Esterase inhibition by synergists in the western flower thrips Frankliniella occidentalis.

BACKGROUND: Western flower thrips (WFT), Frankliniella occidentalis (Pergande), is among the most important crop pests in the south-eastern region of Spain. Its increasing resistance to insecticides constitutes a serious problem, and understanding the mechanisms involved is therefore of great interest. Use of synergists to inhibit the enzymes involved in insecticide detoxification is widely used to determine their responsibility for insecticide resistance. However, they do not always act as intended or expected, and caution must be exercised when interpreting synergist results. RESULTS: Laboratory-selected strains of WFT were used to analyse the effects of the synergists piperonyl butoxide (PBO), S,S,S-tributyl phosphorotrithioate (DEF) and methiocarb on total esterase activity. Significant differences were found, indicating esterase activity inhibition by DEF, a lower effect for methiocarb and a small inhibition of the activity by PBO. Esterase isoenzyme inhibition by these compounds showed a similar result; this assay revealed an extreme sensitivity of Triplet A (resistance-associated esterases) to DEF. In an in vivo assay carried out with these compounds at different incubation times, only DEF caused posterior in vitro esterase activity inhibition, with a maximum effect 1 h after treatment. CONCLUSION: In this work, only DEF shows true synergistic inhibition of WFT esterases.

Esterase isoenzymes and insecticide resistance in Frankliniella occidentalis populations from the south-east region of Spain.

BACKGROUND: Frankliniella occidentalis (Pergande) is among the most important crop pests in the south-east region of Spain; its increasing resistance to insecticides constitutes a serious problem, and understanding the mechanisms involved is therefore of great interest. To this end, F. occidentalis populations, collected from the field at different locations in south-east Spain, were studied in terms of total esterase activity and esterase isoenzyme pattern. RESULTS: Individual thrips extracts were analysed by native polyacrylamide gel electrophoresis (PAGE) and stained for esterase activity with the model substrate alpha-naphthyl acetate. Significant correlations were found between resistance to the insecticides acrinathrin and methiocarb and the presence of a group of three intensely stained bands, named Triplet A. For each individual thrips extract, total esterase activity towards the substrates alpha-naphthyl acetate and alpha-naphthyl butyrate was also measured in a microplate reader. Insects possessing Triplet A showed a significantly higher alpha-naphthyl acetate specific activity and alpha-naphthyl acetate/alpha-naphthyl butyrate activity ratio. This observation allowed a reliable classification of susceptible or resistant insects either by PAGE analysis or by total esterase activity determination. CONCLUSION: The PAGE and microplate assays described can be used as a monitoring technique for detecting acrinathrin- and methiocarb-resistant individuals among F. occidentalis field populations.