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This article was published in 1951
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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.

Prevention and Control of Body Strike of Sheep

G. J. SHANAHAN, B.Sc.Agr., Entomologist, Department of Agriculture. N.S.W.

Prior to 1947 no serious attempt was made in Australia to find a means of combating body strike of sheep. The neglect of this aspect of the blowfly problem was due to (1) the bias in the past towards finding an answer to crutch strike which can be troublesome in most years, whereas serious body strike usually occurs at intervals of five years or longer; (2) the lack of promising insecticides and (3) the difficulty of obtaining a definite indication of the value of any particular body strike preventive method from field trials.

From 1947 onwards the body strike problem has been investigated with considerable energy, for the main factors which delayed the discovery of an effective remedy no longer applied. With the crutch strike problem virtually solved, research institutions were able to devote attention to developing a method of combating body strike, using promising chemicals, namely DDT and BHC, and in the past three years serious waves of body strike occurred.

The results from this programme from late 1947 to autumn 1950 formed the basis of the body strike control measures which were given in the Joint Blowfly Committee Pamphlet (Sept., 1950). It was stated that good protection from body strike would result when sheep were either sprayed of jetted with 1.0% DDT or 0.05% gamma isomer BHC, and by passing them through a swim dip containing DDT or BHC at not less than four times ordinary dipping strength.

All concerned with the publication of this pamphlet realised that further information was required before the limits of the value of DDT and BHC, as body strike preventives, would be known. The opportunity was taken over the past six months during which blowflies have been spasmodically active, to increase our knowledge of the body strike prevention problem.

One very important contribution in this connection was the demonstration that DDT and BHC will give at least six weeks' protection from body strike providing the insecticides are applied correctly. Unanimity of opinion has not been reached concerning the best manner of applying the insecticides. The writer's impressions of the value of the various methods, of which a brief description is given, are presented in this article on body strike control. These impressions are based not only on the results from recent field tests but also on all the information available from Australia and overseas from insectary and field trials on the use of DDT and BHC as body strike preventives.

Before commencing this account an explanation is given of the manner in which DDT and BHC prevent body strike. These explanatory remarks are presented now because they have a bearing upon subsequent remarks regarding the effectiveness of the application of the insecticides by various methods.

REASONS WHY DDT AND BHC PROTECT SHEEP FROM BODY STRIKE

Whilst these insecticides have several common properties, each possesses a distinct characteristic, partially shared by the other, which enables both materials to protect sheep from body strike. Sheep which have been treated with DDT are protected for two main reasons, the first of which is the more important. Firstly, the chemical prevents oviposition, and secondly, being toxic to the adult flies, it reduces the blowfly population. BHC shares these characteristics with DDT, but its value in preventing oviposition is much less pronounced. This insecticide protects sheep both by its contact and fumigant action against maggots.

Theoretically, it therefore would seem possible to protect sheep from body strike by merely treating the tip of the wool with either insecticide. But surface application is insufficient, unless some degree of penetration is obtained, for a gravid female fly on being attracted to the susceptible sheep quite frequently prefers to lay her eggs in the wool some distance from the tip. If a particular treatment leaves an ineffective barrier of insecticidal residue, a partial or complete breakdown is inevitable when very favourable conditions for strike occur.

The need for something more than treating the tip of the wool will be appreciated when it is realised that "breaks" in the fleece appear, particularly after rain, thus exposing zones of untreated wool in which the female blowflies are able to deposit their eggs, from which larvae can hatch subsequently. These larvae set up strike lesions, unhindered by the barrier of insecticide above.

The above remarks explain how DDT and BHC protect sheep from body strike when the insecticides are applied by means of spraying, dipping and jetting at the concentrations recommended by the Joint Blowfly Committee. If these materials are used at strengths several times higher than those that are recommended, for the conventional treatments just mentioned, it is possible, although not established, that a surface application of high concentrations of DDT or BHC, for example by fogging, might protect sheep from fly attack by drastically reducing the fly population for a time.

METHODS OF APPLYING PREVENTIVE TREATMENTS

The available methods of applying the preventive treatments can be classed into two categories, namely, surface application to and deep penetration of the fleece. The aim in surface application is to treat, evenly, the tip of the fleece although some penetration into the wool results. The objective in deep penetration is to wet the wool to the skin. Saturation of the fleece is not obtained although something approaching this state is achieved.

The terms "spraying" and "power spray dipping" of sheep occur repeatedly throughout this article. Spraying implies the application of a treatment either by means of spray equipment, or in a power spray dip with modified nozzles which give a reduced output of the selected body strike preventive material. The dipping of sheep in power spray dips implies their treatment in these units with the ordinary overhead nozzles.

A.—TREATMENTS WITH DDT AND BHC AT LOW CONCENTRATIONS

In this section reference is made to the value of applying DDT and BHC at concentrations not in excess of 1.0% and 0.05% gamma isomer, respectively, as body strike preventives. One's impressions of the value of the various available methods of applying the preventive treatments at these concentrations are given under two headings, surface application and deep penetration.

1.—SURFACE APPLICATION: SPRAYING

Sheep can be sprayed individually by hand or collectively in groups. Hand spraying is carried out most conveniently an a long narrow race; preferably provided with a grated floor. The operator can stand outside the race. The wool along the back from the poll to the rump and half-way down the sides of the sheep must be treated. Engine-driven jetting or spray plants are essential if large numbers of sheep are treated, although manually-operated equipment can be used. A handpiece fitted with two nozzles, which give either a rosette or fish-tail spray, has been found satisfactory for applying the insecticides.

A pressure of 100 to 150 lbs. is adequate for spraying. Further work is required to ascertain whether the wool can be wetted sufficiently by much higher pressures and the desirability of increasing nozzle apertures for speeding up spraying times.

The manufacturers of power spray dips recently have devised alternative nozzles for use with their plants for body strike control. The modified nozzles give a reduced output of spray preparation. The run-off is not used. If a power spray dip is not available, the sheep can be treated in groups by arranging a spray. e.g.. a Pope Rotor sprayer, for which the insecticidal preparation is supplied by either a jetting or spray plant or small centrifugal pump, above a pen about 14 feet square. A catching pen in a shearing shed is ideal for the purpose.

When the sheep are sprayed in groups it is essential to ascertain the delivery rate from nozzle(s) so that the time of treating each pen of sheep can be determined.

One operator can hand-spray up to 250 sheep per hour, whilst 400 to 600 sheep per hour can be treated in a power spray dip with the modified nozzles. The speed of spraying depends upon the rate of application.

Good protection from body strike has been obtained when 1.0% DDT is applied at the rate of one-sixth gallon per head during a moderate fly wave. This volume is really insufficient to meet all emergencies and the recommendation is made that sheep should be treated at the rate of ¼ to ½ gallon per sheep, depending on the size of the sheep and the length of the wool.

Spraying at a rate of approximately ¼ gallon per sheep with any of the methods described in this section will do little more than wet the tip of the wool. If there is an indication that the blowflies will be very troublesome, the aim should be to apply DDT at the higher application rate of ½ gallon per head, particularly with sheep in long wool. The increased rate of application will result in a more satisfactory wetting of the wool and reduce the incidence of body strike during the period for which the treatment is effective.

The surface application of BHC at 0.05% concentration by any of these means will reduce the incidence of body strike also, but this insecticide will not give as effective protection as DDT. Under severe fly wave conditions a surface application of BHC at the rate of ¼ gallon per head will not give a satisfactory degree of protection from body strike. Superior protection can be obtained by increasing the rate of application to approximately ½ gallon per head. Sheep with several months' growth of wool should not be sprayed with less than this volume of BHC preparation.

The degree of protection afforded by either treatment possibly can be improved by any means which increases the quantity of insecticide which is left in the fleece following treatment. The amount of DDT and BHC which is retained in the fleece can be increased also by reducing the run-off. This can be achieved by lightly spraying the sheep with water or insecticidal preparation before treatment. Run-off can be reduced also by the addition of a wetting agent to the spray material.

2.—DEEP PENETRATION

(a) Plunge and power spray dipping. Body strike can be prevented effectively by treating sheep in either DDT or BHC in plunge or power spray dips. Whilst the wool of sheep, treated in this way, is not wetted to the skin, providing sheep are dipped well, either method will give an adequate degree of saturation of the fleece to provide good protection against body strike. This method has much to commend it for there is nothing new for the pastoralist to learn beyond the realisation that the insecticides should be used at several times the concentration of that used for normal dipping against lice and ked.

DDT can be used at a concentration as low as four times dipping strength (0.3 to 0.4% concentration). The BHC treatment should be given at a concentration not less than six times ordinary dipping strength (approximately 0.05% gamma isomer). Reliable dip formulations should be selected and additional concentrate should be added in accordance with the manufacturer's instructions.

The treatment of sheep in either a power spray or plunge dip doer not wet the wool to the skin. However, the writer feels reasonably convinced that greater protection from body strike will be obtained from a preventive treatment in a plunge or power spray dip than is obtained by surface spraying, unless the volume of insecticidal preparation which is applied during the spray application is sufficient to give a degree of wetting of the wool comparable to that which follows dipping.

The application of the insecticides by this means has the advantage that sheep with fly strike can be treated. Most of the strikes will be freed of maggots, but subsequent inspection of the previously "struck" sheep is necessary, particularly when hairy maggots are present. If the position arises where resort is made to dipping of sheep with strikes, BHC should be the insecticide of choice, until more is known of the value of treating "fly struck" sheep with DDT.

The treatment of sheep by dipping has two major objectives. Firstly, if sheep in long wool are treated, staining of the fleece might occur with resultant lowering of its value. Secondly, whilst losses of sheep following dipping are not common, heavy mortalities and post-dipping disorders occurred when sheep heavily infested with grass seed are treated in a plunge of power spray dip.

The danger of post-dipping losses is increased when graziers, mainly due to lack of time, and expense, in times of major fly waves, decide to treat their sheep in dip preparations which have been left standing for days. Losses cannot be overcome always by the use of bactericidal and bacteriostatic agents.

(b) Jetting. Good protection from body strike can be obtained by jetting with 1.0% DDT and 0.05% gamma isomer BHC. A powered plant, capable of maintaining a pressure of about 80 lbs. per sq. inch. is essential where large numbers of sheep require treatment. When applying the insecticides in this way the writer prefers a five jet nozzle. The sheep are treated in a long narrow race, preferably holding at least 40 sheep, and fitted with a grated floor. The operator works in the race. Each sheep is jetted by making a sweep, with the nozzle close to the wool and pointing slightly forward, firstly along the mid-line of the back from just behind the hips to the poll. A sweep is then made to each side of the mid-back. In addition, the wool on the sides of the neck and shoulders is treated. The rate of application depends upon the growth of wool and size of the sheep. A little more than gallon per head is required for sheep with up to three months trom shearing. From three to six months off shears, approximately ½ gallon per head will give the desired degree of saturation of the wool, whilst a little more than 1 gallon per head is required for sheep with more than six months' growth of wool.

Jetting is laborious and reasonably slow procedure. One man is really working to treat 100 sheep per hour. Since the operator stands in the race with the sheep waterproof clothing should be worn for comfort and protection.

The quantity of run-off can be reduced and the amount of insecticide which is retained in the fleece can be increased by the addition of a wetting agent to the jetting formulation.

Jetting with either 1.0% DDT or 0.05% BHC will give good protection from body strike irrespective of the severity of the fly wave. Some strikes will occur, especially away from the treated zone of wool, but the writer's experience has been that jetting will give superior protection to surface spraying.

Whilst there is an indication that DDT can be used at a concentration as low as 0.3%, sheep should not be treated with BHC below 0.05%.

B—TREATMENTS WITH DDT AND BHC AT HIGH CONCENTRATIONS

Investigational work is required to ascertain whether body strike can be prevented by the application of a low volume of an insecticidal formulation containing a high concentration of either DDT or BHC.

Fogging is one method of applying the insecticides in this manner. Although a number of graziers have been using fogging machines for combating body strike there is no experimental evidence to support the view that this development will give good control. In a trial where a severe fly wave occurred a little more than six weeks from treatment, fogging with either 10% DDT or 2% gamma isomer BHC or 5% DDT / 1% BHC in combination failed to afford satisfactory protection. The 10% DDT treatment was the most effective but the degree of protection given by it was markedly inferior to that obtained by spraying or jetting with either 1.0% DDT or 0.05% gamma isomer BHC.

Whilst fogging will not give good control of body strike when a fly wave is experienced six weeks from treatment, further work is required to show whether it will give protection for any time. An investigation is required not only to establish the maximum period of protection but also to ascertain whether the adult fly population can be reduced drastically following the fog treatment of sheep. DDT appears to be more promising than BHC for the prevention of body strike by fogging.

The procedure of fogging sheep is outlined in a separate article in this Journal.

DEGREE OF PROTECTION

Whilst the writer's impressions of the value of the various methods of applying DDT and BHC as body strike preventives already have been given, it is considered advisable to make a brief comparison of their merit. It is felt that jetting and dipping, either in a plunge or power spray dip with ordinary nozzles, will give better protection than surface spraying. However, the degree of protection obtained from spraying depends upon the rate of applicatior and the selection of the insecticide. If the decision is made to spray with a low rate of insecticidal preparation, DDT will give better protection than BHC.

Further work is required to establish the value of applying high concentrations of DDT and BHC by fogging, or any other means, as body strike preventives.

LENGTH OF PROTECTION

The period for which sheep can be protected from body strike depends largely upon the method of application and the severity of any subsequent fly waves. Jetting and dipping in either a plunge or power spray dip with either DDT or BHC at the recommended concentrations will give at least six weeks' good protectior from body strike.

Surface spraying with 1.0% DDT at a rate of application in accordance with the suggestions made earlier in this article also will give this period of protection.

There is reason to believe that surface spraying with BHC at 0.05% gamma isomer concentration will not give good prevention of body strike unless the wool is well wetted by the treatment.

MATERIALS

There are many suitable DDT and BHC concentrates which have been prepared for sheep dipping or horticultural purposes, which, can be used for surface spraying and jetting. A reliable dip preparation of either insecticide should be selected if the preventive treatments are given by means of a plunge or power spray dip with ordinary nozzles.

The opinion is expressed that BHC, which is normally available ar various dipping preparations at country stores or is already on hand, has greater utility as a body strike preventive than DDT for the latter insecticide is rarely procurable in quantity at short notice from suppliers in the country.

Highly concentrated solutions of DDT and BHC alone or in combination can be purchased for the fog treatment of sheep.

DANGER TO SHEEP AND OPERATORS

Whilst sheep can be treated safely with 1.0% DDT and 0.05% gamma isomer BHC, workmen should not expose themselves unduly to either insecticide. Protective waterproof clothing should be worn for any treatment, especially jetting, which would otherwise result in the men working in clothes which are saturated with the insecticidal preparation.

The danger to sheep and operators of exposure to fogs containing DDT or BHC is mentioned in a separate article on fogging in this journal.

ADDITIONAL COMMENT

Due regard should be paid to the normal precautions which apply to the dipping of sheep, when body strike preventive treatments are given. The suggestion is made also that sheep should be allowed to dry in the shade as far as possible, especially on hot days. The rate of evaporation will thus be reduced and enable the insecticide to spread through the wool.

TREATMENT OF "FLY STRUCK" SHEEP

Under some circumstances, particularly on large properties, situation occasionally arises in which it is necessary to mass treat a large number of sheep with strikes. The value of plunge and power spray dipping in this regard already has been mentioned.

It was stated also that treatment, even with BHC, cannot be relied upon to free all the strikes of maggots. A large number of strikes can be freed of maggots completely if milk oil fluid is added to the BHC preparation at the rate of 1 pint to 25 gallons. The combination treatment can be given conveniently in a power spray dip with ordinary nozzles or by jetting. Where jetting is employed every care should be taken not to direct the jet on to the broken skin. The wool around the strike should be saturated and the excess material should be worked into wool above it.

FUTURE RESEARCH WORE

Although it is now clear that body strike can be prevented with DDT and BHC providing the insecticides are applied correctly. there are many aspects of the problem which require investigation. This work will be aimed at discovering the best method of applying the minimum quantity of insecticide necessary to give good protection. The benefit to be gained from incorporating wetting agents in materials for use in body strike prevention should be explored.

There will be great interest in any work which attempts to show whether the adult fly population can be reduced drastically, with an accompanying decrease in the incidence of strike, by the application of a formulation at high concentration, e.g., DDT, by fogging or spraying.

Most of the work on the Australian body strike problem has been undertaken with DDT and BHC. Toxaphene, chlordane and dieldrin are also promising in this regard and should be tested in the field.

 


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