Replacement for DDT may be available in New Zealand soon

Press, 20 April 1979, Page 9

Replacement for DDT may be available in New Zealand soon

When Britain’s “Sunday ‘Times” trumpeted the arrival of NRDC 143 in January 1975, it was not just a case of sensationalism, for being reported was the discovery of a long sought-after chemical insecticide which had all the qualities of DDT, but none '6f its drawbacks. In the next few years *NRDC 143, known commercially as “Ambush,” Will make its mark on New' Zealand agriculture, according to the manufacturer.

DDT had been the first chemical to revolutionise the control of insect pests of agricultural and public health importance. Until its development in the early 19405, nearly all insecticides could be placed in one of three categories: fumigants, stomach poisons or contact poisons. ‘ To be effective, stomach Poisons like lead arsenate or Paris green must be eaten by insects whereas contact poisons, such as petroleum oils and pyrethrum, must be sprayed directly on to the target pests.

The significance of DDT was simple: For the first time, inanimate objects Could be treated with an insecticide which would kill insects merely walking or landing on the treated surface at a later time. DDT was the first truly residual and practically useful insecticide because of its persistence.

Its only fault was that it was too persistent, as became obvious in due course.

DDT accumulates in the food chain — being stored in the body fat and excreted in the milk. It is estimated that an average United States citizen carries about 100 to 200 mg of DDT in his body and the occurrence of it 'in the environment — in the soil, for example of forests where it has never been applied, and in the air — led to an increased public awareness of the possible damaging effects of pesticides to the environment, and to the ban of DDT for agricultural use in most countries.

Parallel with the development of DDT and other organochlorine compounds, other chemical groups were also investigated. Particular emphasis was placed on organo-phos-phorus and carbamate compounds, but after

many years of research and expenditure, no compound was found which was as effective as DDT.

By the late 19605, because of the absence of effective and economic alternatives, resistence to DDT and the oganochlorines had developed into a serious problem.

With increasing awareness of some other undesirable side-effects of these compounds, many scientists felt that the research and development programmes to find alternative pesticides had to be re-examined and intensified. The world was waiting for a new' discovery of the scale of DDT, with . none of its drawbacks.

In 1948 work had begun at Rothamstead in England under the auspices of the National Research and Development Corporation, to produce synthetic pyrethroids. The original discoveries followed much of the pattern of natural pyrethrins and were not altogether successful — except for use in fly spray indoors.

Natural pyrethrum is probably the oldest insecticide used on a wide scale. It is a flower extract of different wild or cultivated species of Chrysanthemum (Pyrethrum'). Although the pyrethrins present in pyrethrum are very active against a wide range of insect species and safe to mammals, they are very rapidly broken down in the field and are of limited practical use.

The race for the discovery of the first stable, pyrethroid went on and in 1973. what shows every sign of becoming the most important advance in pest control since the advent of DDT was discovered: The NRDC 143 series of pyreth r o i d insecticides, stable synthetic analogues of naturally occurring compounds with sensational activity against insects and low toxicity to higher animals. NRDC 143 has been given the chemical name of permethrin and the' trade name of “Ambush.” . "

“British' scientists- invent super pest-killer” — was the front page headline when “The Sunday Times” announced the discovery of permethrin on Januarv 12. 1975. “NRDC 143, in fact, comes so close to the specification for an ideal

chemical pesticide that it is spoken of with awe in the industry,” said the article. “Ambush” acts essentially as a contact and stomach poison. Furthermore it’ exerts significant anti-feeding effect against many pests even after the concentration ■ on plant surfaces has dropped to sub-lethal levels. The real, nature of this effect has not yet been explained but it has been observed in the field that, although larvae of corn earworm, tomato fruitworm and diamond-back moth were present on the plants very little damage had been caused, and later the pests died, probably because of the combined effect of poisoning, starvation and dehydration. No Other pyrethroid has so fat been reported as having anti-feeding effect.

“Ambush” is non-syste-mic. meaning the chemical is not taken up by the plant tissue, and thus diluted: it remains where the caterpillars come into contact with it. Also one can expect that there is virtually no danger of finding undesirable levels of residues. But it also means that all surfaces where pests harbour and feed should be sprayed.

“Ambush” is non-vol-atile and virtually no losses occur through evaporation. As a very rough guide, the vapour pressue of permethrin at normal temperatures can be taken

to be comparable to that tzf’ DDT, and about 250 times less than that for methomyl. This means that the active substance is not carried away by the wind and that it has no unpleasant smell.

“Ambush” and other synthetic pyrethroids act on the insect nervous system. The exact mode 'of action is not yet fully understood, but if appears that the compounds interact with nerve membranes, blocking ion channels, and so preventing normal conduction of the nerve impulse. This is similar to the mode of action of DDT. The sensory nervous system is probably one of

the first sites affected, and this would explain the insect repellent properties of “Ambush,” shown by reduced feeding and egg laying. It also penetrates quickly to the site of action, resulting in quick knockdown. Although py-

rethroids can also affect the nervous systems of other animals, penetration and other factors prevent them reaching the nerve and this explains their relative selectivity to insects. •

“Ambush” is a broad spectrum insecticide, but exhibits the highest degree of activity against leaf eating- caterpillars. It also controls beetles, weevils and boring or sucking insects, but in this case the lack of systemic activity and fumigant effect may be a slight disadvantage and the rate advantage over conventional and more specific products may not be as extreme as on caterpillars. “Ambush” can reduce the hatching rate considerably if egg clusters have' been sprayed.

“Ambush” is relatively safe to humans and other mammals when compared with nearly all currently available insecticides. An ave r a g e-sized human would have to consume or absorb through the skin more than half a litre of “Ambush” 50 E.C. td receive a lethal dose. “Ambush” is more than 1000 times more toxic to insects than to rats, while malathion, considered to be one of the safest in-* secticides, is only 22 times

mote toxic to insects than rats. The selectivity of pyrethroids is far higher than the selectivity of any commonly used organochlorine, organo-pitos-phorous or carbamate insecticide.

While highly toxic to fish, .pyrethroids use is unlikely to pose any real hazards to marine life, as excessive concentrations would be required over creeks, rivers or lakes to affect aquatic animal life. Contamination of this magnitude could only be the result of gross misuse of the insecticide, such as the injection of large quantities of near chemical into streams, rivers or lakes.