POSSIBLE SIDE-EFFECTS WITH CHEMICALS

Press, Volume CIII, Issue 30478, 27 June 1964, Page 9

POSSIBLE SIDE-EFFECTS WITH CHEMICALS

The opinion that the organophosphates should not be fully accepted for use under agricultural insecticide regulations in New Zealand until their possible side-effects on soil and plant life had been clarified was expressed this week by Mr J. M. Kelsey, officer-in-charge of the Lincoln sub-station of the Entomology Division of the Department of Scientific and Industrial Research.

Because the organophosphates broke down Very rapidly and were extremely safe from a residue point of view, Mr Kelsey said that there was currently a world-wide Swing away from use of the hydrocarbons to the organophosphates and in this context some were now fully registered for use in New Zealand.

Mr Kelsey’s concern is based on trial work against grass grub and porina at Halkett in central Canterbury. Here it has been observed that just over a year after treatments of two organophosphates—diazinon and thimet or phorate as it is now known —had been made in June, 1962, there was a marked deterioration in the pasture cover and the treated plots were actually inferior to the untreated control. It is suspected that a similar effect may also be associated with the use of some other organophosphates. Thirteen months after treatment with these materials at 21b active ingredient to the acre, there was a herbage reduction of 69 per cent on the diazinon plots and 82 per cent on the phorate plots compared with 52 per cent on the control plots.

It has been observed by Mr Kelsey, and also some overseas scientists, that plant growth is actually stimulated for a period after application of some of these organophosphates. This has also been noticed by farmers who have visited the Halkett trials. This effect, which shows up within a month of treatment, is characterised by a marked deep green colouration of the herbage, and it persists for about six months. Mr Kelsey has developed a hypothesis to account for the stimulation and subsequent deterioration effects, but he emphasises that it is as yet unproved. He believes that some of the organophosphates may be killing soil and nitrifying bacteria and the decomposition of these bacteria may be resulting in a temporary Increase in the nitrogen available to the plants, which would account for the stimulation effect. The subsequent deterioration effect would be caused by the reduced nodulation of legumes, such as clovers, with a slow reduction in the size of the legumes. In the field this was exactly what was happening, he said.

The effect on the clovers was not so much a reduction in their numbers as a reduction in the size of plants and of their leaves. Ultimately the effect on the pasture would be a loss of herbage or fodder plants and of seed yield from clovers. Even more serious, if his hypothesis was correct, • was the long-term effects on soils. If there was this effect on soil bacteria, which were largely responsible for the build-up of humus, the whole soil-plant biotic relationship could be upset and this would be accentuated by the fact that organophosphates would have to be applied annually for control of grass grub and porina, as they had a short effective life, and it had been shown over 17 years of trials in Canterbury that these pests reinfested the same soils year after year. Mr Kelsey said there was a suggestion that where there had been a ■ deterioration in the plots treated with organophosphate, there had since been some recovery of the clover on the margins with plots that had not had organophosphate treatment. In these cases there was a possible slow movement of nodulating bacteria into these plots in soil water. Mr Kelsey has been interested to see that Mr M. M. H. Wallace, of the Entomology Division of the Commonwealth Scientific and Industrial Research Organisation in Western Australia, and a number of others, have noticed the stimulation effect

after the use of these materials. Mr Wallace had conducted pot, laboratory and field trials and had referred to the effect in scientific papers as early as 1961. His work with five or six organophosphates and the nodulating bacteria of several legumes had indicated that there is a zone of inhibition close to the point of application but further outwards there are two zones of stimulation. He had considered that the stimulation effect would be of greater importance in the field than nodulation by bacteria, but Mr Kelsey points out that Mr Wallace’s trials were terminated after three months, which was still in the stimulation phase, and he had written to Mr Wallace to see if he had noted any subsequent deterioration effect in the field. Mr Kelsey said that none of those who had observed the stimulation effect had yet reported seeing the deterioration he had seen at Halkett. This deterioration effect had not been noted where trials had been conducted on a high ferility Nelson soil, he said, and the Soil Bureau was planning to look at the soils from the two areas to see if there were any differences in the two. Mr J. E. Cox, a pedologist of the Soil Bureau in Christchurch, said this week that if Mr Kelsey’s hypothesis was correct, it might be expected that there would be a similar effect on all the soils of the sub-humid parts of Canterbury with a rainfall below 35 inches, except those with high water tables. This would run into millions of acres. Mr Cox said that the soil at Halkett was a Hatfield fine sandy loam. It covered an area of 20,000 acres in Canterbury. In the same group of soils were the Templeton and Paparua, and the Chertsey was closely related.

The plant physiology section of the Crop Research Division at Lincoln is beginning trials aimed at proving or disproving Mr Kelsey’s hypothesis.