Reduction in egg viability resulting from infestations on cattle of hybridized Boophilus ticks and B. microplus (Acari: Ixodidae) at various ratios.

The competitiveness of hybridized Boophilus males, which are 100% sterile, was compared to B. microplus (Canestrini). Hybrid larvae used in the study were the offspring derived by cross-mating B. annulatus (Say) males with B. microplus females. Cattle were infested with a total of 2,500 larvae at ratios of 5:1 and 10:1 (hybrid to pure strain). The reduction in egg hatch resulting from the hybrid males was 68 and 77.5% at the 5:1 and 10:1 ratios, respectively. Both treatment ratios produced egg sterility that were lower than expected, assuming purely random mating, suggesting that hybrid ticks were not as competitive as B. microplus males. At the 5:1 treatment level, genotypic determinations based on isoenzyme analysis indicated that mate pairings involving hybrid males occurred 20-40% less frequently than expected, whereas matings involving pure-strain B. microplus males occurred twice as frequently as expected. At the 10:1 treatment level, mate pairings involving hybrid males occurred 10-20% less frequently than expected, whereas pairings involving pure strain B. microplus occurred 2-4 times more frequently than expected. The results indicated that in a sterile hybrid male program, it would be necessary to increase the ratios of hybrids by approximately 2-fold over the 5:1 or 10:1 ratios to achieve the 80-90% sterility expected, because of the decreased competitiveness of hybrid males.