Long-term exposure of Aedes aegypti to Bacillus thuringiensis svar. israelensis did not involve altered susceptibility to this microbial larvicide or to other control agents

• Published in: Parasites & vectors Vol. 11; no. 1; p. 673
• Main Authors: Carvalho, Karine da Silva, Crespo, Mônica Maria, Araújo, Ana Paula, da Silva, Renata Santana, de Melo-Santos, Maria Alice Varjal, de Oliveira, Cláudia Maria Fontes, Silva-Filha, Maria Helena Neves Lobo
• Format: Journal Article
• Language: English
• Published: England BioMed Central Ltd 29-12-2018; BioMed Central; BMC
• Subjects: Aedes - drug effects; Aedes aegypti; Animals; Aquatic insects; Bacillus thuringiensis; Bacillus thuringiensis - growth & development; Bacillus thuringiensis - pathogenicity; Bacterial Proteins - pharmacology; Bio-assays; Bioassays; Brazil; Catalysis; Control; Cross-resistance; Cry11Aa; Cry4Ba; Detoxifying enzymes; Diflubenzuron; Endotoxins - pharmacology; Enzymes; Esterases; Exposure; Gene expression; Glutathione; Health aspects; Hemolysin Proteins - pharmacology; Infestation; Insecticide Resistance; Insecticides; Insecticides - pharmacology; Integrated control; Kinases; Laboratories; Larva - drug effects; Larvae; Larvicides; Metabolism; Microbiological strains; Microorganisms; Mode of action; Mosquito Control - methods; Mosquitoes; Ova; Pest Control, Biological - methods; Pesticides; Protoxins; Resistance; Sustainable use; Temephos; Testing; Toxins; Transferases; Tropical diseases; West Nile virus; Yellow fever mosquito; Brazil; Cry11Aa; Resistance; Temephos; Diflubenzuron; Cry4Ba; Detoxifying enzymes
• ISSN: 1756-3305; 1756-3305
• DOI: 10.1186/s13071-018-3246-1

Bacillus thuringiensis svar. israelensis (Bti) is an effective and safe biolarvicide to control Aedes aegypti. Its mode of action based on four protoxins disfavors resistance; however, control in endemic areas that display high mosquito infestation throughout the year requires continuous larvicide applications, which imposes a strong selection pressure. Therefore, this study aimed to investigate the effects of an intensive Bti exposure on an Ae. aegypti strain (RecBti), regarding its susceptibility to Bti and two of its protoxins tested individually, to other control agents temephos and diflubenzuron, and its profile of detoxifying enzymes.
The RecBti strain was established using a large egg sample (10,000) from Recife city (Brazil) and more than 290,000 larvae were subjected to Bti throughout 30 generations. Larvae susceptibility to larvicides and the activity of detoxifying enzymes were determined by bioassays and catalytic assays, respectively. The Rockefeller strain was the reference used for these evaluations.
Bti exposure yielded an average of 74% mortality at each generation. Larvae assessed in seven time points throughout the 30 generations were susceptible to Bti crystal (resistance ratio RR ≤ 2.8) and to its individual toxins Cry11Aa and Cry4Ba (RR ≤ 4.1). Early signs of altered susceptibility to Cry11Aa were detected in the last evaluations, suggesting that this toxin was a marker of the selection pressure imposed. RecBti larvae were also susceptible (RR ≤ 1.6) to the other control agents, temephos and diflubenzuron. The activity of the detoxifying enzymes α- and β-esterases, glutathione-S-transferases and mixed-function oxidases was classified as unaltered in larvae from two generations (F
and F
), except for a β-esterases increase in F
.
Prolonged exposure of Ae. aegypti larvae to Bti did not evolve into resistance to the crystal, and no cross-resistance with temephos and diflubenzuron were recorded, which supports their sustainable use with Bti for integrated control practices. The unaltered activity of most detoxifying enzymes suggests that they might not play a major role in the metabolism of Bti toxins, therefore resistance by this mechanism is unlikely to occur. This study also highlights the need to establish suitable criteria to classify the status of larval susceptibility/resistance.