Effect of deltamethrin on germination and virulence of Beauveria bassiana (Bals.) Vuill. on Triatoma infestans (Klug)

Efeito de deltametrina na germinação e virulência de Beauveria bassiana (Bals.) Vuill. em Triatoma infestans (Klug)

Insecta; fungo entomopatogênico; piretróide; manejo integrado de pragas

SCIENTIFIC NOTE

Effect of deltamethrin on germination and virulence of Beauveria bassiana (Bals.) Vuill. on Triatoma infestans (Klug)

Efeito de deltametrina na germinação e virulência de Beauveria bassiana (Bals.) Vuill. em Triatoma infestans (Klug)

Raul Alzogaray^I; Christian Luz^II; Ionizete G. Silva^II; Roberto E. Lecuona^III; Myrian S. Tigano^IV,1

^ICIPEIN-CITEFA/CONICET, Zufsiategui 4380, 1603 Villa Martelli, Bs.As, Argentina

^IIIPTSP/UFG, Caixa postal 131, 74001-970 Goiânia, GO

^IIIIMIZA-CICA-INTA Castelar, C.C. 25, 1712 Castelar, Bs.As, Argentina

^IVEmbrapa Recursos Genéticos e Biotecnologia, Caixa postal 02372, 70849-970, Brasília, DF

RESUMO

Foi avaliada a ação do inseticida deltametrina sobre a capacidade de germinação e virulência do fungo entomopatogênico Beauveria bassiana (Bals.) Vuill. contra ninfas de 3° estádio de Triatoma infestans (Klug) (Hemiptera: Reduviidae). Após 12 horas em cultura submersa, observou-se uma diminuição significativa da germinação de conídios de B. bassiana em proporção ao aumento da concentração do inseticida. Na mais alta dose aplicada, 550 mmg/ml, foi observado retardamento da germinação com 24 e 36 horas e interrupção do desenvolvimento do fungo após esse tempo. A exposição de ninfas de 3° estádio a concentrações subletais de deltametrina não alterou de forma significativa a virulência de B. bassiana.

Palavras-chave: Insecta, fungo entomopatogênico, piretróide, manejo integrado de pragas.

Chagas Disease, a chronic parasitic disease caused by the protozoan Trypanosoma cruzi (Chagas), is a serious health problem in Latin America (WHO 1991). It is normally transmitted to humans by blood-sucking triatomes (Hemiptera: Reduviidae). In Argentina and Brazil, Triatoma infestans (Klug) is the main vector of Chagas Disease (Schofield 1994), and its control is extensively made by pyrethroids, particularly deltamethrin (Zerba 1989). As an alternative to chemical control, the entomopathogenic fungus Beauveria bassiana (Bals.) Vuill. was shown to be a potential candidate for biological control of triatomes (Luz & Fargues 1997). The simultaneous application of entomopathogenic fungi and reduced doses of chemical insecticides was first suggested by Telenga in 1964, based on the observation of epizootics which occurred after chemical treatments against insect pests (Ferron 1985). Several other studies showed that advantageous interactions, synergism and potentiation, can be observed when fungus and insecticides are simultaneously applied (Pristavko 1966, Ferron 1985, Barjan et al. 1995). Synergism is a toxicological interaction between two or more toxic agents, where the effect of the joint action is significantly higher than the sum of the individual effects. Potentiation is a particular form of synergism, where one of the involved chemicals is applied at a non-toxically dose (Eaton & Klaassen 1996). This study was carried out to verify the effect of flowable deltamethrin on in vitro B. bassiana conidial germination and to determine if the virulence of this fungus against T. infestans could be potentiated by its combination with subletal concentrations of deltamethrin.

The tests were realized with a strain of B. bassiana (CG306 from Embrapa Culture Collection, Brasília, DF), originally isolated in Brazil from Thyanta perditor (Fabr.) (Hemiptera: Pentatomidae). This strain was selected based on its high virulence against T. infestans in low humidity conditions (Luz et al., unpublished). A commercial formulation of deltamethrin (flowable, 2.5% AI; AgrEvo, San Isidro, Argentina) was used for the tests. This formulation was specifically developed for control of Chagas Disease vectors. Germination tests were carried out with five concentrations of deltamethrin, 5.5, 55.0, 181.5, 363.0 and 550.0 µg/ml, in 50 ml aliquots of liquid complete media (1 mg FeSO₄ , 0.5 g KCL, 1.5 g KH₂PO₄ , 0.5 g MgSO₄ x 7 H₂O, 6 g NaNO₃ , 1 mg ZnSO₄ , 1.5 g hydrolysed caseine, 0.5 g yeast extract, 10 g glucose, 2 g peptone, and 1 liter of distilled water). Each aliquot was inoculated with B. bassiana conidia to a final concentration of 10⁶ conidia/ml. The percent germination was determined on the basis of the number of conidia producing germ tubes, that were at least twice as long as the length of the conidial diameter. One hundred conidia/replicate were counted. The procedure was repeated 4 times. Bioassays were conducted with 5-10 day old third instar nymphs of T. infestans, that were not fed after the last molt. Insects were obtained from a laboratory colony, where they were fed on chicken and maintained at 25°C. Two sublethal concentrations of flowable deltamethrin were selected for the tests: 0.5 and 5.0 ng a.i./cm², the latter being the concentration affecting 10% of the treated individuals (EC₁₀) 72h after application (data not shown). Filter paper disks (Whatman #1.7 cm diameter) were impregnated with a dilution of the flowable formulation in sterile distilled water (SDW). The treated filter papers were dried for 24 h at room temperature and placed at the bottom of plastic containers (7 cm diameter and 4 cm high). Groups of 20 nymphs were continuously exposed to each film. The containers were covered with gauze tied by rubbers, and kept at 26°C. At 24 h of exposure, nymphs were submersed in fungal suspensions of 10⁵, 3x10⁵, 10⁶, 3x10⁶, 10⁷, 3x10⁷, and 10⁸ conidia/ml. After conidial application, nymphs were replaced on the insecticide-treated filter papers, and maintained at 26°C and 50% relative humidity. Control insects were submersed only in SDW. The same conidial concentrations were applied to nymphs exposed to filter papers treated with SDW. The procedure was replicated 4 times for each treatment. Nymphal mortality was recorded daily for 15 days after the fungal application. Conidia concentration to kill 50% of the treated insects (LC₅₀) was calculated by probit method (SAS Institute 1985). Survival time distribution was analyzed by Wilcoxon test (SAS Institute 1985).

The percentage of conidial germination decreased significantly as a function of deltamethrin concentration (Fig. 1). At the highest deltamethrin concentration (550 µg/ml), a delay in the formation of the germ tube was observed and the germination rate at 24 h was 5.3 ± 2.6 %, compared to 78.8 ± 4.3 % in the control. Fungal development was totally interrupted after 36 h incubation. Lower deltamethrin concentrations did not affect fungal germination (>98%) after 24 h. Regarding fungal virulence at 10 days after conidial application, the LC₅₀ values ranged from 2.1 to 3.3 x 10⁵ conidia/ml (Table 1). No significant differences (P > 0.05; based on overlap of 95% C.I.) were observed between LC₅₀ values for nymphs exposed to the combinations of 0.5 or 5.0 ng/cm² of deltamethrin with B. bassiana, and B. bassiana alone. Survival time 50% varied from 5 to 7 days for the highest and lowest conidial concentration, respectively (Table 2). In all concentrations tested, no significant differences were observed between the distribution values for deltamethrin treated and untreated insects (Wilcoxon test, P > 0.05). Despite the lack of potentiation by sublethal concentrations of deltamethrin on the virulence of B. bassiana against T. infestans, the evaluation of the association with other chemical insecticide should be pursued. Otherwise, other isolates of B. bassiana should also be tested in association with deltamethrin.

Acknowledgements

To Ms. Celia M. T. Cordeiro (Embrapa Recursos Genéticos e Biotecnologia) for assistance with statistical analysis and Dr. Bonifácio Magalhães (Embrapa Recursos Genéticos e Biotecnologia) for critical review of the manuscript. This research was funded by Centro Brasileiro-Argentino de Biotecnologia through a cooperative agreement between Brazil and Argentina.

Literature Cited

Received 19/XI/97. Accepted 14/IX/98.

• Barjan, C., A. Fedorko & K. Kmitowa. 1995. Reactions of entomopathogenic fungi to pesticides. Pol. Ecol. Stud. 21: 69-88.
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• Ferron, 1985. Fungal control, p. 313-346. In G. A. Kerkut & L. I Gilbert (eds.), Comprehensive insect physiology, biochemistry and pharmacology, vol. 12. Oxford, Pergamon, 849 pp.
• Luz, C. & J. Fargues. 1997. Temperature and moisture requirements for conidial germination of the an isolate of Beauveria bassiana, pathogenic to Rhodnius prolixus Mycopathologia 138: 117-125.
• Pristavko, N. 1966. Processus pathologiques consécutifs a l'action de Beauveria bassiana (Bals.) Vuill. Associée a de faibles doses de D.D.T., chez Leptinotarsa decemlineata Say. Entomophaga 11: 311-324.
• SAS Institute, Inc. 1985. SAS/STAT Guide for personal computer 6.0. Cary, SAS Institute, Inc.
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• Zerba, E. N. 1989. Chemical control of Chagas disease vectors. Biomed. Environ. Sci. 2: 24-29.

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Corresponding author

Publication Dates

• Publication in this collection
  05 June 2006
• Date of issue
  Dec 1998

History

• Accepted
  14 Sept 1998
• Received
  19 Nov 1997