Diet quality widely affects the survival, development, fecundity, longevity, and hatchability of insects. We used the greater wax moth Galleria mellonella (Linnaeus) to determine the effects of the antifungal, antibiotic terbinafine on some of its' biological parameters. The effects of terbinafine on malondialdehyde (MDA) and protein carbonyl (PCO) contents and the activity of the detoxification enzyme, glutathione S-transferase (GST), in the midgut of seventh-instar larvae of G. mellonella were also investigated. The insects were reared on an artificial diet containing terbinafine at concentrations of 0.001, 0.01, 0.1, and 1 g. The survival rates at all development stages of G. mellonella were significantly decreased at all terbinafine concentrations. The females from a control diet produced 82.9 ± 18.1 eggs; however, this number was significantly reduced to 51.4 ± 9.6 in females given a 0.1 g terbinafine diet. The highest concentration of terbinafine (1 g) completely inhibited egg laying. Terbinafine significantly increased MDA content and GST activity in the midgut tissue of seventh-instar larvae in a dose-dependent manner. Relative to controls, these low dietary concentrations of terbinafine significantly increased midgut PCO content; a 0.1 g terbinafine concentration raised PCO content from 155.19 ± 21.8 to 737.17 ± 36.4 nmol/mg protein. This study shows concentration-dependent effects on the biological traits of the greater wax moth G. mellonella, including the oxidative status and detoxification capacity of the midgut. Low terbinafine concentrations may be possible for use as an antifungal agent in insect-rearing diets.