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This article was published in 1966
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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.
Change in the Response of the Primary Sheep Blowfly to Organophosphorus Insecticides
G. J. SHANAHAN, B.Sc.Agr., M.Sc., Ph.D., Special Entomologist, N.S.W. Department of Agriculture, Rydalmere
Organophosphorus insecticides have been the choice to combat flystrike of sheep in Australia since 1957 when dieldrin-type resistance was established in field populations of Lucilia cuprina, the blowfly which initiates most cases of flystrike. Since then surveillance tests have been conducted almost each year to seek evidence of any change in the susceptibility of the blowfly to organophosphorus (O.P.) chemicals. The discriminating dose approach was employed in these tests—this technique enables an early diagnosis of an altered response of field populations of flies to any insecticide which has been widely used for flystrike prevention.
The essential details of the discriminating dose technique follow. Samples of flies from cultures formed from larvae removed from sheep with flystrike are treated with a single concentration of an insecticide. This concentration, which may vary from insecticide to insecticide, is based upon the exposure of groups of flies from a known susceptible culture to a range of insecticidal concentrations. By this means a concentration to give 100 per cent mortality of flies in the susceptible culture is determined. Now, when surveillance tests are conducted in a routine manner or there is suspicion from the field that a change in the susceptibility of flies in the field has occurred, samples of flies are treated with the known concentration which discriminates against all susceptibles. Even one survivor is frequently sufficient to indicate a change in response towards a given insecticide. Quite often, as occurred with dieldrin resistance which arose in 1957, and late last year with the organophosphorus chemicals, a significant proportion of individuals in samples of flies from the field-established cultures survive the respective discriminating levels of insecticide. Under these circumstances, there is little doubt that the flies in the field have developed an added ability to withstand the insecticides in question.
The next exercise is to determine the order of change of response. The comparison is usually made at the L.D. 50 level with susceptible and suspect strains. L.D. 50 is an abbreviation for Lethal Dose 50 and refers to the amount of insecticide required to kill 50 per cent of flies in a strain. The order of difference between the L.D. 50 for a susceptible and a suspect strain is the important consideration once flies have been found with a change in response to a particular insecticide.
TWO CONCEPTS: INSECTICIDE RESISTANCE AND TOLERANCE
It is generally accepted amongst resistance workers that the exposure of insects to adverse environmental conditions, and this includes contact with insecticide used in control programmes, often causes a slight change in the response range of a species to an insecticide. The differences which apply with variation in the size of insects or from culture to culture, differences in the response of the sexes or variations in response with age are excluded from this comment. The change in response in mind at the moment results from the perpetuation of the species through the most hardy individuals in the field populations of insects. It is a heritable characteristic with a polygenic explanation and, in the absence of the adverse environmental factors, such an altered strains returns to full susceptibility in a few generations. This phenomenon has been termed vigour tolerance and is characterised by affording additional protection of a species to unrelated groups of chemicals. It is seldom of a high order, less than five times above normal at the L.D. 50 values.
Resistance, on the other hand, is usually decisive—the order of difference of the L.D. 50 values for the susceptible and suspect strains is above five times normal and it may be several hundredfold above normal; this was the position with dieldrin resistance in L. cuprina; there was no economic room to enable the dieldrin concentration to be increased to provide prevention of flystrike. This type of situation is a true resistance, a heritable characteristic, and in most instances detoxification by the insect explains the resistance, and to give this more meaning it may be well to add that most field developed resistances are due to single genes, thus removing the need for a threshold effect which could apply where resistance was due to polygenes.
ORGANOPHOSPHORUS TOLERANCE IN THE BLOWFLY
In October/November, 1965, flies from field samples taken from the Dubbo and Binnaway areas survived discriminating doses of diazinon, Nemacide, Nankor and Lucijet. The initial recognition of a possible change in susceptibility of the blowfly to the organophosphorus chemicals just named was part of a surveillance programme commenced in the Spring with the aid of field officers of the Department of Agriculture. Towards the end of the year it was clear that the change in susceptibility of the flies was widespread, particularly in the central west, north and northwestern regions of the State. Subsequently, flies of this type were diagnosed from the south-west of the State and more recently from Queensland and Victoria.
Answers were required to three queries.
(1) Could flies in heterogeneous cultures be selected for a high order change towards diazinon, which is the most widely used fly preventive?
(2) Was the order of change similar towards diazinon, Nemacide, Nankor and Lucijet?
(3) Do larvae from diazinon selected strains possess an increased ability to set up strikes on sheep well treated with the insecticide just mentioned?
The selection programme indicated that the suspect strains were unable to develop a high order change of response to any of the four insecticides, including the selective agent diazinon. This was a comfort and was in marked contrast to the sequence which applied several years previously when the problem arose with dieldrin and related insecticides. Next it was found that the change in susceptibility embraced all four insecticides; it was of a similar order, around three to four times above normal. This was most interesting in that if true resistance was involved, it would have been expected that Nankor, which is a di-methyl O.P., would not have been involved, whereas diazinon, Nemacide and Lucijet would have all sustained a change in response; these latter insecticides are di-ethyl O.P.s. Each new case of insecticide resistance requires investigation in full; the lack of division between a change in response to di-methyl and diethyl O.P. insecticides in L. cuprina was contrary to the situation which normally applies in houseflies and mosquitoes. There are diazinon resistant houseflies present in Australia, for example, but these flies have no resistance whatsoever to malathion which, like Nankor, is a di-methyl O.P.
The comparatively low order of change in response of L. cuprina to the four registered O.P. chemicals, together with the similarity in the change to the di-methyl and di-ethyl O.P.s, suggested that a tolerance and not a true resistance had arisen. Since the dichotomy between tolerance and resistance is largely based upon an arbitrary difference of order of change in susceptibility centring around the fivefold mark above normal, it would be ill-advised to place the present problem with L. cuprina towards the O.P. insecticides firmly in one category or the other. The final answer will come from the field once severe fly waves are encountered.
Due to the severity of the present drought, it has not been possible to obtain sufficient field data to determine the limits of organophosphorus insecticides in terms of flystrike prevention now that a change in response to these chemicals has undoubtedly developed in field populations of the blowfly throughout the State. An answer to the third query, namely have larvae from O.P. selected strains an increased ability to set up strikes on treated sheep, has been obtained from semi-field work in which small groups of sheep were treated with the four O.P. insecticides mentioned in this article at concentrations recommended by the makers. Whilst larvae from the selected strains were able to set up strikes following artificial implants sooner than those from susceptible strains, the difference in the period of protection was not great. It appears from this work, which is in the process of being repeated, that sheep thoroughly jetted with diazinon, Nemacide, Nankor or Lucijet will be protected for six to eight weeks from breech strike and eight to ten weeks from body strike. Graziers would be well advised to think in terms of the minimum rather than the maximum period of protection indicated from this work.
Thorough treatment with O.P. insecticides at the recommended concentrations may provide an answer to the present problem of the change in response of the primary sheep blowfly to O.P. chemicals. Not less than ½ gallon of fluid for the treatment of a breech and not less than ¾ gallon for a combined breech and body strike jetting should be used. The volume of fluid will vary depending upon the length of wool and size and age of the sheep, but it should not be less than the amounts mentioned.
Semi-field tests using a few sheep, on which artificial strikes are attempted, are a conventional means to determine the length of protection afforded by insecticides, used in sheep blowfly prevention. It is well to mention that the periods of protection obtained against artificial strikes are two to three times more than that obtained in the field. One explanation for this large difference could be that female flies deposit their eggs in a communal fashion in the fleece of sheep. Large numbers of larvae on hatching from batches of eggs deposited by several flies, provided they can establish themselves on the skin, would tend to dilute the effects of the insecticide. This behaviour pattern of the fly, together with the community feeding habits of the larvae, emphasises the need for thorough treatment with O.P. insecticides now that a change in susceptibility of the blowfly to this group of insecticides is present.
The term O.P. tolerant is suggested instead of O.P. resistant to explain the recent development in the blowfly. It is to be noted that the companion word "vigour" has been dropped from the concept vigour tolerance already explained. Tolerant as used here refers to a change in susceptibility of a reasonably low order but insufficient to cause a major breakdown in the effectiveness of an insecticide when used in a thorough fashion at concentrations recommended before the onset of the problem. The studies conducted to date are inadequate to indicate whether an increasing O.P. tolerance will follow the continued use of this class of insecticides for sheep blowfly prevention. Bio-chemical studies at present planned will show whether tolerant flies are able to degrade O.P. chemicals.
ALTERNATIVE CHEMICALS
In an attempt to classify the O.P. tolerance in the blowfly, laboratory tests have been conducted with a range of insecticides and it has been found that the flies have a tolerance at a similar level to insecticides such as bromophos ethyl and methyl and parathion ethyl and methyl, to which the flies have not been exposed. This is quite normal in the sense that an insect species which develops resistance to a certain group of materials is simultaneously tolerant or resistant to similar members of this group. Preliminary work indicates that an unrelated material belonging to the carbamates, Butacarb, is lethal to O.P. tolerant larvae and, for this reason, may have a role as an alternative insecticide. Further field testing is required to establish its full merit.
CONCLUSION
Despite the diagnosis of O.P. tolerant primary sheep blowflies, studies to date indicate that sheep well jetted with diazinon, Nemacide, Nankor or Lucijet will be afforded sufficient protection to carry them through major fly waves. Emphasis is to be placed upon thorough jetting to ensure that all portions of the fleece on susceptible sites are thoroughly saturated with jetting liquid. A reduced period of protection may result, but this should not be less than about six weeks for breech strike and about eight weeks for body strike. Butacarb is a possible alternative, but its final value will have to be established in the field.