RESISTANCE TO D.D.T.

Press, Volume CVI, Issue 31161, 10 September 1966, Page 8

RESISTANCE TO D.D.T.

The area in MidCanterburj’ in which grass grubs had been shown to have resistance to D.D.T. was not large in relation to the whole area, Mr F. C. Allen, an officer of the research divison of the Department of Agriculture, told a meeting on the grass grub and porina problem in Christchurch last week.

Mr Allen said that areas where resistance had been shown, on the basis of topical tests of grubs carried out by the Entomology Division at Nelson, were in the Ashburton county on the north bank of the Rangitata, between the Rangitata and the Ashburton, the Dromore area, one site on the south bank of the Rakaia, the north bank of the Rakaia, in the lower reaches, one site at Waikari and one near Motunau. But he said that there were large areas where it was very difficult to find grubs and these included the WillowbyEiffelton area, also Lauriston and the Methven district. But in the Methven district plenty of porina could be found.

Mr Allen said he did not think that at the moment resistance was restricted to soil

types. There was a concentration on the Lismore type soil, but this was possibly due to the fact that these soils had been brought to a state of high production with the use of D.D.T.

Dr J. M. Hoy, director of the Entomology Division of the D.5.1.R., said that there were areas where two applications of D.D.T. had been made two years apart but there were resistant grubs. In other areas where D.D.T. had been applied for 16 years there was no resistance. The resistance picture was not developing on a district basis—it was rather on a paddock basis. This made prediction of the eventual spread of resistance very difficult.

Mr Allen said that there had been one case where resistant grubs had spread from one property across a road and 200 or 300 yards into the paddock on a neighbouring property.

There were long lists of pest insects in all parts of the world that had developed resistance to synthetic insecticides, said Mr P. G. Fennemore, of the Entomology Division. This could well follow in some areas here so that D.D.T. might have an indefinite useful life, but because resistance developed in a limited area was no reason for abandoning the use of D.D.T. elsewhere.

Was the extended period of porina damage this season a product of a growthy season or a new problem, the scientists were asked. Mr J. M. Kelsey, of the Entomology Division, Lincoln, said that there were still a lot of late species of porina about this year. He did not think that this was something that had suddenly arisen but a normal fluctuation within populations. He was personally doubtful of the value of putting on insecticides at this stage. Last year there had been a big reduction in the natural population in untreated areas. Where a' trial had been put down in March there had been 99.3 per cent kill with the best insecticide, but in the control there had been a 76.8 per cent reduction with no treatment —a difference of only 22.5 per cent, but sufficiently high to make the treatment worthwhile in this case. Where a trial had been put down in May, there had been an 88.3 per cent reduction with the best material, but also an 83.4 per cent reduction in the untreated control area—a difference of only 4.9 per cent. The farmer using insecticides in the latter case would only have been wasting his money.

Mr Kelsey said that this natural reduction in grub and caterpillar populations was something that could not be forecast ahead. Dr. Hoy said that porina was a more free ranging pest. It could fly quite considerable distances. The moths could come in and lay eggs. Quite a small population in one year could result in quite a high population in the following year. Pasture management could have a quite marked effect. If a paddock was shut up for white clover and the straw was left on the ground there was a pretty good chance of having porina in the following year. If a paddock was shut up for any purpose, or was free from grazing, Mr Kelsey said that there was likely to be trouble. The moths definitely preferred the longer vegetation. The crushing effect of feet on the caterpillars must be pretty considerable, he said. Caterpillars also could not stand the sunlight. There could be control of this type in October and November and December and January, which were the main egg-laying periods.