Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 11, 1998
Publication Date: N/A
Interpretive Summary: One of the greatest concerns of the United States Fever Tick Eradication Program is the threat posed to the U.S. cattle industry by acaricide resistant southern cattle ticks, which are prevalent in many areas of Mexico and could spread into the U.S. Research is underway to determine the degree of resistance of southern cattle ticks from Mexico to pyrethroid d(P) and organophosphorus (OP) acaricides and our ability to control these parasites with the acaricides available to the eradication program. A pyrethroid resistant strain was repeatedly exposed through successive generations to permethrin, a P acaricide. In 5 generations of selecting for resistance, the degree of resistance of these ticks from Mexico increased 4 times and they exhibited 21 times more resistance than ticks from a normal strain. To determine the effectiveness of permethrin dips for the control of the P resistant ticks on cattle, a comparison was made of the effectiveness of the same treatment against a southern cattle tick strain known to have a normal degree of susceptibility to permethrin. The control achieved against the susceptible strain was 99.2%, but the amount of control of the resistant strain was only 35.7%. These results demonstrate that the use of any pyrethroid acaricides in the eradication program would be imprudent. Tests on OP resistant strains will be reported in the near future.
Technical Abstract: A strain of Boophilus microplus (Canestrini) was selected for resistance to permethrin by pressuring larvae with increasing doses (range 0.05-0.35% active ingredient) through successive generations (generations F2-F7). At the beginning of the selection process (F2) the pyrethroid resistant (PR) strain was 5.4 times more resistant to permethrin than the pyrethroid susceptible (PS) strain, and the level of resistance increased in each successive generation of the PR strain, reaching a resistance factor (RF) of 20.9 in the F7 generation. Thus, in only five generations the level of resistance in the PR strain was increased by 4-fold. The efficacy of permethrin was evaluated by dipping cattle infested with the PS strain and cattle infested with the F7 generation of the PR strain in a vat at 0.057% AI (manufacturer's recommended concentration). Evaluation of the PS strain nshowed that untreated cattle produced significantly (P<0.05) more females with a higher index of reproduction (IR) than treated cattle. Additionally, biological factors (female weight, egg mass weight, and percent egg hatch) associated with the untreated females were all significantly (P<0.05) greater than treated females. Conversely, results obtained from the PR strain showed that tick numbers, IR value, and biological parameters of females in the untreated group were not significantly different (P>0.05) from those of the treated group. The overall control achieved in the PS strain (99.2%) was significantly higher (P<0.05) than that of the PR strain (35.7%). The results of the study as they relate to the United States Boophilus Eradication Program are discussed.