Selection of Bacillus thuringiensis strains toxic to cotton boll weevil ( Anthonomus grandis , Coleoptera: Curculionidae) larvae

Abstract

Preliminary bioassays with whole cultures (WC) of 124 Bacillus thuringiensis strains were performed with neonate larvae of Anthonomus grandis, a major cotton pest in Argentina and other regions of the Americas. Three exotic and four native strains were selected for causing more than 50% mortality. All of them were β-exotoxin producers. The native strains shared similar morphology of parasporal crystals, similar protein pattern and identical insecticidal gene profiles. These features resembled Lepidoptera-toxic strains. Furthermore, these strains showed a Rep-PCR pattern identical to lepidoptericidal strain HD-1, suggesting that these strains may belong to serovar kurstaki. However, some differences were observed in the plasmid profiles and in the production of β-exotoxin. To determine the culture fractions where the insecticidal metabolites were present, bioassays including resuspended spore-crystal pellets, filtered supernatants (FS) were compared with those of WC. Both fractions tested showed some level of insecticidal activity. The results may suggest that the main toxic factors can be found in FS and could be directly correlated with the presence of β-exotoxin. Based on the bioassays with FS and autoclaved FS, the participation of thermolabile virulence factors such as Cry1I in toxicity is neither discarded. In the selected strains, β-exotoxin would be the major associated virulence factor; therefore, their use in biological control of A. grandis should be restricted. Nevertheless, these strains could be the source of genes (e.g., cry1Ia) to produce transgenic cotton plants resistant to this pest.

Molecular Characterization of the cry Gene profile of Bacillus thuringiensis Isolated from a Caribbean Region of Colombia

“…As mentioned above, the presence of cry1 and cry2 genes was not detected in any of the Bt-UPC samples that were toxic to mosquito larvae (Table SII). This result is concordant with results previously reported in Bt samples from Latin America, showing that cry1 and cry2 genes both together and separately are associated with toxicity against Lepidoptera and Coleoptera but not against larvae of Diptera (Uribe et al, 2003;Lopez-Pazos et al, 2009;Santos et al, 2012;Camacho-Millán et al, 2017;Pérez et al, 2016). It is noteworthy that the sequenced cry1 gene segments of the two Colombian B. thuringiensis samples were almost identical to the respective cry1A homolog of the subspecies named Bt kurstaki HD-1 isolated from a commercial insecticidal formulation called Dipel (Abbott Laboratories, North Chicago) (Kondo et al, 1987), differing from the database sequence only in two nucleotides which caused a non-silent mutation from serine to phenylalanine (Fig.…”

“…B. thuringiensis has been isolated from different ecosystems; the soil is one of the habitats in which it is most widely distributed (Arango et al, 2002;Jara et al, 2006;Pérez et al, 2016;Camacho-Millán et al, 2017;Soares-da-Silva et al, 2017). This fact, coupled with the wide variety of climatic regions and high diversity of insects in Colombia, provides opportunities to isolate new strains with toxic activity against Diptera or other insect orders (Ibarra et al, 2003).…”

“…Studies of B. thuringiensis strains isolated from Latin America strongly suggest that there is a heterogeneous geographic distribution of cry genes associated with toxicity against different insect orders (Bravo et al, 1998;Ibarra et al, 2003;Uribe et al, 2003;López-Pazos et al, 2009;Santos et al, 2012;Pérez et al, 2016;Camacho-Millán et al, 2017;Soares-da-Silva et al, 2017). Colombian strains of B. thuringiensis have been isolated mainly from the central region, e.g.…”

“…In Central and South America, several infectious diseases which affect humans, animals and plants are transmitted by insect vectors from all orders (López-Pazos et al, 2009;Hernández-Fernández 2011;Pérez et al, 2016;WHO 2016;Camacho-Millán et al, 2017). Among them are Diptera such as mosquitoes of Ano pheles and Aedes genus involved in the transmission of malaria and dengue, respectively, which cause great impact in human heath (WHO 2016;Soares-da-Silva et al, 2017).…”

“…Among them are Diptera such as mosquitoes of Ano pheles and Aedes genus involved in the transmission of malaria and dengue, respectively, which cause great impact in human heath (WHO 2016;Soares-da-Silva et al, 2017). Among the insect vectors that cause great pests in agriculture in Latin America, are Lepidoptera such as Tuta absoluta which produces great loss in tomato production (Salazar and Araya, 2001;Hernández-Fernández, 2011) and Coleoptera such as Premnotrypes vorax and Anthonomus grandis that cause devastating pests for potato and cotton crops, respectively (López-Pazos et al, 2009;Pérez et al, 2016) . For these reasons it is very important in Latin America to seek more effective, sustainable and ecological methods to improve the campaigns to control and eradicate the various vectors of communicable diseases present…”