An evaluation of the phosphorus insecticides by means of the bio-assay
method has revealed that certain of these compounds combine very good larvicidal
action with outstanding diffusing power in the fleece of living sheep. They compare
favourably in this respect with the insecticides previously tested (Du Toit and
Fiedler, 1953), the best of which was Aldrin with an average period of protection
of 39 weeks. Dieldrin gave a protection of 37 weeks, and gamma B.H.C. 33
weeks. In addition to the properties mentioned, however, on which the duration
of protection depends such compounds must comply with other demands necessitated
by the environmental factors encountered on the bodies of sheep. To start
with such protecting agents must not stain or damage the wool fibres and secondly
they should be stable in a slightly alkaline medium in order to produce the
maximum residual effect in all situations such as even in the soiled wool of the
crutch region. From this point of view Parathion and EPN 300 must be excluded
for practical purposes as they discolour the treated wool imparting to it a strong
yellow-green tint Furthermore, their general use cannot be recommended on
account of their toxicity to warm-blooded animals. The only phosphorus
insecticide which seems to comply with all the requirements of a protecting agent
for sheep against blowfly strike is Diazinon. Its larvicidal action as well as its
diffusing power in the fleece are superior to insecticides like Dieldrin and B.H.C.
which have proved to be very efficient protecting agents in practice. In the light of
the properties studied Diazinon may be expected to produce a longer period of
protection than B.H.C., which is not stable in an alkaline medium; and it may be
found even to surpass Dieldrin in this field of application.
Field experiments to demonstrate the actual period of protection of Diazinon
in comparison with Dieldrin and B.H.C. are well under way in some of the worst
blowfly areas of the Karoo and will be reported on in due course.
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