Resistance To D.D.T.

Press, Volume CII, Issue 30278, 2 November 1963, Page 6

Resistance To D.D.T.

A CONFERENCE was receratly held in Nelson among representatives of the Department of Agriculture, the Fertiliser Manufaictuirers’ Research Association. the agricultural chemical manufacturers, fertiliser manufacturers. the Dominion Laboratory. and the Entomology Division of the Department of Scientific and Industrial Research. to determine the extent of factors affecting the efficiency of D.D.T. application to grasslands. At the annual conference of the New Zealand Grassland Association last week, the director of the Entomology Division (Dr. W. Cottier) said that, except in certain areas, field officers of the Farm Advisory Division of the Department of Agriculture had considered that where D.D.T. was properly mixed and applied it appeared to be as effective as ever. South Island problem areas were in the Riwaka and Brooklyn valleys and an area of about 2000 acres near Reefton in the Nelson district and the lower Wairau area in Marlborough. No area in Canterbury was listed. Dr. Cottier said it had been found in certain areas, one of which was near Patea, that grubs definitely required more D.D.T. to kill them and it had become pretty clear that all areas where this sort of difficulty was being experienced had a history of previous D.D.T. treatment, although only over a period of three or four years. In one area at Riwaka in Nelson it had been found in laboratory tests that 100 times the normal dose would not produce a satisfactory

kili, and when field experiments had been tried on this site no kill could be obtained with D.D.T. in either the year of application or the following year. Kills could, however, be obtained with other chemicals, such as some of the organophosphates. T n certain areas of pumice country in the Rotorua area it had for s»me time been the experience that it was difficult to secure a satisfactory kill of grassgrub with 21b of D.D.T. to the acre. It had been shown that in some of these soils D.D.T. did not

penetrate more than half an inch. This was a physical effect caused by absorption of the D.D.T. on the soil particles so that the D.D.T. did not penetrate deeply enough to reach the grassgrubs feeding on the grass roots below.

In the last 12 months it had been suggested that the presence of protozoa in the gut of the grassgrubs had reduced the efficiency of DDT. At Riwaka where they had been dealing with grubs that resisted D.D.T. treatment it had been found that most of these grubs were heavily infested by these protozoa and it had been found that a strain of grassgrub in another area in Nelson that was normally susceptible to D.D.T. was only lightly infested by the protozoa. Cases had, however, since been found where grubs were normally susceptible to D.D.T. and contained heavy infestations of protozoa. “We still do not know the exact answer, but we believe from the field evidence we have that protozoa are not necessarily connected with difficulty to kill with D.D.T.,” saad Dr. Cottier.