Monitoring of insecticide resistance in field sampled buffalo flies

Summary

Buffalo flies were collected from yarded cattle with a sweep net and transferred into disposable plastic Petrie dishes containing filter papers impregnated with discriminating concentrations of various insecticides. Mortality was counted after a two hour interval and the percentage survival calculated and used as an indicative or equivalent measure of percentage resistance.

The discriminating concentrations used in the 2000 survey were the same as those used in surveys conducted for the Meat Research Corporation in 1994 and 1995 by Agrisearch Services Pty Ltd. Two discriminating concentrations were used for diazinon and for chlorfenvinphos, because of the greater interest in any change in resistance to these organophosphate insecticides. Information was also gathered about current buffalo fly management practices being used on the cattle properties surveyed.

2000 Survey Results Summary

A total of 26 farms were sampled for buffalo fly bioassays throughout the whole of New South Wales and Queensland in the 2000 survey. Also, due to reasons of practicality all the assays conducted during the 2000 survey were carried out over a limited time (4-5 days for each region). Given the limited sample size and time frame of this 2000 survey, some caution should be exercised when interpreting the results.

The 2000 survey indicated the possible appearance of diazinon resistance in buffalo flies in the northern New South Wales' region for the first time as measured using the filter paper bioassay technique. On the northern New South Wales' properties sampled in 2000 (3), the average percentages of buffalo fly survival (ie 26.4% and 67.7%) at the respective 1994 and 1995 diazinon DCs were considerably higher than the corresponding levels of survival in assays in 1994 (0.5%) and 1995 (9.5%). These degrees of increased diazinon tolerance were not seen in any other sampled regions in 2000, particularly in south eastern Queensland - where a similar result may have been expected. Nor had similar levels of diazinon tolerance ever been seen in the previous surveys (ie 1994 and 1995).

A concerningly high level of resistance to the other organophosphate insecticide, chlorfenvinphos was also found in assays in the northern New South Wales region. In 2000, the average percentages of buffalo fly survival to respective chlorfenvinphos DCs (ie 43.7% and 80.9%), were significantly higher than the corresponding levels of survival in the 1994 (2.5%) and 1995 (21.5%) assays. In other regions there was a slight rise in the level of chlorfenvinphos tolerance in buffalo flies sampled in the north Queensland region, however there were unchanged and/or low to moderate levels of tolerance to chlorfenvinphos assayed in the south eastern and far north Queensland regions.

Continued high levels of resistance to the synthetic pyrethroid insecticides were generally indicated in buffalo flies in all regions of New South Wales and Queensland. Slightly reduced levels of tolerance to deltamethrin and cyhalothrin may indicate that resistance genes in the population have become diluted.

The questionnaire of current management practices towards buffalo fly carried out in 2000 was taken from a relatively small number of properties and is therefore limited as far as drawing too many conclusions about the wider grazing community. The 2000 survey did indicate an interesting link between fly control management practices and the development of insecticide resistance. That is, that in the regions where organophosphate resistance percentages was suggested, the recent use of these same insecticides had been commonplace. As well, application efficiencies of these insecticides - particularly chlorfenvinphos through back spray and back rubber - were generally not good. Lack of thorough spraying of animals with diluted insecticides was identified as a problem in the 1994 and 1995 survey. The relative cheapness of the organophosphate insecticides (containing chlorfenvinphos and diazinon) that are intended for use through back spraying may lead to less care being exercised with their use, eventually contributing to under- dosing and the selection for resistance.

By way of contrast, in the south eastern region of Queensland where chemical abstinence and keen interest in dung beetles welfare were reported, levels of organophosphate resistance indicated increased only slightly since 1995. It also appeared encouraging that the general reduced usage of deltamethrin and cyhalothrin insecticides may have contributed towards an overall decrease in the levels of resistance to these chemicals. The main message to come from the 2000 survey is that continued care needs to be taken with the use of insecticide products to help prevent (or delay) the onset of insecticide resistance. To help achieve this aim it is felt that there needs to be a co-ordinated industry-wide approach, with regular monitoring of buffalo fly resistance via current and/or newer technologies, to evaluate agreed resistance management strategies.