D.D.T. ON THE FARM

Press, Volume LXXXII, Issue 24866, 4 May 1946, Page 3

D.D.T. ON THE FARM

MORE EXPERIENCE NEEDED BRITISH EXPERTS’ VIEWS Two useful opinions on the value of DJD.T. to farmers have recently been received from England. Both record that certain uses have been established, as valuable, but both give a warning that more experimentation must yet be done before the full possibilities, or possible dangers, of the substance are known. In a broadcast from the 8.8. C., Dr. C. l Potter said: “Starting with farm buildings, DJD.T. may prove a great boon by keeping flies in check in dairy barns and similar places. Exposed surfaces in the building are sprayed to give them a coating of DJD.T. and flies settling on this coating are killed. The coating may remain effective for several weeks or even months. “Turning from farm buildings to farm animals, D.D.T. has also shown promise in protecting sheep and cattle from various pests. Encouraging results have been obtained against tsetse fly, buffalo fly, and sheep blowfly, also against some of the tick parasites. Plant Pests “I find it difficult to make any generalisations about the usefulness of D.D.T. against plant pests, each case has to be treated on its merits. D.D.T. is not a cure-all. For instance, two major pests of fruit are codlin moth and red spider mite. DJD.T. has given encouraging results against codlin moth but has practically no effect on red spider, in fact it may increase the attack by red spider because it kills insects that prey upon the mites. Another instance of a similar kind is found with the brassicas or cabbage family. D.D.T. gives excellent control of flea beetles that attack young brassicas and the caterpillars that attack them later on, but it seems relatively ineffective against the cabbage aphis, although it appears to kill some of the insects that parasite it and prey upon it.

“The instances I’ve given only reflect in a more extreme way, the fact, that with TJ.D.T. as with all other insecticides, some insects are much more resistant than others. If the concentration is high enough DJD.T. will probably kill the active stages of most insects, but then so will insecticides. I don’t think that there are many insects that can resist the pyrethrins, if the dose is large enough, and the pyrethrins will kill at least some insect eggs while the available evidence indicates that D.D.T. does not kill eggs though the insect on

hatching may be killed. Another point to consider is that D.D.T. is rather slow in action. This doesn’t matter much with most insects, since they are disabled before they can do much damage, and are not able to reproduce before they die. With the aphides, however (those insects commonly known as greenfly and blackfly) the insects that form a colony produce living young, and reproduction of live insects may occur after the parent has been treated with D.D.T. Whether it is because of this factor or some other, my own experience so far has been, that it is not satisfactory against some species of aphides, for instance the cabbage aphis that I have already mentioned, and black bean apfos. By this I mean that it has not given satisfactory results in getting rid of an existing infestation, it may prove useful in preventing attack. “If you bear these reservations in mind, I can go*on to say that tests all ovej the world on a wide variety of plant pests have indicated that in very many instances D.D.T. compares favourably with the insecticides in current use and in some instances, for example, apple blossom weevil and green vegetable bug, has provided a possible chemical method of Control where none existed previously.

Two Important Considerations “Before D.D.T. is used on a large scale two important matters have to be considered. ’The first is the affect of D.D.T. on beneficial insects and the general balance of nature. And the second is its effect on human beings and warm-blooded animals generally. Bees are some of the most important beneficial insects. While it has been shown that D.D.T. can kill bees in the laboratory at concentrations that are likely to be used in practice, there is evidence that under field conditions they are not seriously harmed. However, more evidence is required before anything definite can be said. The effect of D.D.T. on the insects that parasitise and prey upon insect pests also requires further study. Experiments leave little doubt that some of these insects are very susceptible to D.D.T. and the consequences of their destruction might well be serious. “The balance of Nature is delicate and -intricate, and it seems that a substance like D.D.T. which has the, at present, unique property of being a powerful contact poison that can remain active over a long period of time should not at first be used in a wholesale and indiscriminate way but introduced cautiously with careful observations of its effect, not only on the pest on which it is being used but on the animal population as a whole. “Finally a word on its poisonousness to man and domestic animals. When dissolved in certain liquids D.D.T. can be absorbed through the skin and cause unpleasant symptoms and it seems likely that if eaten in large enough amounts it is poisonous. But,, where used with ordinary care there seems little danger that a single dose will be taken that will cause serious effects. However, there is some evidence that repeated doses of small amounts taken over a period may accumulate and cause trouble.”

Field Experiments Summing up an article in which much the same conclusions as Dr. Potter’s are reached, a writer in “Agriculture,” the journal of the Ministry of Agriculture, says: “Although a great deal of experimental work has been done in the past year or two. it is only ‘very recently that sufficient quantities of D.D.T. have been available for field experiments on pests of farm and garden crops, and for this reason it has not yet been possible to obtain full information about its possibilities and the best methods of using it for various purposes. Every effort is being made to fill the gaps in our knowledge but experimental work of this kind inevitably takes time. “Farmers and fruit growers who intend to try D.D.T. insecticides should in the meantime regard them as experimental and use them with due care until there has been time for entomologists and chemists to ascertain in more detail how they can be most efficiently and safely employed. We have in D.D.T. a chemical that shows great promise of being a most valuable addition to the toeans available for controlling many insect pests of crops and thus increasing food production. It would therefore be particularly unfortunate if it should be used wrongly in the early stages of its development.”