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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.
Some Recent Developments of Insecticides in Veterinary Practice
N. P. H. GRAHAM, B.V.Sc., Technical Adviser, Wm. Cooper & Nephews (Aust.) Ltd.
The post-war era has seen the development of a continuous stream of new synthetic insecticides by chemical manufacturers all over the world. Only a small proportion of those which are evolved in the laboratory ever reach the stage of commercial production but, even so, the to the list of insecticides in common usage must be bewildering to those who are not in close association with this work.
In order to understand more clearly what is going on, it is as well to realise the normal course of development of those insecticides which eventually come into use in the veterinary field. The chemical is evolved in the laboratory and tested against a group of insects such as house flies, cockroaches, aphids and other insects which can be maintained easily in laboratory colonies, and which are usually agricultural pests. Compounds which show some insecticidal activity with this initial screening are usually then tested against agricultural pests in the field and, while this is going on, problems in relation to manufacture, formulation,etc. are ironed out. It is generally about this stage of development, i.e. after they have been proved effective for agricultural purposes, that the insecticides come under the notice of people in the veterinary field. This may be some years after first official notice has been given to the insecticide.
During this initial stage, there is often confusion because the new insecticide frequently only has a code number and it may be given one or two tentative names before it acquires a standard name. For example, Chlordane was first known as Compound 1068, Dieldrin as Compound 497 and then as Octalox, Toxaphene as Compound 3956,etc. Although this period of stabilisation may last for two or three years, it still frequently happens that insecticides are launched into fairly widespread use before adequate methods have evolved for detecting their presence by analytical methods, and before their chemical structure has been determined precisely.
So we find that in dealing with new insecticides they may become widely used for insect control while there are still many gaps in precise knowledge of their behaviour.
In the veterinary field, we are probably fortunate that the number of insecticides which come into use are limited and, therefore, we are better able to assess their true value.
This insecticide race by the chemical industry started with the successful synthesis of such insecticides as D.D.T., B.H.C., Chlordane and Toxaphene; and in order to understand the trend of insecticide development, it is useful briefly to review the general behaviour of the various loose groups of insecticides.
Some 50 years ago the choice of insecticides was limited to a few inorganic compounds, such as arsenic and sulphur, and to some coal tar derivatives, such as the phenols. In general, those which possessed a good toxicity to insects had to be used at concentrations which approached the danger level as far as the host was concerned. This weakness is familiar to you all. The insecticides which were safe to use, such as sulphur, usually did not have a high efficiency against the insects.
The next advance was the introduction of some materials of vegetable origin, such as derris root, which had a very high toxicity to certain insects and were safe to use. However, most of these had the disability of being comparatively unstable and exerting little or no residual action.
This period also saw the introduction of the first of the synthetic insecticides which, however, had similar properties to the insecticides already available.
During the last war, we saw the introduction of another group of synthetic insecticides, and it is within this group that most of the present developmental work is taking place. These insecticides, which are roughly grouped as the chlorinated hydrocarbon insecticides, started with D.D.T. and have developed to such compounds as Dieldrin and Aldrin. Their main characteristic is that they are compounds having a very high toxicity to insects and a prolonged residual action. They also have a low toxicity to the host. Although in the later members of this group there is a higher toxicity to mammals, this is offset by the high dilutions at which they are used but this factor remains as a hazard for the operator and manufacturer who may handle concentrated products.
In many fields the prolonged residual action which was considered such a desirable feature in this group has brought with it the disadvantage of the selection of resistant strains of insects and, in fact, in some fields it is no longer possible to use them.
This has led to the development of a further group of synthetic insecticides and these are called the organic phosphates. They have an extremely high toxicity to a wide range of insects, little or no residual action but have the disadvantage of a high toxicity to mammals in their concentrated form. Parathion was the first member of the group; the newer members, such as Diazinone, Malathion and Overtran, are designed to reduce the mammalian toxicity even if by so doing some of the insecticidal toxicity is also lost.
This brief review should give you a general outline of what the insecticide chemists are trying to do and will explain to some extent why there is a never-ending stream of new insecticides.
In the veterinary field in this country, it is as well to remember that arsenic is still by far the most popular insecticide for controlling parasites on livestock. The new synthetic insecticides, except in a few specific cases, offer little advantage in controlling most parasites but do definitely have a greater safety factor. However, the average grazier is prepared to risk the losses from arsenic rather than pay the higher price of the newer insecticide formulations.
There are, though, some fields in which the newer insecticides are outstandingly superior.
The control of keds by B.H.C. is one of these and a still more outstanding example is in the use of Dieldrin and Aldrin for the control of flystrike. This latter development has taken place over the last two or three years and has proved so effective that the use of these two insecticides has already largely superseded all other insecticides for this purpose.
The main advantage that these two insecticides possess is the extraordinary way in which they persist on wool. This has been rather startlingly demonstrated by Dr. Lipson of C.S.I.R.O., who has found that wool collected from sheep which have been treated some months previously with low concentrations of Dieldrin is still toxic to clothes moths after it has been subjected to the normal commercial scouring process.
For controlling flystrike, Dieldrin and Aldrin are both excellent larvacides, but so are many other insecticides, and to use them to their best advantage their extraordinary long residual action should be exploited to the utmost, i.e. they should be used for protection rather than as a cure for struck sheep.
As was said earlier, these new insecticides often come into widespread use before they have been fully investigated.
In the case of Dieldrin and Aldrin a number of trials have been carried out in the insectary and extremely long periods of protection (30-33 weeks) have been obtained when the treatment was challenged by artificial strikes. Under these conditions, Dieldrin and Aldrin were equally effective. In field trials, however, the period of protection has been very much shorter and Dieldrin has been found to be more effective than Aldrin. Also in the field, we have found a considerable variation in the period of absolute protection given in different trials and this may be due to the effect of wool length at the time of treatment as suggested by Fiedler and du Toit, 1954 (Onderstepoort Journal of Veterinary Research 26 (3) 405), who found that the length of protection was to some extent proportional to the length of wool when the sheep were treated.
In my own field trials, it has been found that with Dieldrin a minimum of about 9 to 10 weeks complete freedom from crutch or body strike can be expected following treatment by jetting with concentrations of about 0.025% or 0.05%. There was also a definite reduction of the strike rate for a further 4 or 5 weeks. When lower concentrations were compared with higher concentrations, there was no appreciable reduction of the period of absolute protection; but during the breakdown period a higher strike rate can be expected with the low concentration. With rams' heads no really satisfactory tests have been carried out owing to the difficulty of holding experimental groups long enough, but the general experience is that treatment gives about six months protection. During some of these trials six inches or more of rain was recorded and the results remained satisfactory. There is one other feature worth noting and that is that during the period of partial protection the strikes are often restricted in size and carry only a few maggots (5-10). A similar picture was seen during the breakdown period with Arsenical preparations but in this case the breakdown was of comparatively short duration.
During the above tests, Aldrin treated groups were included from time to time and the comparative results were as follows:
TEST 1.
Merino ewe weaners treated for body strike. Dieldrin 0.1%, Aldrin 0.1%. No strikes over a six week period. Controls 40%.
TEST 2.
Merino lambs jetted at marking time. Dieldrin 0.3%. Aldrin 0.3%. Sheep, examined 14 weeks later. Dieldrin, 12% struck but only one extensive strike; Aldrin, 14% struck and about 1/3rd extensive strikes.
TEST 3.
Merino ewe weaners (unmulesed) jetted on crutch. Dieldrin 0.05%. Aldrin 0.05%. First strike recorded in the Dieldrin group 11½ weeks after treatment and in the Aldrin treated sheep 9 weeks after treatment. Strikes to 14th week after treatment: Dieldrin group 14%, Aldrin group 27%. Controls 46%.
TEST 4.
Merino ewe weaners jetted on crutch. Dieldrin 0.1% and 0.025%, Aldrin 0.1%. No strikes were recorded in the 0.1% Dieldrin group for 18 weeks. First strike recorded in 0.025% Dieldrin group 14 weeks: in 0.1% Aldrin group 12½ weeks. Strikes recorded up to 18 weeks: 0.025% Dieldrin group 7%, 0.1% Aldrin group 9%. Controls, 63 strikes per 100 sheep: including 30% between 12½ and 18th week.
My colleagues in England have observed a similar difference in their field trials, and one trial carried out by the Queensland Department of Agriculture also showed the same relative trend.
In assessing the relative value of Dieldrin and Aldrin much stress is laid on the lower cost of Aldrin, but the comparison is always made at equal concentrations. The above tests, however, indicate that at equal concentrations Aldrin is inferior to Dieldrin and in the 4th test, Dieldrin gave the same results as four times the concentration of Aldrin. In this latter test, costs would have favoured Dieldrin by virtue of the higher dilution used. It is probable that the concentrations we have been recommending for Dieldrin are higher than they need be for most practical purposes as changes in climatic conditions and shearing or crutching generally determine the period for which protection is desired.
In designing these tests, we have assumed that all adult ewes will be mulesed and the treatments tests were designed to protect lambs and weaners from marking till such times as they could be mulesed. By also providing an economic prophylatic treatment against body strike in weaners and on lambs heads, we feel that the blowfly problem has at last been solved effectively, but there is no doubt these insecticides will be exploited in other veterinary fields.
It is difficult to see how the present standard of application can be raised effectively as long as we are confined to the external application of insecticides by dipping or jetting. In this respect the most promising line of investigation is the systemic use of insecticides by giving them either orally or by subsutaneous injection. Already there have been an impressive number of successes by this method of application but, unfortunately, with the present insecticides the margin of safety is rather small.
Even the rapid advance in the chemistry of insecticides will do little to solve various parasitic problems unless attention is paid to the method of application.
In conclusion, we are making continuous advances with the evolution of insecticides but it must be remembered that any insecticide is only as good as the man who applies it.