BIOCHEMICAL AND PHYSIOLOGICAL DIFFERENCES IN THE MALATHION CARBOXYLESTERASE ACTIVITIES OF MALATHION-SUSCEPTIBLE AND MALATHION-RESISTANT LINES OF THE SHEEP BLOWFLY, LUCILIA-CUPRINA

Three malathion carboxylesterase (MCE) phenotypes have been described previously in Lucilia cuprina adults: low, intermediate, and high. The MCE specific activities of adults with the intermediate and high phen otypes are 21- and 33-fold higher than those with the low phenotype, b ut the high phenotype is almost 1000-fold more resistant to malathion than either of the other phenotypes. Here we show that MCE activity al so peaks in Day 4 larvae from three lints representative of these phen otypes. The MCE activity of the low-line larvae is only 2.7- and 4-fol d lower than those of the intermediate and high-line larvae, respectiv ely. The relatively high MCE activity of the low-line larvae is largel y explained by their crop activity. Assays in the presence of esterase inhibitors reveal three distinct types of MCE activity across the thr ee lines and two developmental stages. Activity in the low-line adults is not completely inhibited by paraoxon and is classified as a subcla ss II carboxylesterase. The MCE activities in the larvae of the low an d intermediate lines and adults from the intermediate line are inhibit ed by low concentrations of paraoxon, classifying them as subclass I c arboxylesterases. The MCE activities in the high-line larvae and adult s are also classified as subclass I carboxylesterases, but they are mo re sensitive to inhibition by triphenyl phosphate than those in the ot her two lines. These data suggest that MCE in the malathion-resistant high line may be structurally different from the MCEs in the malathion -susceptible intermediate and low lines. (C) 1996 Academic Press, Inc.