Grub Resistance In Mid-Canterbury

Press, Volume CVI, Issue 31101, 2 July 1966, Page 9

Grub Resistance In Mid-Canterbury

From results to hand so far it appeared as though resistance to D.D.T. in grass grubs existed in a number of pockets in MidCanterbury, the principal one of which was in the area between the Hinds and Rangitata rivers on the Lismore type soils, Mr F. C. Allen, a field research officer of the Department of Agriculture, said this week.

Mr Allen was commenting on a survey of grass grubs in the Ashburton County being undertaken by the research division of the Department of Agriculture in conjunction with the Entomology Division of the Department of Scientific and Industrial Research. Samples of grubs from the area between the Hinds and Rangitata rivers varied in their tolerance to D.D.T. as measured by laboratory tests, he said. Their reaction to D.D.T. ranged from highly tolerant to very susceptible. While the tolerant samples predominated in this area, cases had been found where grubs from neighbouring paddocks were tolerant on the

one hand and susceptible on the other hand. Another smaller pocket of tolerance existed in the Dromore district, said Mr Allen. Elsewhere in the county tests indicated that the grubs were fully susceptible to D.D.T. A point which had still not been fully resolved was the relationship between the laboratory tests of the grubs and field use of D.D.T. Laboratory tests had shown grubs to be susceptible in spite of the fact that farmers were getting poor results with the use of D.D.T., which had been applied regularly over the years. As had already been pointed out in previous articles on

these pages, failure of control could depend on a number of factors. Poor results had been caused by poor mixing, applying straight superphosphate in error thinking it to be D.D.T. super, faulty application in the paddock, failure of the D.D.T. to break away from pellets or prills, the trapping of the D.D.T. in the surface layers of the soil so that it did not penetrate to where the grubs were working and application of the material too late to allow it to become available. From cases where fanners had had good results over the years it appeared that there might be other factors than resistance of the grub to D.D.T. coming in to complicate the issue. However, from the farmer’s point of view it did not matter what the cause of the poor results was—the consequence was loss of production often at a vital time of the year. A number of chemicals—organophosphates—were showing considerable promise for

control and from the point of view of initial kill they could be more efficient than D.D.T. However, their effective action was limited by rather special conditions at time of application. They had a shorter life than D.D.T. and unless they came into actual contact with the grubs in a matter of a few days they were likely to be ineffective. Also because of this short life it seemed improbable that they would give more than one season’s control of the grubs. Further work was being undertaken to find out whether, by varying times of application in relation to the life cycle of the grub, it might be possible to gain more successful control. A further difficulty lay In the fact that these materials were more expensive per unit of active ingredient than D.D.T, and this, combined with the shorter residual life of the material, meant that cost of treatment would be considerably higher, said Mr Allen.