GRASS GRUB AND PORINA CONTROL

Press, Volume XCVII, Issue 28606, 7 June 1958, Page 9

GRASS GRUB AND PORINA CONTROL

Failures Of D.D.T. Mixes Reported Reports of failures of both wet and dry mix D.D.T. treated superphosphate to control grass-grub and subterranean caterpillar or porina have been coming into the entomological research station of the Entomology Division of the Department of Scientific and Industrial Research at Ashburton from all parts of the South Island. Indications are that the fault lies in commercially-prepared mixes.

Mr J. M. Kelsey, officer in; charge of the station, said thisweek that the complaints which) came mainly from Mid-Canter-1 bury, but also from North Canterbury, South Canterbury, Otago' and Southland, all related toi applications made in the autumn; of last year or since then. Treatments given to pastures on dry) land areas last year should have! been effective for three years! against both grass grub and porina and on irrigated or high rainfall areas for .two years against porina and three years against grass grub, said Mr Kelsey, but they had failed within a year. Even where applications had been made this autumn, which should have resulted in pasture plants sending down new root systems, it had been found that grass grub damage was continuing. On past experience this indicated that there was not DDT in the ground. Mr Kelsey said that the possible explanations for this situation were that the mixes had been carelessly applied. DDT was not present in the mix in the right quantity, or that possibly the grubs had built up a resistance to DDT. Long Experience Dealing with these possibilities, he said that many of the complaints had come from better than average farmers who had had up to seven years’ experience of the methods of applying the material. The fact that there had been no report of failure where farmers had prepared their own mixes indicated that it was the preparations of fertiliser works which were in question. The only way of checking on these mixes was to secure samples for chemical analysis from sacks which farmers had left over, but in only one case where complaints had been made to the station did the farmer still have some of the material left over. Samples of this mix will be submitted to the Dornin ion Laboratory for chemical analysis.

For the last three years no analyses had been taken of commercially prepared mixes in the South Island, said Mr Kelsey, but. of a batch of material recently taken from two fertiliser works in the North Island, it had been found that five out of nine samples were carrying less than the required quantity of DDT The possibility of the grubs having developed an immunity to DDT was ruled out by the experience of the station in trials in both the North and South Islands last year which showed that if the DDT was present in the correct amounts and in the requisite particle sizes good control was achieved. However should at any time such a resistance develop there were two alternatives available which were equally effective as grub killing agents These were Dieldrin and Lindane, which were respectively 25 and 50 times as toxic as DDT, bui which could still be used safely if recommended practices were followed.

Satisfactory Application

Discussing the application of DDT superphosphate to ensure satisfactory results, Mr Kelsey said it was essential that the insecticide should reach the soil surface and to that end the material should be applied to close grazed dry pasture. If it was not possible to wait for close ’ grazed pasture to dry, stock should be kept off after treatment until rain washed the insecticide on to the ground. Half an inch of rain was sufficient to wash insecticides off wet pasture if it was continuous, but once pasture had dried with insecticides on it three inches of continuous rain would be required to dislodge them It vyas also recommended that materials should be broadcast to obtain an even distribution over the whole soil surface. For insurance against attack and as a routine retreatmerit Mr Kesley said that the wet mix was by far the best proposition as there was no doubt about the uniformity through the mix of the DDT that had been added. In the wet mix process DDT in dry form or in a wet slurry is added to the dry crushed rock phosphate, and during the process in which the phosphate is sub jected to the sulphuric acid treatment the D.D.T. is carried very evenly through the batch ul superphosphate, but it reforms again on and in the superphosphate particles, which are relatively coarse compared with the drv mix particles. Mr Kelsey said that fertiliser works had been asked to keep available dry mix material for use in treating an existing grub infestation, as provided there was a little rain to wash it on to the ground it gave an almost immediate reaction and was four to six weeks quicker than, the coarser particle wet mix. Farmers should not be perturbed by grub populations increasing after treatment, said Mr Kelsey, for once the grubs had come into contact with the insecticide they no longer fed on the root systems and the value of the treatment should rather be assessed in the light of the response of the pasture.

When Mr Kelsey’s statement referred to him. Mr G. A. Claridge. business manager for Kempthorne Prosser and Company, who was speaking of the Hornby works, saiu that while occasional complaints were received from farmers it could, only be assumed that the material had been incorrectly applied. Against |hose who complained there must be

many farmers who had successful results and in the first four months of the current financial year sales of D.D.T. fertilisers had reached an all time high point being equal to the total of sales in the whole of the last financial year. Farmers would not be buying it in this way if it did not produce results, he said Since a batch mixer had been installed at Hornby two years ago Mr Claridge said that mixes had been good and with the process now in use it seemed practically impossible to put out a product that was not properlymixed. Care was always taken to check the para para isomer content of each shipment of D.D.T. to see whether it was up to strength and if it was not extra D.D.T. was added to make up for the lower strength. Mr Claridge said that his company had always co-operated with the Entomology Division in its requirements for particle size and it had carried out Mr Kes ley’s recommendations by using coloured D.D.T. to indicate whether there had been a good mix. If it was necessary to vary the source of purchase of D.D.T., samples of the new material were always submitted to Mr Kesley for examination.