Mortality of Oryzophagus oryzae (Costa Lima, 1936) (Coleoptera: Curculionidae) and Spodoptera frugiperda (J E Smith, 1797) (Lepidoptera: Noctuidae) Larvae Exposed to Bacillus thuringiensis and Extracts of Melia azedarach

BIOLOGICAL AND APPLIED SCIENCES

Mortality of Oryzophagus oryzae (Costa Lima, 1936) (Coleoptera: Curculionidae) and Spodoptera frugiperda (J E Smith, 1797) (Lepidoptera: Noctuidae) Larvae Exposed to Bacillus thuringiensis and Extracts of Melia azedarach

Diouneia Lisiane Berlitz^*; Aline Oliboni de Azambuja^I; Alessandra Sebben^I; Jaime Vargas de Oliveira^II; Lidia Mariana Fiuza^I,II

^ILaboratorio de Microbiologia e Toxicologia; Centro de Ciências da Saúde; Universidade “Vale do Rio dos Sinos”; Av. Unisinos, 950; C.P.: 275; 93022-000; São Leopoldo - RS - Brasil

^IIEstação Experimental do Arroz; Av. Bonifácio Carvalho Bernardes, 1494; 94930-030; Cachoeirinha - RS - Brasil

ABSTRACT

Oryzophagus oryzae (Costa Lima 1936) (Coleoptera: Curculionidae) and Spodoptera frugiperda (J E Smith, 1797) (Lepidoptera: Noctuidae) cause important crop losses in southern Brazil. Control is possible by the use of the bacteria Bacillus thuringiensis and extracts of Melia azedarach. This study aimed to evaluate the mortality, in vivo, of O. oryzae and S. frugiperda submitted to two isolates of B. thuringiensis and the aqueous extract of M. azedarach. The LC₅₀ for O. oryzae due to bacteria was 5.40μg/mL (Bt 2014-2) and due to plant extract 0.90μg/mL. For S. frugiperda, the Bt 1958-2 bacterial suspension (1.10¹⁰UFC/mL) caused a 100% of corrected mortality, showing that the purified Cry proteins caused a CL₁₀ of 268μg/mL five days after the treatments, and M. azedarach toxins caused a CL₅₀ 173μg/mL four days after the treatment. Corrected mortality for O. oryzae and S. frugiperda in the interaction between the bacterial and plant toxins were 11 and 6%, respectively. In the PCR analysis of B. thuringiensis isolates, DNA fragments were enlarged and corresponded to the cry1 and cry2 genes for Bt 1958-2. Thus, it could be concluded that the usage of Bt 2014-2 active against O. oryzae larvae; Bt 1958-2 for S. frugiperda and, for both the insect species, M. azedarach aqueous extract could be used.

Key words: Alternative pest control, entomopathogenic bacteria, insecticidal activity, botanical insecticidal

INTRODUCTION

Twelve million tons of rice, Oryza sativa (Linnaeus) were produced in Brazil in 2010, much of which in the south region, where average productivity was 6,412 kg ha^-1. The highest productivity is in the State of Rio Grande do Sul with 6,410^-1 (Conab 2010). Losses due pests are often large, if untreated. The rice water weevil Oryzophagus oryzae (Costa Lima) (Coleoptera: Curculionidae) is an important pest in rice crops. Its larvae attack the roots, causing losses up to 30% (Martins et al. 1993; Sosbai 2010). Another important pest in rice crops is the fall armyworm, Spodoptera frugiperda (J E Smith) (Lepidoptera: Noctuidae) that can cause grain losses of up to 24% (Cesconetto et al. 2005).

The control of these pests is mainly by the use of chemical pesticides that, in addition to killing the target species, affect the natural enemies, cause intoxication of producers and can leave residues in the environment. Alternatives to chemical controls are very important, such as microbial control with Bacillus thuringiensis (Berliner) and botanical pesticides as those found in Melia azedarach (Linnaeus) are promising. B. thuringiensis is a Gram-positive bacterium that yields crystals during the sporulation and made up by proteins synthesized by cry genes (Höfte and Whiteley 1989; Schenpf et al. 1998; De Maagd et al. 2001; Crickmore 2005). This entomopathogen has an insecticidal spectrum for more than 10 insect orders, including coleopteran and lepidopteran, according to the presence of cry genes and the proteins synthesized in the isolates (Schenpf et al. 1998; De Maagd et al. 2003).

The use of plant extracts with insecticidal activity is increasingly important in alternative pest control. For example, M. azedarach (Meliaceae), or Chinaberry has insecticidal properties (Huang et al. 1996, Ventura and Ito, 2000; Brunherotto and Vendramim 2001) due to various compounds, such as salanalin, meliaterin, and meliacarpinin. Some other compounds from this plant have insecticidal activity against beetles (Curculionidae, Tenebrionidae, and Chrysomelidae), as well as Lepidoptera (Huang et al. 1996; Bohnenstengel et al. 1999; Carpinella et al. 2003; Charleston et al. 2006; Nathan 2006).

The objective of this study was to assess the toxicity using in vivo assays of two new isolates of B. thuringiensis Cry proteins and an aqueous extract of M. azedarach as possible control agents for the rice pests O. oryzae and S. frugiperda.

MATERIAL AND METHODS

Bacillus thuringiensis

Isolates origin and preparation

The Bt 2014-2 and Bt 1958-2 isolates were isolated from the soil samples in rice growing areas in the State of Rio Grande do Sul. The first isolated was used because it had insecticidal activity against O. oryzae (Pinto and Fiuza 2003). The latter isolate was grown in the USUAL+G medium at 180 rpm and 28ºC for 48 h (De-Barjac and Lecadet 1976). Then the bacterial suspension was centrifuged at 5000 rpm and cells were counted using Neubauer chamber and optical microscopy, and the concentration was adjusted to 1.10¹⁰ UFC/mL.

Cry proteins purification

Proteins were obtained from the isolates (Bt 2014-2 and Bt 1958-2), grown in the USUAL+G medium, up to 90% of cell lysing as estimated using phase contrast microscopy. Protein purification was carried out through the sucrose gradient (67 and 79%) in a refrigerated centrifuge (82,700g, 4ºC, 1 h), where bands were separated and collected from the gradient from which proteins were dissolved in alkaline-buffer (pH 10) (Fiuza 1995). Bacterial protein dosages followed Bradford (1976).

Detection of cry genes through PCR

The Bt 1958-2 isolate was grown in Agar Nutrient at 30ºC for 12 h, after which total DNA extraction was carried out following Hansen and Hendriksen (2001) primers were used for that isolate, which enlarged the fragments of classes of cry1 and cry2 genes; the PCR analyses were performed according to description of Ben-Dov et al. (1997).

Reactions were carried out in a final volume of 25 μL with 5 μL DNA added to the reaction buffer, to which 0.2 mM from each dNTP, 0.2 - 0.5 mM from each primer and 0.5 U from Taq DNA polymerase were added. The amplification was performed in thermocyclator (PTC -100, MJ Research, Inc.) and a 35-cycle program for each reaction. In each cycle, the samples were denatured for 1 min at 94ºC and ringed to the primers for 40-50 seconds at 57ºC after which extension of PCR products was carried out at 72ºC for 50-90s. For the positive control B. thuringiensis aizawai HA3 was used, and the negative control was similarly prepared without the addition of DNA. The enlargement by-products were analysed in agarose gels at 1.5%, bleached in ethidium bromide, photographed under UV and compared with the molecular weight marker 100pb (Invitrogen^®).

Toxins from Melia azedarach extracts

For the extraction of toxins from the plants of M. azedarach, leaves were collected from the branches 3 – 4m above the ground in October (2004) at UNISINOS, and were dried at 40 ºC during 48 h. Ten grams of dried product was crushed and mixed in 100mL sterile distilled water (10% aqueous extract) at 4ºC following Bradford (1976).

Target Insect Pests

Oryzophagus oryzae – rice water weevil

Second and third instars of O. oryzae were collected from rice paddy in the Rice Experimental Station of Instituto Riograndense do Arroz (IRGA) in Cachoeirinha, RS and kept in assay tubes and glass basins with water and rice plants in a B.O.D chamber with temperature, relative humidity and photoperiod controlled (28 ±2 ºC, 75 ±2 % RH and a 12h photo phase).

Spodoptera frugiperda - fall armyworm

Second instar caterpillars of S. frugiperda were collected from the corn fields in Novo Hamburgo, RS, and maintained on Poitout artificial diet (Poitout and Bues 1970) under the controlled conditions at 28 ±2 ºC, 75 ±2 % RH, and a 12h photo phase.

Bioassays with target insects

Oryzophagus oryzae

The experimental protein concentrations were determined through a pilot assay with the bacterial and plant toxins, using five dilutions from 0.5 to 34 μg/mL for B. thuringiensis and 0.025 to 257 μg/mL for M. azedarach. The Lethal Concentration (LC) was determined in sterile assay tubes with 1mL protein solution and disinfected rice plant, into which 5 larvae of O. oryzae were added. Three replicas using 20 insects/treatment resulted in 720 insects tested. In the controls, proteins were substituted by the sterile distilled water. Assays were kept under controlled conditions (28 ±2 ºC, 75 ±2 % RH, 12h photo phase) and the mortality was quantified seven days later.

In the assays for the interaction of bacterial and plant toxins, LC₅₀ of B. thuringiensis and M. azedarach were used to determine the experimental concentrations and mixed 1:1 (v/v). Assays were carried out as described above.

Spodoptera frugiperda

For the 1.10¹⁰ UFC/mL suspension assay, individual second instar S. frugiperda caterpillars were kept on acrylic miniplates (35 mm wide) with artificial diet containing 100 μL of bacterial suspensions. Pilot assay protein concentrations ranged from 0.36 to 3.608 μg/mL for M. azedarach, and from 0.028 to 281μg/mL for B. thuringiensis. Individual caterpillars of S. frugiperda were kept on acrylic miniplates with a rice-leaf disc (10 mm) containing 10 μL of each treatment tested. In the control, sterile distilled water was used instead of bacterial suspensions. The treatments comprised 30 insects/concentration and three replicas each for a total of 1080 insects. Bioassays were done at 28 ±2 ºC, 75 ±2 % relative humidity and 12h photo phase. Leaf discs were substituted by the artificial diet 48h after the treatment. The mortality was quantified daily for seven days after the treatment.

For the interactions of bacteria and plants, the M. azedarach LC₅₀ was used as 1.10⁵ UFC/mL for B. thuringiensis, mixed 1:1 (v/v), and assays followed the procedures described above.

Statistical Analysis

Corrected mortality (CM) was estimated following Abbott (1925). Lethal Concentration (LC) and Lethal Time (LT) were estimated with Probit Analysis using the program Polo-PC LeOra 1987 (Haddad 1998). Mortality was compared among the treatments with the analysis of variance ANOVA and Tukey (α = 0.05) using Systat program.

Protein assessment in SDS-PAGE

Isolate suspensions and proteins and aqueous extract of M. azedarach were analyzed for the proteic profile in poliacrilamid gel at 10%, following Laemmli (1970). Protein bands were compared to the molecular weight marker (Invitrogen^®) using Kodak Digital Science 1D program.

RESULTS AND DISCUSSION

The mortality of S. frugiperda and O. oryzae with the purified protein is shown in Table I. This showed higher mortality of O. oryzae by B. thuringiensis purified proteins in comparison with the control (F = 4.43, P = 0.008). Therefore, as apparently O. oryzae has not been studied with respect to B. thuringiensis purified proteins, these results seemed promising for its control with this bacteria. Similar results were also found with the botanical extract (F = 8.92, P = 0.0001). However, these results were in contrast of those for the curculionid Premnotrypes vorax (Hustache) (Uribe et al. 2003), in which none of the extracts with the genes cry1, cry3, cry7 or cry8 were effective.

The extracts of M. azedarach were tested against different beatle species (Carpinella et al. 2003). Doses of 2mg/cm³ were used for Sitophylus oryzae (Linné, 1763) (Coleoptera: Curculionidae) and Priocyphus bosqui (Hustache) (Coleoptera: Curculionidae), and doses of 0.4 mg/cm³ for Pantomorus leucoloma, (Boheman, 1840) (Coleoptera: Curculionidae), which inhibited their feeding within 24h after the treatment. The mortality of adults and larvae of the Gonipterus scutellatus (Gyllenhal) (Coleoptera: Curculionidae), a herbivore on Eucaliptus sp., after azadiractin ingestion was 73%. However, B. thuringiensis kurstaki did not affect the larvae and adults mortality (Santolamazza-Carbone and Ana-Mágan 2004).

Corrected mortality in S. frugiperda due to Bt 1958-2 isolate was high, yet when Cry proteins were purified, the isolates had no insecticidal activity (F= 2.40, P = 0.99) against the leaf caterpillar, with an LC₁₀ of 268 μg/mL within five days after the treatments (Table I). This low mortality with purified Cry proteins could be due to the resistance mechanisms, including the changes in allele frequencies of specific genes (Heckel et al. 2007), or the production of other B. thuringiensis toxins. Among the latter are “Vip” proteins, in which the “Vip3A” is important due to its toxic effects on Lepidoptera, including S. frugiperda (Schnepf et al. 1998; De Maagd et al. 2003). Testing five-strain supernatant of B. thuringiensis against the fall armyworm, Barreto et al. (1999) found mortality between 65-90 % caused by “Vip” proteins. Thus, mortality due to the bacteria could be higher than that due to Cry proteins alone. The criteria to determine the insect specificity in the case of VIPs are not completely understood (Bhalla et al. 2005).

The entomopathogen includes other virulent factors, such as beta-exotoxins, alfa-exotoxins, hemolysins, enterotoxins, chitinases, and phospholipases (Höfte and Whiteley 1989; De Maagd et al. 2003). However, the real role of each factor in bacterium virulence is still unknown and so it is difficult to understand the toxic spectra of the isolates that synthesize more than one protein. To completely understand the resistance of B. thuringiensis, it is necessary to complement the results with the studies about biochemistry and physiology of target insect (Heckel et al. 2007). The CL₅₀ of 173μg/mL of S. frugiperda in the extract of M. azedarach was after four days of the treatment (F = 21.52, P = 0.0001) and action could be due to its influence on feeding and behavior (Carpinella et al. 2003). Breuer et al. (2003) found a decrease of 31% in the activity of C cholinesterase in S. frugiperda, when the extract of M. azedarach (0.01%) was mixed in the artificial diet. Similarly, three new meliacarpin compounds from of the leaves against S. litorallis (Boisduval, 1833) (Lepidoptera: Noctuidae), showed a LC₅₀ of 2.36; 1.19, and 0.48 ppm for the compounds 1, 2, and 3, respectively, when compared with 0.32 ppm for azadiractin (Bohnenstengel et al. 1999).

The mortality of S. frugiperda was due to the compounds such as meliacarpin and its by-products aqueous extract from M. azedarach. Also, Chinaberry leaves at 5% extract, resulted in > 90% mortality in Tuta absoluta (Meyrick, 1917) (Lepidoptera: Gelechiidae) (Brunherotto and Vendramim 2001). The aqueous extract of M. azedarach together with Neemix 4.5^® was effective in the control of Plutella xilostella (Linné, 1758) (Lepidoptera: Plutelidae) (Charleston et al. 2006).

Corrected mortality (ca. 11% in O. oryzae, F= 0.76, P = 0.588) in the simultaneous application of bacterial and plant toxins was similar to that of the controls (Fig. 1). Corrected mortality was 6% in S. frugiperda and therefore also similar to that of controls (F = 0.52, P = 0.754). These results suggested antagonism when Cry proteins of B. thuringiensis were used with the aqueous extract of M. azedarach. The ether extract from the Chinaberry leaves had a powerful antimicrobial activity against B. cereus (Marquez et al. 2003). In contrast, assays using a combination of Taxodium distichum and B. thuringiensis increased the mortality in stored-grain pests (Sabbour 2003).

The fragments of cry1 and cry2 gene classes were found in PCR analysis of the Bt 1958-2 isolate (Fig. 2). Also, the fragments of cry3 class have been found in Bt 2014-2 isolates (Pinto and Fiuza 2003). It is known that cry3, cry7, and cry8 gene classes synthesize the proteins with insecticidal activity for the Coleoptera and cry 1 and cry 2 gene classes synthesize the proteins with insectidal activity for Lepidoptera (De Maagd et al. 2003).

The assessment of the profile of Cry B. thuringiensis proteins and M. azedarach aqueous extract in polyacrilamide gel (10%) showed predominant bands of 209kDa for the proteins of Bt 2014-2 and Bt 1958-2 isolates, and 50kDa for the plant insecticide (data not shown).

In the light of few information for controlling O. oryzae with B. thuringiensis and M. azedarach, the present results suggested that this pests could be effectively controlled with these alternative extracts. Thus, field trials should be done to test how these extracts could be applied and field mortality rates should be estimated. This could be an important first step in establishing their use in the management of rice culture pests. Additionally, the genes of the subclass isolates of B. thuringiensis should be examined because they might provide control for the larvae of O. oryzae that live in the water and is difficult to control by the standard methods (pulverized chemical pesticides). This could be a better option to systemic insecticides that, in addition to be left out in the soil and water, could also be found in the rice grains in traces.

As for S. frugiperda, studies have concentrated on the use of new isolates of B. thuringiensis and cry genes with the insecticidal activities. A study of 58 subspecies of B. thuringiensis in S. frugiperda found that the caterpillar mortality was 80 % (Polanczyk et al. 2003). Also, in a test of 77 entomopathogenic strains, only four isolates were toxic to the army caterpillar (Silva et al. 2004). This suggested any level of resistance by S. frugiperda to the entomopathogen and therefore more studies would be needed to address this issue, especially due to the importance of this polyphagous phytophagous pest.

The results showed that Bt 1958-2 isolates were effective against S. frugiperda caterpillars and were potentially useful for its control, despite the low toxicity associated with the synthesized Cry proteins. Bt 2014-2 isolate or the purified Cry3 proteins could also be useful for the control of O. oryzae larvae. The aqueous extracts of M. azedarach was toxic to both the pests (O. oryzae and S. frugiperda) and had high potential as a botanical insecticide. Interestingly, the combined use of these two extracts reduced their toxic effects and so they should not be used together for the pest control.

Received: February 14, 2011;

Revised: June 21, 2011;

Accepted: June 05, 2012.

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Publication Dates

• Publication in this collection
  09 Oct 2012
• Date of issue
  Oct 2012

History

• Received
  14 Feb 2011
• Accepted
  05 June 2012
• Reviewed
  21 June 2011