Some Aspects Of Grass Grub Control

Press, Volume CVIII, Issue 31760, 17 August 1968, Page 8

Some Aspects Of Grass Grub Control

Certain factors in the grass grub control programme were clarified during the twentyfirst conference of the Weed and Pest Control Society in Auckland last week.

In a combined project the Department of Scientific and Industrial Research and the Department of Agriculture surveyed the incident of DDT resistance in the Ashburton County. This area has the longest history of DDT usage, and it was expected to be the earliest to show any true resistance. The laboratory techniques devised at Entomology Division, Nelson, were used to determine the level of this factor. The whole county was sampled for grubs on a two mile and a half square grid. Of 108 pastures sampled, 40 per cent were found grub-free. Of the remaining 60 per cent, 24 carried populations that would not be controlled by the standard 21b DDT normally used. This amounts to 40 per cent of the grubinfested areas, or 22 per cent of the total paddocks sampled.

Another paper of biological as well as practical interest covered the attempts be-

ing made from the Entomology Division at Nelson to attack the adult beetles with insecticides. In a first attempt to do this there was no clear-cut success. Equipment intended to apply ultralow volume (ULV) sprays arrived late from overseas and it was reported that the workers involved feel that treatments should be applied earlier in the season in future trials.

Requirements for successful attack against beetles are recognised as being fairly exacting to achieve economic levels of success, and this approach cannot offer any immediate hope for relief from grass grub problems. With the clarification of these points, discussion of papers on chemical control centred round a deliberate endeavour to answer the question of a Federated Farmers representative: “What can a farmer do now to gain control?”

With the attempts to answer this question on the basis of biology or management largely without result, the meeting accepted control measures at present as having three facets:—

(1) Where resistant populations are known to be pre‘ sent the farmer may, by permit, apply lindane at reasonable cost, and with a fair assurance of success. (2) Where dairy stock is grazed DDT may, by regulation, no longer be applied in any form. Later discussion in a residue session gave very good reasons for this restriction. Hope for this farmer seems to lie in diazinon, though limited amounts of fensulfothion are available, efficient, and permitted under strictly regulated conditions intended to minimise the toxic hazards of this material. Both materials are much more costly than DDT, and the best levels of control obtained with diazinon seem to be achieved regularly when this material is applied as a spray during rain. This results in the deposit reaching the zone of grub activity almost as soon as applied, and before any breakdown occurs.

(3) Pastures infested with grubs still susceptible to DDT, and intended for grazing by other than dairy stock, may still use certain regulated forms of DDT, a remedy that is still both effective and inexpensive.

Almost all organophosphate insecticides now under trial for this pest offer a lesser degree of control than DDT. Most of the reports on trials expressed some opinion on the timing of application of insecticides, and it was recommended by those offering materials for use on dairy pastures that action be taken as early as March against known infestations if pasture is to be effectively retained. Such applications gave 70-80 per cent retention of good pasture, according to dosage, whereas untreated controls were reduced to 45-50 per cent cover.