FLY RESISTANCE TO CHEMICALS EXTENDING

Press, Volume C, Issue 29436, 11 February 1961, Page 8

FLY RESISTANCE TO CHEMICALS EXTENDING

There has been a good deal of trouble with fly strike in lambs and ewes in North Canterbury this season. Certainly the weather—moist and reasonably warm—has been favourable for this but it also seems that blowflies in this district also are now developing a resistance to the insecticide dieldrin, a phenomenon which has already been reported from several parts of the country—notably the Wairarapa, Hawke's Bay, Gisborne, and Nelson.

In North Canterbury flies have recently struck lambs and pre-lamb shorn ewes carrying some weight of wool after tip spraying with dieldrin. The trouble has occurred while the insecticide has been diffusing down the fibre and before it has reached the base of the fibre. The strike has actually occurred below the insecticide.

The Farm Chemicals Company, of Port Mapua, near Nelson, set up an insectary for breeding flies to enable its research staff to study resistance of blowflies. It recently reported that some flies now have 200 times the resistance to dieldrin and aldrin that they had formerly. This organisation says in a recent publication that resistance in New Zealand has been found only in the small green blowfly, Lucilia sericata, but it seems probable that eventually resistance will be found in this species in most areas subject to blowfly attack.

In the absence of resistance dieldrin and aldrin give more positive and lasting blowfly control at lower cost than any other chemical at present available, according to this publication, but where resistance does occur alternative chemicals are available, which if properly used, will give economic control. Two organophosphorous insecticides are mentioned.

One of the main problems arisirfg from the use of alternative chemicals concerns their persistence, it is stated in the article. Experience has shown that shower or plunge dipping with dieldrin

and aldrin will give blowfly protection throughout the season while dusking and tip spraying has been proved to give several weeks’ protection. The new insecticides mentioned give a comparable period of control on clean sheep but do not remain as effective where wool is soiled by dung and urine. The suggestion here is that treatments may have to be repeated in seasons or districts where blowfly attack persists over a long period and at the same time a system of jetting may have to be adopted with a relatively high dosage of dip chemical being applied to areas subject to attack, so saving wastage by treating some areas unnecessarily. The article says that the problem of resistance to chemicals confronts mankind wherever insects have to be controlled. . . . When D.D.T. was first used for control of house flies it was’ so effective that in some areas methods of hygiene previously used were relaxed so that when flies resistant to D.D.T. developed they found extensive breeding grounds and multiplied in numbers greater than ever before.

It was important to avoid this mistake in dealing with sheep blowflies. Sheep should

be kept clean over fly strike periods. When clipping wool on a struck area all larvae should be collected and destroyed. If a struck sheep, which has died, is buried mature larvae, L. sericata, can still pupate and emerge from a considerable depth. It is therefore better to leave the animal exposed for a short period so that the secondary flies (e.g. Chrysomyia) can attack the primary L. sericata. Will flies also become resistant to the organophosphorous compounds? Aleady it has been reported that there has been some resistance in Australia to diazinon. The degree of resistance to these materials is not likely to be as great as that towards dieldrin and aldrin as’ overseas work suggests that while resistant flies can tolerate 200 or 300 times a normal dosage of chlorinated hydrocarbon insecticide they can only develop a resistance of a few times the normal dosage of organophosphorous c o mpounds. However, whatever the future holds it is certain that new chemicals will have to be introduced if control of pests is to be maintained., In North Canterbury many farmers are reported to be going back to plunge and shower dipping to ensure that their sheep get completely saturated with insecticide.