Resistance Mechanisms of the Diamond Back Moth, Plutella xylostella (Lepidoptera: Plutellidae) to Phosalone

Diamond back moth, Plutella xylostella L. (Lepidoptera,Plutellidae), is the cosmopolite insect pest of cruciferous plants causing significant injury to the plants of this family. Although many integrated approaches have been proposed and developed for DBM management, the most common method of controlling this pest is chemical control. Organophosphates (OPs) are mainly used to control the agricultural pests in Iran, especially DBM. This study was performed to determine and compare some toxicological and biochemical properties of detoxification enzymes (ESTs and GST) and cholinesterase between two resistant (Esf-R) and susceptible (Ard-S) field populations of P. xylostella.

The phosalone-susceptible (Ard-S) population was collected from Ardabil, Ardabil province, Iran and the phosalone –resistant (Esf-R) population from Esfehan, Esfehan province, Iran. The toxicity of insecticides was measured using a standard leaf-dip bioassay. To determine the role of metabolic degradation as a mechanism for phosalone resistance in DBM, PBO (piperonyl butoxide), TPP (triphenyl phosphate) and DEM (diethyl maleate) were bioassayed for synergistic activity with phosalone (Kodwo and Tanaka, 2005). EST and GST assays were determined based on the method of Van Asperen (1962) and Habig et al. (1974) with minor modifications. AChE activity and its kinetic parameters were measured with two artificial substrates, ATC and BTC, along with the modified method of Ellman et al. (1961). Statistical analyses were evaluated by LeOra software (1978) through ANOVA followed by Tukey test.

According to the bioassay results, the (Esf-R) population showed a significantly high resistance to phosalone compared with Ard-S population (17-fold). Diethyl maleate (DEM) and triphenyl phosphate (TPP), as glutathione S-transferase (GST) and esterase inhibitors, increased phosalon toxicity on both resistant and susceptible populations, but the synergistic ratio in the resistant population was higher than that of susceptible one. This confirms the greater role of esterase and GST enzymes in phosalone resistance. Metabolic resistance mechanisms to phosalone were surveyed by biochemical assays. The results indicated that specific activities (SA) of GST, α-esterase and ß-esterase were 2.1-, 2- and 1.7-fold higher in the resistant populations than those of susceptible population, suggesting higher expression of GST and esterase enzymes in resistant population. Furthermore, target site insensitivity was surveyed by biochemical assay. Kinetic parameters of acetylcholinesterase (AChE) on the hydrolysis of acetylthiocholine iodide (ATC) showed no change in the affinity of AChE of resistant population to this substrate and the phosalone resistance mechanism was not related to altered AChE active site.

The results distinctly indicated that metabolic detoxification mechanisms such as GST, esterases created phosalone resistance in the Esf-R and AChE structure were not involved in resistance. According to the result, use of synergists can be helpful for suppressing the phosalone resistance.