Aspects of Insecticide Resistance in New Zealand Strains of the Sheep Blowflies, Lucilia Cuprina and Lucilia Sericata

Abstract

A treated surface bioassay was used to detect insecticide resistance to five organophosphorus insecticides (diazinon, chlorfevinphos, chlorpyrifos, dichlofenthion and propetamphos), the synthetic pyrethroid, deltamethrin, and the insect growth regulator, diflubenzuron, in larvae of field strains of Lucilia cuprina, and diazinon in Lucilia sericata from 1990 to 1997. Organophosphate resistance was widespread in both species of blowfly and L. cuprina exhibited side-resistance between diazinon, chlorpyrifos, dichlofenthion and propetamphos but no cross-resistance could be demonstrated with diazinon and deltamethrin, or between diazinon and diflubenzuron. Phenotypic expression of resistance appeared to be influenced by environmental factors such as season, region, sheep density and year. The relative efficacies of four of the above OPs and the extent to which the OP resistance status of L. cuprina affected this, was assessed by comparing a susceptible strain of L. cuprina, and a strain known to exhibit OP side-resistance, in a series of in vivo trials. All four treatments provided 19 – 21 weeks protection against susceptible larvae but chlorfenvinphos provided the longest protection (16 - 17 weeks), followed by propetamphos (15 - 16 weeks), dichlofenthion (10 - 13 weeks) and diazinon (9 - 13 weeks), against the resistant strain. Penetration of insect cuticle, non-oxidative and oxidative mechanisms that might contribute to resistance to organophosphate insecticides were examined in larvae and adults from field isolates of L. cuprina. Only non-oxidative mechanisms were considered with respect to L. sericata. The basal mechanism of resistance appeared to be an altered esterase (the E3 esterase) in L. cuprina and was also suspected to contribute to OP-resistance in L. sericata. Adult and larval strains of L. sericata showed no relationship between level of GST conjugating activity and resistance to diazinon, suggesting that these enzymes do not constitute a mechanism of OP-resistance in this species. Contrasting this was the observation of a strong relationship between the level of GST conjugating activity, with respect to model substrate DCNB, and resistance to diazinon in both adult and larval strains of L. cuprina. This relationship was supported by a study of the OP-conjugating properties of partially purified glutathione S-transferases from resistant L. cuprina larvae. Neither an increase in acetylcholinesterase content nor an insensitive AChE in the central nervous system of larvae of L. cuprina and larvae or adult L. sericata appear to have been selected for in the resistant field strains examined. There did however appear to be a positive correlation between resistance to diazinon and susceptibility of AChE inactivation by tetrachlorvinphos in L. cuprina adults. The significance of this factor was investigated further by multiple regression and it was concluded that this did not have a main effect on resistance in adults. Samples of L. cuprina were further characterised by RAPD-PCR to determine intra-and inter-strain variation. Key features of this analysis were the calculation of phylogenetic similarities in an attempt to better define a 'field strain', the mode of dispersal of the species throughout New Zealand as well as to identify genetic markers of high level insecticide resistance. No markers of resistance were identified but this study emphasised the need to treat each sample as a separate strain as sampling from the same area at different times showed that samples were not identical suggesting there is variable gene flow in and out of areas and populations. Based on biochemical identification of resistance mechanisms, consideration was given to the use of potentiating pairs of insecticides as a method of prolonging the life of some existing OP insecticides. It was shown that prospects for using such pairs is limited due to the widespread OP-resistance observed in L. cuprina. Finally, rather than reiterate discussion of resistance mechanisms, the conclusion of this thesis looks towards applying knowledge of resistance mechanisms effectively within the framework of integrated pest management system (IPM). It discusses areas of weakness in knowledge that need to be addressed and emphasises the need for a susceptibility reservoir if IPM is to succeed at a national level.