Esterase-based metabolic resistance to insecticides in heliothine and spodopteran pests

Abstract

IntroductionCarboxylesterases have been implicated in metabolic resistance to several classes of insecticide in a wide variety of pest insect species. [1][2][3] The classes most commonly involved are synthetic pyrethroids (SPs) and organophosphates (OPs), and carbamates (CBs) are also involved in several cases. There are also occasional reports of carboxylesterase phenotypes associated with resistance to benzoylureas, the oxadiazine indoxacarb, the diacylhydrazine tebufenozide, the organochlorine endosulfan and the proteinaceous Cry1Ac toxin of Bacillus thuringiensis that is expressed in transgenic crops. Figure 1 gives structures for some key compounds among these various classes of insecticide.It might be expected that carboxylesterases would be involved in SP resistance because the great majority of SPs are carboxylesters. However, no precise molecular mechanisms for esterase-based SP resistance have yet been elucidated in any insect. OPs are generally phosphotriesters rather than carboxylesters but nevertheless they bind many carboxylesterases with high affinity and this provides a basis for two forms of resistance, both now elucidated at a molecular level in several species. One form involves substantial overexpression of the esterase, allowing for effective sequestration of the insecticide. The other involves specific mutations in the active site of the enzyme which convert it to an OP hydrolase (the so-called "mutant ali-esterase" mechanism). CBs are carbamic acid (NH 2 COOH) derivatives, in which a functional group has been added as an ester; these esters are chemically similar to carboxylesters. Carbamates bind carboxylesterases with quite high affinity, but as yet the molecular mechanisms by which a (mutant) carboxylesterase confers CB resistance have not been elucidated in any species. Of the other insecticide classes occasionally linked to esterase-based resistance only indoxacarb has ester bonds (two Elevated esterase activities and increased band intensities of multiple esterase isozymes after electrophoresis are commonly associated with resistance to organophosphate, pyrethroid and carbamate insecticides in various heliothine and spodopteran pests. One possible explanation for this involves a 'master regulator' mutation in a more general chemical stress response. An association between elevated esterase activities and isozyme intensities has also been reported for resistance to the Cry1Ac toxin of Helicoverpa armigera. The basis for this is unclear albeit some involvement of esterases could be mediated by the toxin's affinity for N-acetyl galactosamine glycans on certain gut-expressed esterases in this species.

Overexpressed esterases in a fenvalerate resistant strain of the cotton bollworm, Helicoverpa armigera

“…While target site insensitivity is clearly also involved, enhanced detoxification is consistently implicated in the resistance, including both oxidative and hydrolytic metabolism, mediated by cytochrome P450s (P450s) and carboxyl/cholinesterases (CCEs) respectively (McCaffery, 1998). However the relative importance of the two detoxification systems appears to vary among populations from Australia, Asia and Africa (Gunning et al, 1996(Gunning et al, , 1999Martin et al, 2002;Yang et al, 2004;Achaleke et al, 2009;Abd El-Latif and Subrahmanyam, 2009;Farnsworth et al, 2010).…”

Testing the evolvability of an insect carboxylesterase for the detoxification of synthetic pyrethroid insecticides

“…There are many cases of metabolic resistance to synthetic pyrethroid (SP) insecticides in which synergism by esterase inhibitors or elevated esterase activity in resistant strains, as measured by in vitro assays with artificial substrates like naphthyl acetate, implies a contribution from esterases to the resistance phenotype (Farnsworth et al, 2010;Oakeshott et al, 2005Oakeshott et al, , 2010b. More specifically, native PAGE analysis of some of these cases has shown greater staining intensities in individual esterase isozyme bands, or in some cases in multiple bands with distinct electrophoretic mobilities.…”

The classification of esterases: an important gene family involved in insecticide resistance - A review

“…Esterase-derived metabolic resistance is a major problem in vector populations because it has already been correlated with resistance to the two major insecticide classes that are currently used, i.e., OPs and pyrethroids (Li et al 2007, Hotelier et al 2010, Bass & Field 2011. Furthermore, metabolic resistance mechanisms are known to cause cross-resistance among insecticide classes and with other chemicals used for insect control (Oakeshott et al 2005, Li et al 2007, Nauen 2007, Carvalho et al 2010, Farnsworth et al 2010, Van Leeuwen et al 2010.…”