Energy Metabolism of Eucalyptus-Boring Beetles at Rest and During Locomotion: Gender Makes a Difference

We studied metabolic rates during rest, maximal running exercise and tethered flight in the long-horned eucalyptus-boring beetles Phoracantha recurva and P. semipunctata. Simultaneous measurement .of rates of O₂ consumption and CO₂ production indicated that closely approximated and hence was a good index of aerobic metabolic rate. The resting metabolic rate (RMR), peak during running-wheel locomotion (MR_run) and factorial scope during running (MR_run/RMR) are similar to published values for several other insect taxa. MR_run was repeatable for most test groups over intervals of 48–96 h. Studies of P. semipunctata show that MR_run is relatively insensitive to changes in ambient temperature (T_a) between 20 and 30 °C, whereas resting metabolic rate increases with T_a with a normal Q₁₀ (2.4). Consequently, factorial scope declines at the higher T_a: mean factorial scopes for male and female P. semipunctata are 17.7 and 13.6 at 20 °C versus 8.9 and 5.5 at 30 °C, respectively. Flight activity requires a considerably greater metabolic rate than terrestrial activity: at T_a values of 20–30 °C, the mean factorial scope for flight activity of male P. semipunctata is 72 (range 36–110). Nevertheless, our measurements of flight metabolic rate in Phoracantha spp. are considerably lower than predicted from allometric equations for other insects. Our most interesting finding was that males of both species had a substantially and significantly higher MR_run and aerobic scope than females. The gender differences in MR_run are consistent with differences in activity levels of males and females during mate-seeking behavior.