Ecology of lice on sheep. VI. The influence of shearing and solar radiation on populations and transmission of Damalinia ovis

Abstract

Many D. ovis are found more than 1/4 in. from the skin of the sheep, and 30-50% of a louse population may be lost when the sheep is shorn. The lice near the tip of the fleece come to the tip quickly when it is shaded and warmed, particularly when the fleece is short, and consequently lice, mainly adults and stage III nymphs, spread rapidly from sheep to sheep which are in close contact. All nymphal and adult stages of D. ovis were killed when exposed to 48°C for 60 min, 50°C for 30 min, or to 55 or 60°C for 5 min. Some lice died when exposed to 45°C for 4 hr but the main effect was on oviposition, and females exposed for only 2 hr laid fewer eggs. Most eggs were killed when exposed to 45°C for 4 hr, 47°C for 2-4 hr, or 49°C for 4-1 hr. The intensity of solar radiation during the summer in Australia can result in a temperature gradient within the fleece of sheep from c. 45°C near to the skin to 65-70°C at the tip of the fleece within 5-10 min of exposure. Many lice in the distal parts of the fleece are killed as lethal temperatures develop, and the number of eggs laid by survivors may be reduced. On newly shorn sheep even lice and eggs near the skin may be killed, as the temperature near the skin can rise to 45-52°C. Reasonably heavy infestations may be maintained on sheep kept permanently in the shade, and it appears that the cumulative effect of repeated mortalities due to solar radiation prevents an increase in numbers of D. ovis during the summer.