Digest

Pacific

Planters'

Use Of Dithane

D ETHANE is a unique chemical related to but different from earlier dithiocarbamate fungicides, and it has come into prominence in Jamaica where it has proved itself successful for controlling Sigatoka spot disease on bananas.

Interest in Fiji is focused on the effectiveness of Dithane, and plant pathologist Dr. Keith Graham is currently carrying out trials with the chemical used with an oil spray.

Dithane has a long residual action and it adheres well to foliage. On bananas it is generally advised to use it 2-4 lb per acre.

Dithane has also shown itself effective against early and late blight, anthracnose and leaf mould in potatoes and tomatoes. At the first sign of these diseases the chemical should be applied H lb per 100 gallons and repeated at 7-10 day intervals.

Further applications should be made if blight conditions or diseases are severe.

On turf grasses Dithane will con trol fusarium mould, rhizoctonia, and helminthosporium leaf blight if used 6-8 oz per 1,000 sq. ft in sufficient water for coverage.

Dithane is compatible with most insecticides, including DDT.

Soyabean Trials

TOURING experiments conducted by the Department of Agriculture in the BSIP 110 varieties of soya beans were planted. The number in cluded the 11 Tanganyika and two Surinam varieties which have already been planted for several years in the Solomons, the 89 CSIRO varieties which have now been planted twice and eight varieties which have re cently been introduced from Australia and Fiji.

As a result of the experiments, which were carried out with one early and one late planting, 70 varieties have been discarded and 40 varieties are being kept on for obser vation, of which 14 will be planted in variety trials.

The performance of the original Tanganyika and Surinam varieties

was again disappointing.

As the plantings in the plots were both very early and very late in the

season, and also considering that in certain periods heavy rain fell, it was concluded that the Tanganyika varie ties should be planted from approxi mately June 1-August 1 and that ex cessive rain at certain periods could cause lodging and seed rotting.

Delphacids Problem

DELPHACIDS have become such a problem in the BSIP, that the Commonwealth Development Cor poration may decide to defer its rice growing expansion programme until it is able to control Delphacids economically.

The BSIP Department of Agricul ture, however, has recently conducted

experiments from which it has con cluded Delphacids are not considered uncontrollable. During the experi ments it was found no damage was done with the chemical used, nor was excessive spraying necessary. The average cost per acre was less than £3.

The standard treatment for Delph acids is Malathion—so at H pint/acre in 20 gallons of water, applied with a motorised pump sprayer. During the Department of Agriculture experiments spraying was done when approximately 10 adult Delphacids were seen on one plant.

Mist blowers were considered to be unsuitable for the application of

the spray due to lack of adequate penetration. If mist blowers have to be used the department advises the engine should be run as slowly as possible with the tap of the spray lance wide open. This results in a fairly coarse spray of 20 gallons to the acre.

The efficacy of control of Delph acids on a large scale appears to de pend on time of application, and complete coverage and penetration of the insecticide. It remains to be seen at this stage whether this is practical and economic on a large scale.

In the meantime Guadalcanal Plains Ltd. reports that treatment was given a heavy infestation of Delph acids with the use of U lb of Sevin per acre through a boom spray.

Forty-eight hours after the applica tion mortality rate in the Delphacids was 100 per cent.

Danger Of Insecticides

TWO questions frequently asked concerning the use of insecticides and other pesticides are, “How toxic is the chemical to humans?” and “Is this insecticide more dangerous than DDT?”

The standards in common use with which to compare toxicities are based on tests with various small animals.

White rats generally are used to get comparisons on the lethal amounts by eating, and rabbits by way of skin absorption.

The amounts are usually given as an LD-50. This means the size of the dose which is lethal to 50 per cent, of the test animals. The LD-50 is usually expressed in terms of milli grams of toxicant per kilogram of body weight of the test animal.

One milligram is equal to .00035 ounces, and one kilogram is equiva lent to 2,2 lb.

Using DDT. for example, the amount which needs to be ingested (or eaten) in a single dose to cause death to 50 per cent, of the test rats is approximately 250 milligrams of technical DDT per kilogram of body weight of the rats.

Thus we say that the acute oral LD-50 of DDT to rats is 250 mg./kg.

Likewise, the acute oral LD-50 of parathion to rats is about 3 mg./kg. while that of malathion is approxi mately 1,500 mg./kg.

From this we see that parathion is considerably more toxic to rats than is DDT or malathion. It does not necessarily follow, however, that parathion is 83 times as hazardous to use as DDT or 500 times as hazardous to use as malathion.

The spray or dust concentration of parathion generally used for insect control is considerably lower than for DDT or malathion. Also, parathion residues break down much more quickly than those of DDT, chlor dane, dieldrin and many others.

In addition, the figures given are for rats, and all animals do not react the same way. And male and female sometimes react differently.

The acute dermal toxicity figures are usually higher than oral toxicity ones. It usually takes much more insecticide on the skin to cause death than it takes to kill by eating the insecticide.

The dermal toxicity figures are probably the more important ones, since in spray or dust applications most of the insecticide contacted is that which gets on the skin.

It is important to realise that the figures are approximate. It cannot be assumed either that the figures given in the following table are also the LD-50’s for humans. The LD-50 for the same chemical will vary con siderably with different mammals, and thus we can use the figures only as an indication of relative toxicity to humans.