Ecology of the cattle tick (Boophilus microplus) in subtropical Australia. III. Modelling populations on different breeds of cattle

Abstract

The numbers of cattle ticks, Boophi1us microplus, were counted on cattle, with different Bos indicus (zebu) content in central and southern Queensland, from November 1977 to May 1982. Concurrently, measurements were made of the survival rates of parasitic stages of the tick and of the success of free-living stages in producing larval progeny. These measurements were related to the tick burdens on the cattle in order to test current understanding of the ecology of the tick.Tick numbers increased exponentially with decreasing Bos indicus content. Tick numbers built up very rapidly on the Bos taurus cattle and resulted in deaths early in the first season, so the herd was replaced with animals of 25% zebu content. The pure zebu animals carried an average of five ticks per animal per day in central Queensland and only one in southern Queensland. The 50% zebu herds carried 65 ticks per day in central Queensland and 1 l in southern Queensland, compared with 79 and 37 on the 25% zebu herds at the two locations. During their brief time in the experiment the B. taurus animals carried 465 and 302 ticks per day in central and southern Queensland respectively.The observed tick infestation levels were estimated to cause annual losses of liveweight gain of 1, 14 and 17 kg in zebu, 50% zebu and 25% zebu animals respectively in central Queensland. Comparative figures for southern Queensland were 0, 2 and 8 kg. Compensatory liveweight gain would have reduced these losses by up to 6 kg. These data relate to animals which were breeding most of the time and hence were less resistant than dry cattle. The results indicate that B. microplus has been reduced to a marginal problem on animals with 50% zebu genes in the region. It is therefore difficull to justify any control measures in southern areas, particularly when they could increase the risk of babesiosis. Owners of B. taurus cattle will continue to be at risk of major losses in productivity.Changes in the tick burden on the herd5 were explained mostly by changes in survival rates of parasitic ticks in central Queensland and by changes in availability of larvae on the pastures in southern Queensland.