Trichlorfon tolerance, risk assessment, and a cross-resistance trend to four other insecticides in Bactrocera zonata (Saunders)

Bactrocera zonata (Saunders) (Diptera: Tephritidae) is a destructive pest of fruits and vegetables in the world. Trichlorfon belongs to organophosphate group and it is used to control wide range of fruit flies especially B. zonata. The present study was planned to characterize the development of resistance risk to trichlorfon and cross-resistance to four other insecticides in B. zonata by selecting a field population for 20 generations. Eight to ten days old fruit fly adults were exposed to insecticides using the fruit dip method. After 20 generations of selection with trichlorfon, B. zonata had a 10.54-fold and 19.67-fold resistance when compared to field and susceptible populations, respectively. This study also showed moderate level of cross-resistance to deltamethrin (11.07-fold), negative cross-resistance to spinosad (0.05-fold) and spinetoram (0.1-fold), but a high level of cross-resistance to fipronil (688.87-fold). The realized heritability (h2) of resistance to trichlorfon, fipronil, deltamethrin, spinetoram, and spinosad was 0.16, 0.29, 0.12, 0.10, and 0.12, respectively. At fixed slope = 2.84, 10-fold increase in trichlorfon resistance would need 8, 5, and 4 generations for their respective h2 = 0.16, 0.26 and 0.36, respectively at 95% mortality. When h2 = 0.16 and slope = 1.84, 2.84, and 3.84, then 11, 8, and 5 generations would be needed for a ten-fold increase in trichlorfon resistance at 95% mortality, respectively. These results suggest that the peach fruit fly has tolerance against trichlorfon under continuous selection pressure. Therefore, continuous use of trichlorfon should be avoided but rotated with spinosad and spinetoram in the field to minimize its resistance development in the future.