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Neotrop. Entomol. vol.33 no.3 Londrina May/June 2004
Compatibility of Beauveria bassiana with acaricides
Compatibilidade de Beauveria bassiana com acaricidas
Renato C. de OliveiraI; Pedro M.O.J. NevesII
IDepto. Ciências Biológicas, Faculdade Assis Guagacz, 85806-095, Cascavel, PR
IIDepto. Agronomia, Centro de Ciências Agrárias, UEL, C. postal 6001, 86051-990, Londrina, PR
The in vitro compatibility of the entomopathogenic fungus Beauveria bassiana (Bals.) Vuil. with 12 acaricide formulations was evaluated. The active ingredients Clorfenapyr, Fenpyroximate, Amitraz, Acrinathrin, Hexythiazox, Abamectin, Pyridine, Dimethoate, Pyridaphethion, Fenbutatin Oxide, Azocyclotin and Cyhexatin were tested in three different concentrations (mean concentration - MC, half MC and twice the MC). The formulations tested affected conidial germination, vegetative growth and sporulation of the fungus in different levels. The formulations more compatible with B. bassiana were Avermectin and the pyrethroids. The acaricides belonging to the organophosphate and organostanic chemical groups, however, drastically affected conidial germination as well as vegetative growth and sporulation.
Key words: Selectivity, fungitoxicity, entomopathogenic fungus, IPM
Visando avaliar a compatibilidade in vitro, entre o fungo entomopatogênico Beauveria bassiana (Bals.) Vuil. e acaricidas, 12 formulações com os ingredientes ativos clorfenapir, fenpiroximate, amitraz, acrinatrim, hexitiazox, abamectina, piridine, dimetoato, piridaphetion, óxido de fenbutatim, azociclotim e cihexatim, foram testadas em três concentrações (concentração Média - CM, metade da CM e duas vezes a CM). Para os parâmetros germinação de conídios, crescimento vegetativo e esporulação, apresentaram diferenças significativas na compatibilidade entre o fungo e as formulações utilizadas. As formulações mais compatíveis com o fungo, para os parâmetros avaliados, foram as pertencentes ao grupo químico da avermectina e piretróide. As formulações pertencentes ao grupo químico do organofosforado e organostânico afetaram drasticamente a germinação de conídios, bem como o crescimento vegetativo e a esporulação.
Palavras-chave: Seletividade, fungitoxicidade, fungo entomopatogênico, MIP
Entomopathogenic fungi are found worldwide associated to insects and phytophagous mite populations, contributing to the biological control of these arthropods on several economically important crops (Carruthers & Hural 1990, Van Der Geest et al. 2000). Seeking to use the potential of these organisms for pest control, commercial products have been developed with entomopathogenic fungi (McCoy & Couch 1982, McCoy 1990, Alves & Pereira 1998).
The success of a pest control program using entomopathogenic fungi, however, depends on conidia survival in the field environment (Benz 1987). Conidia survival may be affected either by environmental factors (Furlong & Pell 1997) or by bio-pesticides and/or chemical products used to protect crop plants (Anderson & Roberts 1983, Loria et al. 1983, Alves & Lecuona 1998).
Many experiments have been carried out aiming to detect pesticides side effects on entomopathogenic fungi (Olmert & Kenneth 1974, Gardner & Storey 1985, Neves et al. 2001). Most of them evaluated only the effects of the products on vegetative growth and sporulation, disregarding conidial germination. However Neves et al. (2001) pointed out the importance of conidial germination in compatibility studies. They emphasized that the inhibition of this initial step will affect the plain development of the fungus in the field because this fungal structure is responsible for initiating the disease on insect pest populations.
According to Anderson & Roberts (1983), germination is also an important criterion to evaluate compatibility of pesticides with entomopathogenic fungi in vitro. Todorova et al. (1998) reinforced the importance of pesticides influence on conidial germination, since those fungal structures are responsible for the occurrence of the first disease foci in the field. This author also emphasized that germination inhibition compromises the efficiency of the pathogen, either for those already present in the agro-ecosystem or for those applied by overflow, associated or not to pesticide products. Duarte et al. (1992) pointed out the importance in considering the antagonistic effect of the pesticides on all developmental phases of entomopathogenic fungi since these products may affect the bio-insecticide potential as well as may reduce the occurrence of epizootics. The knowledge of the compatibility between entomopathogenic fungi and pesticides may facilitate the choice of these products in Integrated Pest Management (IPM) programs where the fungus is an important pest control agent (Neves et al. 2001).
This study aimed to evaluate the in vitro effect of acaricides on the entomopathogenic fungus B. bassiana at different stages of development.
Material and Methods
Entomopathogenic Fungus. Embrapa Recursos Genéticos located in Brasília, Federal District, provided the strain CG424 of B. bassiana. Conidium of this strain were stored at -4°C (in Eppendorf tubes) and subsequently grown and multiplied on PDA (Potato-Dextrose-Agar) medium (25°±1°C; 12h photophase). Conidia produced on PDA were used for germination, vegetative growth and sporulation studies.
Pesticides. Information on active ingredient, trade name, formulation, chemical group and recommended doses of the acaricides as well as the doses used (ppm) are shown on Table 1. For compatibility tests, the acaricides were used in three different concentrations: mean concentration (MC), half MC and twice the MC. For MC determination, the arithmetic mean was computed using the various concentrations of the same acaricide recommended by the manufacturers for field application on different crops, diluted in 100 liters of water per ha.
Conidial Germination. For conidial germination tests, the acaricides, in the pre-established concentration, were diluted in sterile distilled water amended with 0.02% Tween 20 and 1 ml of B. bassiana conidia suspension, standardized to 1x106 conidia/ml. Sixty minutes after components mixture, 0.5 ml aliquots of each suspension was transferred to four 9-cm culture plates (petri dishes) containing solidified water-agar medium and smeared with the aid of a Drigalsky loop. The same aliquot of sterile distilled water standard spore suspension and 0.02% Tween 20, without the acaricides, was used as control. There were four replications (plates) for each treatment. Culture plates were then randomly transferred to an incubator (25 ± 1°C; 12h photophase) for 24h. After incubation, lines were drawn on the external bottom surface of each plate, dividing them into four quadrants, in which approximately 100 germinated or non-germinated conidia/quadrant were counted under light microscope. Data were used to compute percent germinated or non-germinated conidia.
Vegetative Growth and Conidia Production. Standard PDA medium was autoclaved at 120°C and 1.5 atm for 20 min, cooled to 40 ± 5°C and amended with 0.5 g/L streptomycin. The acaricides, in the pre-established concentrations, were then added. Approximately 20 ml of each one of these amended media was poured into eight 9 cm culture plates. The same amount of streptomycin-amended medium without the acaricides was used as control. After media solidification, each plate was inoculated with B. bassiana conidia using the three points method (Alves et al. 1998b). The plates were then randomly transferred to an incubator (25 ± 1°C; 12h photophase) for eight days. After incubation, the diameter of colonies in each culture plate was determined with the aid of a sliding caliper, measuring three times each colony in different directions. The means of the values obtained were used to compute the area of each colony, using the formula pR2.
For conidia production evaluations, one 15 mm disk was cut with a cork borer from the center of 10 (replications) fully sporulated B. bassiana colonies per treatment. Disks were then separately transferred to flat bottom glass vials (8.40 cm tall x 2.30 cm diameter), suspended in 10 ml of a 0.02% Tween 20-amended sterile distilled water and agitated until complete conidia release. Conidia suspensions were subsequently diluted and quantified using a Neubauer chamber.
A completely randomized experimental design was used in all experiments. Data were submitted to ANOVA and means were compared by Tukey's multiple range test (P < 0.05) using SAS (Statistical Analytical System, inc. 1998). The statistical correlation was estimated by means of Pearson analysis.
All acaricides tested caused different levels of inhibition on germination, vegetative growth and sporulation of B. bassiana (Table 2). These levels were mainly dependent on the chemical nature of the compounds as well as on the concentrations used.
Conidial germination was significantly affected by all acaricides used, independently of concentration (Table 2). The formulation with Febutatim Oxide, Azocyclotin, Cyhexatin and Dimethoate drastically reduced (>91%) B. bassiana conidial germination at the three concentrations (MC, half MC and twice the MC). The formulations with Pyridaphethion and Clorfenapyr, similarly, induced high reduction (>88%) at MC and twice the MC concentrations. These two formulations, together with Pyridaben caused, respectively, 83.9%, 78.6% and 70.6% reduction on conidial germination at half MC concentration. The formulations with Amitraz, Fenpyroximate and Hexythiazox also caused significant reduction on conidial germination at the three concentrations used, with percent reductions ranging from 47% to 85%. Likewise, the reductions caused by the formulations with pyretroid Acrinathrin and avermectin Abamectin. In addition, Abamectin also significantly differed from the control at the three concentrations. Nevertheless, the reductions on percent germination for these two formulations were lower than 45%.
As far as the effect of the acaricides on B. bassiana vegetative growth is concerned, results have shown that almost all formulations studied significantly inhibited fungal development (Table 2). However, the formulation with the pyrethroid Acrinathrin at half MC induced a fungal growth 10% higher than the control treatment. Moreover, the vegetative growth inhibition induced by this formulation at twice the MC was not significantly different from the control treatment. On the other hand, the formulations with Pyridaphethion, Azocyclotin and Cyhexatin induced fungal growth inhibition higher than 89%. Dimethoate, although inducing percent growth inhibition higher than 98% at MC and twice the MC, caused only 40.3% reduction at half MC. The remaining formulations induced percent growth inhibition ranging from 21.1% (Clorfenapyr, at half MC) to 80.0% (Pyridaben, at twice the MC).
The formulations with Clorfenapyr, Fenpyroximate, Acrinathrin, Hexithioazox and Abamectin induced intermediate levels of inhibition of sporulation at all concentrations used, ranging from 69.2% (Fenpyroximate at half MC) to 29.2% (Abamectin at MC). However, only data on Clorfenapyr and Acrinathrin at half MC and Abamectin at the three concentrations, were not statistically different from each other although significantly different from the control treatment. The formulations with Dimethoate, Amitraz, Fenbutatin Oxide, and Pyridaben induced levels of sporulation inhibition higher than 75%. Whereas formulations with Pyridaphenthion, Azocyclotin and Cyhexatin totally prevented sporulation (100% reduction) at all concentrations used (Table 2).
A positive correlation (r = 0.80) between vegetative growth and sporulation was observed, indicating that the amount of conidia produced was directly dependent on colony size (Table 2). When germination and vegetative growth data were compared, a positive correlation (r = 0.70) could also be observed. Nevertheless, when a product presents compatibility with vegetative growth of a given fungus it almost never will show the same compatibility level with conidia germination. The formulatons with active ingredient Clorfenapyr, Fenpyroximate and Hexythiazox that induced high levels of inhibition on germination but a relatively low level of inhibition on vegetative growth confirmed this statement.
Conidial germination is a very important step in pest control programs with fungi because the beginning of epizootics is conditioned to the capacity of these structures to germinate on the host. The entomopathogenic fungus success, however, depends on conidial viability (Feng et al. 1994, Batista Filho et al. 1998).
In the germination tests carried out in this study a simulation of procedures used in the field for simultaneous application of pesticides and entomopathogenic fungi, was attempted. These procedures allow to evaluate more completely the effect of the chemicals on the fungus since conidia are fully exposed to the products. Boucias et al. (1988) used similar methodology to demonstrate that the treatment of entomopathogenic fungi conidia with chemical products, either ionic or molecular ones, may neutralize the electrostatic charge of the surface and/or remove the mucous layer covering conidia, thus affecting the substrate recognition process and the transduction of the signal that initiates germination. St. Leger & Cooper (1987) and St. Leger et al. (1991) have demonstrated the role of this coat removal on substrate recognition and on transduction of the signal that triggers germ tube formation in M. anisopliae.
The drastic inhibition induced on germination of B. bassiana conidia by the acaricides may be related to evidences already reported by Moore-Landecker (1982). This author has found that the presence of chemical products may block conidia metabolic functions and thus drastically affect germination. Metabolic blockage in phytopathogenic fungi conidia due to ions accumulation on the surface of the cellular membrane has also been described by Ghini & Kimati (2000). The authors reported that molecules, analogous to prosthetic groups, diffuse to the cytoplasm where they bind to specific receivers affecting membrane permeability and enzymatic synthesis, consequently affecting metabolic processes. The same authors also emphasize that organophosphate compounds directly interfere on cell wall formation due to inhibition of the enzyme that converts phosphatydilethanalamine into chitin. The same mechanism of inhibition was probably responsible for the drastic reduction on B. bassiana conidia germination, vegetative growth and sporulation observed in this study. The organophosphates as well as the organostanic formulations used in the experiments comprise metallic and semi-metallic elements in their composition, which confers to them high ionic activity.
Information on the effect of the pyrethroid Acrinathrin and the avermectin Abamectin on fungi are scarce in the literature. According to Halley et al. (1993) Abamectin has no significant anti-fungal activity. Results of tests conducted by AgroEvo (1998) reported selectivity of Acrinathrin to beneficial insects and natural enemies. However, no reference is made to its effect on fungi.
Another possible explanation for inhibition of conidia germination is the accumulation of chemical compounds against a concentration gradient, starting from diluted solutions, as described by Kimati (1995) for phytopathogenic fungi. In this process the compound molecules get in contact with conidia and slowly diffuse to the inner cytoplasm allowing fungal germination before achieving a lethal dose. According to the same author carboxamide compounds have their primary site of action on succinate oxidation that variably affects cellular respiration and, consequently, the metabolic activity of the fungus.
Concerning the effect of the products presently used on vegetative growth and sporulation, a significant reduction was found in relation to the control treatment. Alves et al. (1998), however, pointed out that results obtained in vitro may be more drastic then what happens under field conditions where several factors may impair the fungal exposure to the chemical compounds. Thus, when the innocuousness of a given product is determined in the laboratory, no doubts that its selectivity under field conditions will stand. On the other hand, the high toxicity in vitro of a given formulation may suggest similar toxicity under field conditions. However for field studies the inhibition of conidial germination should be the key factor to be considered, as discussed by Neves et al. (2001).
Regarding the effect of the chemical formulations studied on different developmental stages of B. bassiana, Abamectin and Acrinathrin presented lower toxicity than the other formulations, mainly when conidial germination was considered. Data obtained in this study may guide future recommendations of these active ingredients in IPM programs where B. bassiana is intended to be used as a control agent of a given mite through inundative strategy. In these cases and when the pathogen and acaricide are used in combinations, compatible products should be prefered for recommendation.
The authors would like to thank CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) for financial support and Dr. Inês C.B. Fonseca for advisory in the statistical analyses.
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Received 24/03/03. Accepted 20/02/04.