Density dependent mortality of the tick, Boophilus microplus, on cattle - further observations.
The value of the density dependent term c in the equation M=aLc relating the number of maturing females (M) of Boophilus microplus (Can.) to the number of larvae infesting cattle (L) was compared in the winter and spring of 1974 in south-eastern Queensland. On Zebu crossbred heifers, the mean values of c in winter (0.87) and spring (0.69) were significantly different (P < 0.05). Likewise, the values of c on Hereford heifers in winter (0.85) and in spring (0.59) were different (P < 0.01). These differences in the values of c were associated with a loss in host resistance in winter. The estimated survival of 20000 larvae was 9.0 and 1.3% on the crossbreds and 28 and 2.8% on the Herefords in winter and spring, respectively. The spring value of the Herefords was exceptionally low and may have been associated with some selection of the group for tick resistance. In the winter experiment, a group of crossbred bulls was compared with the heifers and the value of c (0.73) was found to be significantly lower (P < 0.05) than that on the heifers. Further observations on Australian Illawara shorthorns and Brahmans in both seasons showed that they behaved like the Herefords and crossbreds, respectively.