Susceptibility and esterase activity in citrus red mite Panonychus citri (McGregor) (Acari: Tetranychidae) after selection with phoxim

Changes in susceptibility and esterase activity were measured in the citrus red mite, Panonychus citri (McGregor), which was repeatedly exposed to phoxim, an organophosphorus acaricide, for 15 repeated treatments as potential selection at the 50% lethal concentration. A reference or non-selected strain (SS) was not exposed to any acaricides throughout the experiments. After 15 repeated phoxim selection events, the resistance ratio increased to 18.6-fold and this strain was considered resistant to phoxim and named Ph-R. The activity and specific activity of carboxylesterase (CarE) and acetylcholinesterase (AChE) in Ph-R were all significantly higher than those in SS. The Michaelis-Menten constants (K m) of both CarE and AChE in Ph-R were significantly higher than those in SS, suggesting decreased affinities of the enzymes in Ph-R to the corresponding substrates. In addition, in vitro inhibition of phoxim against CarE of the mite showed that CarE from SS was more sensitive than that from Ph-R, and an in vitro inhibition assay of AChE by phoxim showed that there was a strong linear relationship between inhibiting action and AChE from the two strains. Similar to the situation of CarE, AChE in Ph-R also suggested a lower sensitivity to phoxim. The results suggested that CarE and AChE might be involved in the resistance to phoxim in citrus red mite.