INDIVIDUAL VARIATION WITHIN BREEDS OF BEEF-CATTLE IN RESISTANCE TO HORN FLY (DIPTERA, MUSCIDAE)

Weekly estimates of populations of horn fly, Haematobia irritans (L.), were made on individual purebred Charolais, Chianina, Hereford, Polled Hereford, and Red Poll cows in 1988, 1989, and 1990 with Angus included in 1989 and 1990. During the study period, 94 of almost-equal-to 200 individual cows were classified as either fly resistant or fly susceptible. In general, individuals retained the same classification over the 3-yr study period. The mean number of flies on individual susceptible cows in all breeds was at least twice the number of flies on resistant cows. Susceptible Chianina cows averaged >4.5 times as many flies as resistant cows of the same breed. Individual cows within all breeds classified as fly resistant ranged in age from 2 to 12 yr, whereas fly-susceptible cows ranged in age from 2 to 14 yr; therefore, cow age did not influence the abundance of horn flies on individual cows within breeds. Two-year-old fly-resistant cows were identified accurately according to their horn fly-population density, showing the potential of host resistance in horn fly-population management.