The protection of sheep against blowfly strike. V. The duration of protection of certain insecticides under field conditions

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.