Selection of Beauveria bassiana and Metarhizium anisopliae Isolates to Control Triatoma infestans
Vol. 93(6): 839-846
Christian Luz/+, Myrian S Tigano/*, Ionizete G Silva, Celia MT Cordeiro/*, Salah M Aljanabi/*
Instituto de Patologia Tropical e Saúde Pública, Universidade Federal de Goiás, Caixa Postal 131, 74001-970 Goiânia, GO, Brasil *Centro Nacional de Pesquisa de Recursos Genéticos e Biotecnologia, SAIN Parque Rural, W5 Norte, Caixa Postal 02372, 70849-970 Brasília, DF, Brasil
Twenty three isolates of Beauveria bassiana and 13 isolates of Metarhizium anisopliae were tested on third instar nymphs of Triatoma infestans, a serious vector of Chagas disease. Pathogenicity tests at saturated humidity showed that this insect is very susceptible to fungal infection. At lower relative humidity (50%), conditions expected in the vector microhabitat, virulence was significantly different among isolates. Cumulative mortality 15 days after treatment varied from 17.5 to 97.5%, and estimates of 50% survival time varied from 6 to 11 days. Maintaining lower relative humidity, four B. bassiana and two M. anisopliae isolates were selected for analysis of virulence at different conidial concentrations and temperatures. Lethal concentrations sufficient to kill 50% of insects (LC50) varied from 7.1x105 to 4.3x106 conidia/ml, for a B. bassiana isolate (CG 14) and a M. anisopliae isolate (CG 491) respectively. Most isolates, particularly B. bassiana isolates CG 24 and CG 306, proved to be more virulent at 25 and 30°C, compared to 15 and 20°C. The differential virulence at 50% humidity observed among some B. bassiana isolates was not correlated to phenetic groups in cluster analysis of RAPD markers. In fact, the B. bassiana isolates analyzed presented a high homogeneity (> 73% similarity).
Key words: vector control - Chagas disease - entomogenous fungi - virulence - random amplification of polymorfic DNA
Entomopathogenic fungi are promising candidates for microbiological control of Triatominae (Hemiptera, Reduviidae) because they invade their hosts through the integument. However, relative humidity (RH) and temperature are known to be limiting environmental factors for fungal development on insects (Glare & Milner 1991, Ferron et al. 1991). High rates of infection and a rapid kill of triatomine bugs by the hyphomycete fungi Beauveria bassiana and Metarhizium anisopliae were obtained at humidities close to saturation (Silva & Messias 1985, Romaña & Fargues 1987, Luz 1990, Romaña 1992, Luz et al. 1994). Infection of bugs diminished with B. bassiana at RH below 97% (Luz 1994). Optimal temperatures for fungal development on the insect host range from 16 to 30°C for B. bassiana and M. anisopliae with a faster development at the higher temperatures (Ferron et al. 1991).
Microclimatic conditions in the natural insect habitat may be unfavorable for fungal infection. Humidity in domestic microhabitats of triatomine bugs may be distinctly lower as shown by Luz (1994) in Rhodnius prolixus. To select isolates for biocontrol, intraspecific differences of fungal behaviour related to abiotic conditions in target insect habitats should be considered. In this study, isolates of B. bassiana and M. anisopliae were screened against Triatoma infestans at RH close to saturation and at 50% RH. The effect of conidial concentrations and temperatures were analyzed for some isolates at the lower RH. Polymorphism among some B. bassiana isolates, with different levels of virulence, was investigated by using random amplification of polymorphic DNA (RAPD) analysis.
ACKNOWLEDGMENTS
To Raquel A Mello for technical assistance; Jacques Fargues and Roberto Lecuona for providing fungal isolates; Eugenia M Bettiol for bibliographic searches; Peter Inglis for the English review and Bonifácio Magalhães for critical review of the manuscript.
Fig. 1 | Fig. 2
Table I | Table II | Table III
+Corresponding author. Fax: +55-62-202.3066. E-mail: wolf@ipe.ufg.br
Received 20 February 1998
Accepted 27 July 1998
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