Cattle breed-variation in infestation by the horn fly Haematobia irritans

A study was carried out to assess the resistance of pure and cross-bred gro ups of cattle to the horn fly Haematobia irritans (Linnaeus) (Diptera: Musc idae) in northern Argentina. Pure-bred cattle were Criolla, Iberian Bos tau rus Linnaeus (Artiodactyla: Bovidae) and Nellore, Bos indicus Linnaeus (Art iodactyla: Bovidae). Cross-bred cattle were Hereford, British B. taurus (34 %) x Nellore (66%) and Hereford (66%) x Nellore (34%). All were heifers and animals were maintained in two groups, each containing a mixture of pure a nd cross-breeds. The lowest fly numbers were found on Criolla heifers and t he highest on Hereford x Nellore cross-breeds. However, it could not be det ermined from this study whether this was a consequence of breed and/or size , as Criolla heifers were lighter than the corresponding Hereford x Nellore heifers. Fly numbers on the heifers followed an approximately negative bin omial distribution. However, the ranking of individual animals in their lev el of infestation within subgroups was not consistent. Hence, culling the m ost infested heifers on any given date would at best give only a small impr ovement in H. irritans control.