Selection and molecular characterization of Bacillus thuringiensis strains efficient against soybean looper (Chrysodeixis includens) and Spodoptera species

Karine Silva De Carvalho,Fernando Hercos Valicente,Tatiane Aparecida Nascimento Barbosa,Ubiraci Gomes De Paula Lana

doi:10.1590/1806-9665-rbent-2020-0080

Karine Silva De Carvalho, Fernando Hercos Valicente + Show 2 more

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https://doi.org/10.1590/1806-9665-rbent-2020-0080

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Abstract

ABSTRACT Soybean looper (Chrysodeixis includens) and Spodoptera group are important defoliators insects responsible for significant yield losses in soybean, cotton and maize in Brazil. Bioinsecticides and transgenic plants expressing insecticidal proteins from Bacillus thuringiensis are among the most used sustainable control methods for pest control in agriculture, especially for several lepidopteran species. To provide new components for insect control, this study aimed to select and characterize B. thuringiensis strains toxic to C. includens, S. cosmioides, S. eridania, and S. frugiperda. We performed initial bioassays with fifty B. thuringiensis strains for C. includens and selected four strains – 1608A, 726, 773, and 775E – that caused high larval mortality (100%) to be tested against Spodoptera species. These strains harbored cry insecticidal genes, megaplasmids, absence of β-exotoxin and showed two major proteins about 65 and 130 kDa in SDS-page attributed to Cry protein classes toxic to lepidopteran species. The 1608A and 775E strains also showed high toxicity to Spodoptera species that demonstrate great potential for B. thuringiensis-based bioproduct development and as sources of new insecticidal genes for transgenic crops for multiple pest control relevant in the Brazilian agricultural system.

Highlights

The soybean looper, Chrysodeixis includens [Walker, 1858] (Lepidoptera: Noctuidae), is a polyphagous pest present in several countries of the American continent, with occurrence from the north of the USA to the south of South America
Each B. thuringiensis strain was inoculated into Petri dishes with Luria Bertani (LB) agar medium enriched with mineral salts (FeSO4, ZnSO4, MnSO4 and MgSO4) and incubated at 29°C for 72 h for complete sporulation (Valicente and Mourão, 2008)
Water was used as negative control and HD125 strain as a positive control (β-exotoxin producer). β-exotoxins production by B. thuringiensis strains were determined by larvae growth inhibition or high mortality of S. frugiperda after eight days of inoculation (Pinheiro, 2013)

Summary

Introduction

The soybean looper, Chrysodeixis includens [Walker, 1858] (Lepidoptera: Noctuidae), is a polyphagous pest present in several countries of the American continent, with occurrence from the north of the USA to the south of South America. The Spodoptera genus are composed of important species of defoliator caterpillars, such as S. cosmioides, S. eridania and the fall armyworm, S. frugiperda (JE Smith), responsible for significant losses in maize production in Brazil, African continent, India, and China (Goergen et al, 2016; Guo et al, 2018; Sharanabasappa et al, 2018). Control of these insects is mainly achieved using chemical insecticides, and it requires control actions throughout most of the growing crop seasons. B. thuringiensis is an aerobic Gram-positive bacterium that produces protein crystalline inclusions named Cry proteins during the stationary phase and encoded by different cry genes

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