Bioactivity of pyrogallol against melon fruit fly, Bactrocera cucurbitae

The insecticidal effects of pyrogallol were studied by treating eggs and larvae of the melon fruit fly, Bactrocera cucurbitae (Coquillett) (Tephritidae: Diptera), with various concentrations (1, 5, 25, 125, 625 and 3125 ppm) of the phenolic compound. Although egg hatching decreased following treatment of 0–8-h old eggs with pyrogallol, the decrease was not significantly different from the control. Larval period and total development period declined significantly in 64–72-h-old and 88–96-h-old B. cucurbitae larvae fed on pyrogallol-treated diet. However, in the 44–48-h-old larvae, the larval period and total development period were not affected by pyrogallol treatment at any of the tested concentrations. None of them survived up to the pupal stage at the highest concentration. Number of pupae formed and adult emergence decreased significantly in all larval instars following feeding on pyrogallol-treated diet. The analysis of enzymes in 64–72-h-old larvae treated with LC40 concentration (16.21 ppm) of pyrogallol at three time intervals, i.e., 24 h, 48 h and 72 h, showed significant induction in the activities of ascorbate peroxidase (APOX) and glutathione S-transferases (GSTs) at 24 h but a decrease was observed following prolonged treatment. On the other hand, superoxide dismutase (SOD) and peroxidases (POX) activity remained suppressed during the initial treatment interval but increased with prolonged treatment in 136–144-h-old larvae. The catalase (CAT) activity was suppressed at all treatment durations whereas glutathione reductase (GR) activity was not affected by pyrogallol treatment. An increase in the activities of ascorbate peroxidase, superoxide dismutase, peroxidases and glutathione S-transferases indicates an induction of defensive response of the melon fruit fly to the toxic effects produced by ingestion of pyrogallol. Although the effects of the compound on enzyme activity were tested on second instar, it would be interesting to see the effects on other instars too.