CHEMICAL CONTROL OF BLOWFLY STRIKE INTRODUCTION S.G. GHERARDI* Blowfly strike is one of the major problems confronting the sheep industry in Australia with the total cost of control for the industry for 1977/78 approaching $55 m (Brideoake 1979). Breech, pizzle and poll strikes can be effectively controlled by suitable management practices, but body strike relies exclusively on jetting with synthetic insecticides. However as shown by Shanahan and Hughes (1979), the Australian sheep blowfly LuciZia cuprina (Wiedemann) has developed resistance to these insecticides after extensive field use. Although widespread resistance has developed to the currently available organophosphorus (OP) insecticides, they still continue to provide reasonable protection against blowfly strike (Shanahan and Roxburgh 1976). The reported failures of the OP insecticides cannot be explained by resistance factors but may be attributed to inadequate methods of application (Shanahan and Roxburgh 1975; O'Flynn and Green 1980). The lack of new insecticides and the possible development of.different mechanisms of resistance to the OP insecticides has stimulated a search for alternative methods of control. The CSIRO Division of Entomology is investigating genetic methods of biological control that use the sheep blowfly for its own destruction (Foster & Whitten 1974; Foster 1979), and the NSW Department of Agriculture is developing methods for controlling and reducing fleece moisture (Hall 1979). As these programmes are still in their developmental stages, insecticides will continue to form an integral part of programmes for sheep blowfly control. Thus future research by the chemical industry for new insecticides that are both safe and effective should continue if body strike is to be effectively controlled ORGANOPHOSPHORUS INSECTICIDE RESISTANCE IN THE AUSTRALIAN SHEEP BLOWFLY LucILm CUPRINA (WIEDEMANN) - - N. MONZU*, J. MOULDEN* and S.G. GHERARDI* Organophosphorus insecticides were introduced for sheep blowfly control in 1957 to circumvent the high resistance which had developed to the chlorinated hydrocarbon insecticides. The development of resistance to OP insecticides by the Australian sheep blowfly has been slower and more complex than to the organochlorines. However, OP resistance had become widespread by 1966 (Shanahan and Hart 1966). Arnold and Whitten (1976a) have shown that resi#stance to OP insecticides in L. cuprirza is controlled by semi-dominant alleles at two loci, Rop-1 and Rop'2, situated on chromosomes 4 and 6 respectively. Rop-2 has a single resistant allele which is designated R2 Larvae which are homozygous for each of the above alleles have resistance to diazinon, an OP insecticide. The R,, resistant allele was derived from a laboratory selected strain,.but has not yeelbeen detected in the field. *Department of Agriculture, South Perth, W.A. 6151. 101
The implant studies on sheep using the method of Shanahan and Roxburgh (1974a) have indicated that OP chemicals should give a substantial degree of protection against all strains, except perhaps the Rlc (Shanahan and Roxburgh 1976). RESULTS A summary of the results from the most current field population resistance studies throughout Australia are presented in Table 1. TABLE 1 The resistance to diazinon shown by LuciZia cuprinu populations, and the approximate length of field protection (weeks) expected in each Australian State. 102
The mean RF recorded for each State is correlated to the approximate length of field protection offered to sheep when jetted with an OP insecticide. For instance, in New South Wales, with a mean RF of 16.7, an approximate length of protection of 2-4 weeks is possible, while in Western Australia, with a mean RF of 11.5, a field protection of 4-6 weeks can be expected. The results of the population resistance studies in Western Australia are recorded in Table 2. TABLE 2 The mean resistance factors (RF) of LuciZia euprina populations obtained from dry inland and wetter southern areas of Western Australia double Populations from the wetter southern areas of the State that of the drier inland 1ocalities. DISCUSSION showed a RF The different OP resistance levels existing in L. euprina populations in Australia can be explained in terms of variation in the frequency of occurrence of susceptible and resistant alleles. Furthermore, it was shown that the most common resistant allele in the field was Rop with extremely low frequencies of Rop 1 and R alleles (Arnold and Whitten %6b). Two recent studies have also con!2irmed zhe low frequency of the susceptible allele in various resistant populations (Hughes 1978; O'Flynn and Green 1980). Populations of L. euprina have not currently developed the maximum level of resistance possible with an RlA type mechanism even after a long period of exposure to insecticide; rather a plateau appears to have been reached in New South Wales in 1966 and no significant increase has occurred since (Hughes 1978). Shanahan and Roxburgh (197433) subjected field strains to heavy diazinon pressure and were able to select a maximum RF of 50 fold after 6 generations, attributable to the RlA allele; However, upon removal of selection pressure at generation 8, the level of resistance slowly fell, reaching about 20 fold by generation 25. There appears to be a -disadvantage associated with the resistant gene allele, since removal of insecticide pressure results in reversion of the population back to reduced resistance (Arnold 1972). The varying frequencies of susceptible and resistant alleles are a direct consequence of insecticide usage in the fi,eld (Shanahan and Roxburgh 1974a; Arnold and Whitten 1976b). An equilibrium therefore exists between two delete-' rious genes. The deleterious side effects associated with the RlA allele naturally tend to reduce the frequency of this gene in the population, when insecticide pressure is absent. However; genes conferring susceptibility or low level resistance are lethal in the presence of high insecticide concentrations. The maximum level of resistance possible due to the RlA mechanism, can therefore only be reached in the field if high selection pressure is constantly 103
maintained. The components of the equilibrium are the insecticide concentration to which the population is exposed and the frequency of exposure of the population to the insecticide. Although OP insecticides have been used in Western Australia for as long as. in New South Wales, the maximum level of resistance recorded in New South Wales has not been attained in Western Australia. It is likely that because of the shorter blowfly season in Western Australia, the blowfly populations are not under the same intensive insecticide selection pressure as are the populations in New South Wales. The lower frequency of insecticide use in Western Australia could therefore tend to favour the retention of susceptible alleles in the population. This hypothesis is supported by the present resistance studies in Western Australia which show that the RF in the wetter southern areas is double that in the drier inland localities (Table 2). It is generally accepted that in the dry inland areas of W.A. breech strike is the main type of strike and that it usually occurs for a relatively short period of the year. In the wetter southern areas which are subjected to frequent summer rains, both breech and body strike occur regularly and this is associated with an increase in the use of insecticides. The maximum levels of RF attainable have been set by the resistance type present and also by the pattern of insecticide usage. An increase in insecticide usage will be reflected in an observed increase in resistance; however the phenomenon is reversible. Any drastic change will probably be the result of the development of a different resistance mechanism. The most essential part of blowfly control programmes must aim at reducing the frequency of exposure of populations to the insecticide. For this reason management practices such as mulesing, crutching and ringing should form the basis of the fly control programme. Insecticides should remain useful for strategic control of body strike in particular situations. INSECTICIDE APPLICATION TO CONTROL BLOWFLY STRIKE IN SHEEP J. HERDEGEN*, P. HOPKINS** and R. ROBERTSON** This paper reviews current technology in the light of practical findings, and highlights developments which may determine future procedures for the external application of chemicals to control blowfly strike in sheep. The types of insecticides available to producers are continually changing, but there is little doubt that the efficacy of any one product depends on the efficiency with which it is applied. The cost of applying insecticides is a minor part of the total cost of fly strike control. The overall exercise embraces costs of mustering, insecticide and depreciation of jetting equipment. From estimates of current labour, operating and chemical costs, the cost of mustering is the major component of insecticide application. When a considerable amount of money is spent on mustering the sheep, it is important to obtain maximum benefit from their I treatment. Possibly the greatest cost however, is that from loss of sheep.and wool due to delays in, or lack of, prevention and treatment. * Department of Agriculture, Cowra, N.S.W. 2794. ** Queensland Department of Primary Industries, Box 46, G.P.O., Brisbane, Old- 4001, 104
TECHNIQUES OF EXTERNAL APPLICATION Spiralling costs of labour have necessitated the introduction of a degree of automation. Hand jetting technology (which incorporates a combination of high volume, high pressure, and large droplet size) has therefore been extended to automatic jetting races that operate on the same basic principle. These apply chemicals to the dorsal, ventral and posterior regions of the animal. The flow of insecticide is automatically controlled by the passage of unrestrained sheep in the race. Another technique recently developed,is the air mist system which uses low volume, low pressure, and small droplet size. FJith this method an air blast parts the wool and carries an insecticide mist to deposit chemical near the skin level. FIELD TRIALS Table 3 shows the results of field trials conducted in New South Wales to compare the efficiency of hand jetting with that of automatic jetting. TABLE 3 A comparison of hand jetting and automatic jetting to control fly strike in two and four tooth wethers Although the bulk of strikes occurred between days 45 and 55 after treatment, all treatment methods reduced- flystrike (P<O.Ol). Results for the combined age group showed no significant differences between treatment methods. However, differences did occur in the susceptible 2 tooth flock (PcO.05). This resulted from a greater number of strikes (regardless of type) in race B when compared to hand jetting (P<O.Ol). Further field evaluation is necessary to consolidate these results. Current field trials with the air mist system have been limited to artificial challenge because of the lack of natural field strike (Mills et al 1979). 105
SOME ADVANTAGES AND DISADVANTAGES OF CURRENT SYSTEMS The main advantages of hand jetting are the thoroughness of treatment that can be achieved and the fact that individual sheep may be treated according to requirement. However the advantages of an automatic jetting race are the decreased labour requirements and the speed of sheep handling. This reduces the costs from sheep and wool losses due to delays in treatment. The advantages of the air mist system are thatit opens the,wool all over the body to place a finely misted solution near the skin, and incorporates a conveyor to introduce a degree of objectivity not previously found in other techniques. This enables a predetermined volume of insecticide to be deposited over any part of the animal's body. Number of nozzles (up to 8), jet size l-4 L/min), air displacement (up to 55 m3/min) and conveyor speed (220-1200 sheep per h) can all be individually regulated to meet particular requirements. Hand jetting is an arduous, labour demanding and time consuming method of treating animals effectively. Also there is a greater risk of the operator being exposed to the chemical. Disadvantages of automatic jetting races relate to the problems of thoroughly treating long-woolled sheep, controlling the passage rate of sheep past the nozzles and treating all parts of the sheep. The major disadvantage of the air mist system is the capital investment involved when a conveyor race is incorporated into the unit. POSSIBLE FUTURE DEVELOPMENTS External Application Automatic jetting races in future may be designed to control body strike and to incorporate a system of controlling the passage of sheep past the nozzles, thereby making the jetting procedure more objective. The problems of penetrating long woql and increasing the period of protection may be at least partly circumvented by the formulation of insecticides which readily diffuse along- the staple, or have a systemic effect. Another possible means of furthering the air mist technology is the incorporation of an electrostatic grid to produce negatively-charged aerated particles,(either water or powder) which are attracted to the opposite (positive) charge of the skin. This development would further facilitate the deposition of active ingredient of any chemical at skin level. Systemic Administration The systemic administration of chemicals to combat fly strike warrants consideration since the larvae feed on the tissues and associated sera in the epidermis. This type of approach would herald a completely new era of fly control technology though its use would depend in part on the extent to which the systemic route of administration produced undesirable chemical residues in the sheep's body. This problem may be circumvented by the use of non-toxic compounds such as growth regulators and metallic salts which when administered systemically to the sheep may not cause residue problems, but will interfere with the development of larvae and afford the animal a degree of protection against myiasis. An obvious advantage of any systemic medication is that it provides an excellent degree of cover all over the animal and would effectively control poll, pizzle, crutch or body strike. 106