a large variation in adult body mass (1 to 7 g) with an expected effect on flight Metabolic Rate (MR). Measuring MR poses a formidable challenge in free-flying insects who can’t breathe into masks or be trained to fly in controlled settings. Consequently, flight MR has been predominantly measured on hovering or tethered insects flying in closed systems. In this study we applied the ‘bolus injection of isotopic 13C Na-bicarbonate’ method to explore the relationship between intraspecific body size and power expended during tethered steady forward flight and free flying B. rufomaculata. While the method was previously used solely on vertebrates, we report a 90% accuracy of the method in our beetle when compared to direct respirometry. We found that free FMR and tethered FMR increased in relation to body mass as 99·𝑀0.54 and 36∙M^0.56 respectfully showing that while the free FMR is ~3 times higher than FMR recorded in tethered flight, the mass exponent remains unchanged. This mass exponent of ~0.55 is lower than the expected exponent for interspecific variation in body mass of M^0.74 to M^0.98.