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

Abstract

BackgroundBacillus 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.MethodsThe 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.ResultsBti 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 (F19 and F25), except for a β-esterases increase in F25.ConclusionsProlonged 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.Electronic supplementary materialThe online version of this article (10.1186/s13071-018-3246-1) contains supplementary material, which is available to authorized users.

Resistance Development to Diflubenzuron and Bacillus Thuringiensis Subsp. Israelensis of Culex Pipiens f. Pipiens and Culex Pipiens f. Molestus (Diptera: Culicidae) and its Effects on Their Winter Survival

“…pipiens f. pipiens that implemented a similar protocol [35]. Despite some sporadic reports describing considerable resistance levels of wild mosquito populations against Bti [22,[36][37][38] long-term studies under both laboratory and eld conditions provide no support to these data [39] and references therein. The low to moderate levels of resistance against Bti after long periods of applications in natural settings or intensive laboratory selection have been primarily attributed to the synergistic action between the three Cry and the Cyt toxin as previously mentioned.…”

New traps for the capture ofAedes aegypti(Linnaeus) andAedes albopictus(Skuse) (Diptera: Culicidae) eggs and adults

“…Thus, if the larvae hatch from the eggs laid by the females in the reeds, they will not develop into adult form [32,33]. In addition, the effect of Bti comes from four major synergistic toxins (Cry4Aa, Cry4Ba, CryllAa and CytlAa), which may reduce the likelihood of selection of resistant target organisms [41,[59][60][61][62], besides not cause damage to other organisms (except Chironomidae and Simuliidae)) due to their high specificity for mosquitoes [40,63].…”

A long‐lasting biological larvicide against the dengue vector mosquito Aedes albopictus

“…Bti has been proven to be highly selective and safe for the control of Aedes mosquitoes during integrated pest management in recent decades 6,7 . This is primarily due to the low risk of resistance development 8,9 and safety for other organisms, including humans. However, owing to the impact of environmental variables (e.g.…”