Long-term population studies on the cattle tick (Boophilus microplus) on untreated cattle selected for different levels of tick resistance

Abstract

Census data were collected at weekly intervals on tick populations on untreated cattle over a period of 6 years in central Queensland, near Rockhampton. Hereford x Shorthorn (British; HS) heifers and two herds of Africander (Zebu) x [Hereford and Shorthorn] (AX) heifers, one with animals of high resistance (AXH) and the other of medium (AXM) tick resistance, were grazed in separate paddocks. A herd of AX steers was added to the trial when the heifers commenced calving in 1972–73. Data were collected from the heifers for three years prior to their first calving and then for a further three years whilst they were breeding. There were large differences between the numbers of ticks in different years and a consistent seasonal pattern, the numbers being lowest from July to September and highest in the April to June quarter. The AXH herd had fewest ticks for most of the experiment, but after the first year there was little difference between the infestations on the AXM and HS herds. The resistance of all herds, measured by artificial infestations with larvae, increased during the first 3 years but declined after the cattle began breeding, and fluctuated from year to year. The resistance of some heifers did not stabilize until the animals were more than a year old, and that of some animals drifted up or down over long periods. The loss of resistance of a small proportion of the cattle in the AXH herd was responsible for the advantage of that herd being substantially reduced. Both the ranking for resistance of animals and the mean resistance of the herds, assessed with artificial infestations, agreed with the rankings and tick population sizes observed from tick counts on the cattle in the field. Changes in the mean resistance of the herds in spring in different years were highly correlated with concurrent field infestations, which suggests that changes in host resistance have important effects on tick populations. Tick counts on the calves up to weaning showed no correlation with concurrent counts on their dams, but the ranking of the counts on the calves was consistent over the 6 month period.