Abstract:
A large number of field trials under close supervision was conducted in the blowfly areas of the Karoo to investigate certain biological aspects of blowfly
strike as well as the protection afforded to sheep by the insecticides Diazinon,
Dieldrin, Aldrin and B.H.C. under natural conditions.
(1) The biological factors rendering sheep susceptible to blowfly strike are
reviewed.
(2) Observations on the development of strikes in sheep treated with an
insecticide as compared with unprotected animals are reported.
(3) The larvicidal value of the compounds tested are discussed, as well
as the factors influencing the duration of protection afforded by them.
(4) Dusts and wettable powders gave longer protection than emulsions in
sheep with clean and dry crutches, whereas emulsions and wettable
powders gave better results in soiled sheep.
(5) The protecting agent must be evenly distributed throughout the vulnerable
areas of the fleece right down to skin level to ensure the maximum
degree of protection.
(6) No significant difference in duration and degree of protection was
observed with quantities of between 0•45 and 2•85 gm. of the active
ingredient per crutch region. This is probably due to this factor
being overshadowed by others.
(7) Insecticides gave better protection when applied to long wool than
to short wool. The crutching of soiled breeches is discouraged, therefore.
(8) Soiled crutches exercise an influence upon the uniformity of the application
and, consequently, upon the duration and the degree of
protection. This is particularly evident in the case of B.H.C. No
significant differences could be detected between Diazinon, Dieldrin
and Aldrin.
(9) All insecticidal compounds afford a considerably shorter period of
protection on young lambs than on adult sheep with an equal or
even shorter length of wool.
(10) The incidence of blowflies (fly population pressure) exercises the most
significant influence upon duration of protection afforded by any of
the insecticides. The relationship between fly population pressure
and duration or protection follows the equation of a hyperbola.
(11) The duration of protection afforded by Diazinon, Dieldrin and Aldrin
showed no significant differences under equal conditions, whereas
B.H.C. was markedly inferior.
(12) The Mule's operation affords adequate protection as long as the
incidence of flies is low and the wool fairly short. With higher fly
population pressures the resistance of "muled" sheep to fly strike
decreases rapidly.
