Abstract
The first monitoring of the resistance to two organophosphates (methidathion and ethion) and one pyrethroid (cypermethrin) insecticides presented by three field collections of different Rhynchophorus ferrugineus (Olivier) populations was performed in this study. The LD50 doses of three insecticides against newly moulted eighth-instar R. ferrugineus larvae were estimated through toxicity bioassays, in which the larvae were fed on artificial diet supplemented with different individual doses of cypermethrin, methidathion or ethion. A population of R. ferrugineus collected from Wadi Ad-Dawasir was chosen as the reference strain, because of its susceptibility to each of the tested insecticide. Compared with the susceptible strain, populations sampled from Al-Ahsa and Al-Qatif showed 8.72 and 4.51-folds increase in resistance to cypermethrin and 3.77 and 2.85-fold increases in resistance to ethion, respectively. The LD50 values of cypermethrin (1.62 ppm), methidathion (3.15 ppm) and ethion (9.12 ppm) in the susceptible population (Wadi Ad-Dawasir) were used to investigate the feeding performances and physiological impacts on eighth-instar R. ferrugineus larvae. The larvae showed no resistance to the most potent insecticide (methidathion) resulting highest growth reduction. However, moderate resistance was observed from red palm weevil population collected from Al-Qatif and Al-Ahsa against cypermethrin resulting lowest reduction in the efficacy of conversion of ingested food (ECI) and digested food (ECD). In addition, glutathione-S-transferase (GST) activity assay further strengthen our findings. Results suggest that enhanced GST activity likely contributes to cypermethrin and ethion resistance exhibited by the R. ferrugineus populations sampled from Al-Qatif and Al-Ahsa. Toxicity bioassays, dietary utilization experiments and GST assays examined in this study support the use of methidathion against R. ferrugineus as the most potent insecticide against all the sampled populations. However, cypermethrin and ethion resistance appears to be evolving in the R. ferrugineus populations from Al-Qatif and Al-Ahsa, justifying the concern of farmers regarding the low efficacy of insecticides. (C) 2016 Friends Science Publishers