Location: Screwworm ResearchTitle: Virulence of Mexican isolates of entomopathogenic fungi upon Rhipicephalus-Boophilus microplus larvae and the efficacy of conidia formulations to reduce larval tick density under field conditions) Author
Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/24/2010
Publication Date: 6/10/2010
Citation: Angel-Sahagun, C.A., Lezama-Gutierrez, R., Molina-Ochoa, J., Pescador-Rubio, A., Skoda, S.R., Cruz-Vazquez, C., Lorenzoni, A.G., Galindo-Velasco, E., Fragoso-Sanchez, H., Foster, J.E. 2010. Virulence of Mexican isolates of entomopathogenic fungi (Hypocreales: Clavicipitaceae) upon Rhipicephalus-Boophilus microplus (Acari: Ixodidae)larvae and the efficacy of conidia formulations to reduce larval tick density under field conditions. Veterinary Parasitology. 170(3-4):278-86. Interpretive Summary: Cattle fever ticks (CFT), Rhipicephalus (Boophilus) microplus, are capable of carrying and transmitting ‘babesia,’ a blood parasite commonly called Cattle Fever and deadly to cattle. Although the ticks were eradicated from the U.S. in 1943, the vigilance against the pest in the quarantine zone cannot end; they continue to exist, as do babesia, just across the border in Mexico. The protection of the U.S. cattle industry from a recurrence of bovine babesiosis currently consist of the use of organophosphate insecticides (dips) applied to the cattle or the vacating of infected pastures. Other approaches to control CFT could enhance the ability to reduce/eliminate populations in the quarantine zone. Here we evaluated 53 Mexican isolates of two species of fungi for their pathogenicity to CFT. The isolate designated as Ma 14 of Metarhizium anisopliae had the highest virulence in laboratory tests. Four application formulations of the Ma 14 fungal isolate were then tested under field conditions. Formulations consisting of wheat bran were consistently effective against the CFT in the field. Further testing is necessary but the judicious application (in habitats favorable to CFT development) may be effective in eliminating ticks from vacated pastures and may give further advantage of reducing CFT populations in and near the quarantine zone.
Technical Abstract: The first objective was laboratory evaluation of the virulence of 53 Mexican isolates of fungi against larvae of Rhipicephalus (Boophilus) microplus. Thirty three isolates of Metarhizium anisopliae var. anisopliae (Metschnickoff) Sorokin (Hypocreales: Clavicipitaceae) and 20 isolates of Isaria (Paecilomyces) fumosorosea (fumosoroseus) (Wize) (Eurotiales: Trichomaceae) were tested on 7-d old larvae under laboratory conditions. Larvae were immersed in a suspension containing 108 conidia/mL and the CL50 values were estimated. Then, field tests were conducted to determine the efficacy of formulations of the isolate with the highest virulence. M. anisopliae (Ma 14 isolate) was formulated with four vehicles: Tween, Celite, wheat bran, and Citroline (mineral oil) and applied on pasture-beds of Cynodon plectostachyus (L.), at a dose of 2x109 CFU/m2. In the first trial, M. anisopliae was applied on plots naturally infested with larvae; in the second trial, tick populations in the experimental plots were eliminated and then re-infested with 20,000 7-d old larvae. In the laboratory, all M. anisopliae isolates infected larvae with a mortality range between 2 and 100%; also, 13 of 20 I. fumosorosea isolates caused mortality rates between 7 and 94%. In the first field trial, 14 days post-application, conidial formulations in Celite and wheat bran caused 67.8 and 94.2% population reduction, respectively. In the second trial, the Tween formulation caused the highest larval reduction, reaching up to 61% (28 days post-application). Wheat bran formulation caused 58.3% larval reduction (21 days post-application) and was one of the most effective. The vehicles have a large impact on the effectiveness of conidial formulations.