BackgroundThe control of Aedes aegypti is usually based on chemical insecticides, but the overuse of these compounds has led to increased resistance. The binary toxin produced by Lysinibacillus sphaericus in the final stages of sporulation is used for mosquito control due to its specificity against the culicid larvae; however, it has been proved that Ae. aegypti is refractory for this toxin. Currently, there is no evidence of the use of L. sphaericus vegetative cells for mosquito biocontrol. Therefore, in this study, the vegetative cells of three L. sphaericus strains were assessed against a field-collected Ae. aegypti, resistant to temephos, and the reference Rockefeller strain.ResultsVegetative cells of L. sphaericus 2362, III(3)7 and OT4b.25 produced between 90% and 100% of larvae mortality in the reference Rockefeller strain. Effective concentrations of each L. sphaericus strain for the four larval stages ranged from 1.4 to 2 × 107 CFU/ml. Likewise, a consortium of L. sphaericus assessed against a field-collected Ae. aegypti resistant to temephos and the Rockefeller strain caused 90% of larvae mortality. Concentrations of L. sphaericus consortium that resulted in larvae mortality of field-collected and Rockefeller Ae. aegypti ranged from 1.7 to 2.5 × 107 CFU/ml. The vegetative cells of L. sphaericus have no effect on the Ae. aegypti eggs and pupae.ConclusionsThe vegetative cells of L. sphaericus are effective against Ae. aegypti larvae, meaning that it could be used in the biological control of these mosquito species. Since the L. sphaericus consortium was effective against temephos-resistant Ae. aegypti, vegetative cells could be an alternative to overcome insecticide-resistant populations. Further studies, should be conducted to reveal the mode of action and the toxic principle of L. sphaericus vegetative cells.

Given its toxicity against culicid larvae, Lysinibacillus sphaericus is used for the biological control of mosquitoes such as Culex sp. and Anopheles sp. The toxicity factors currently reported for L. sphaericus include the Binary toxin, Mtx toxins, and the S-layer. However, Aedes aegypti is refractory to the Binary toxin, the most toxic larvicidal protein of L. sphaericus. Until now, there are no evidences of the hemolytic and chitinolytic capacity of L. sphaericus. Herein, the expression of the hemolysin D (hlyD) and the chitin-binding protein genes of L. sphaericus III(3)7, OT4b.25, and 2362 was quantified. Gene expression was assessed 24 and 48 h after field-collected and Rockefeller A. aegypti larvae were fed with the bacteria. The hlyD gene showed the highest expression at 24 h whilst the expression of the chitin-binding protein gene increases at 48 h. The highest hlyD expression was seen in the III(3)7 strain and the highest chitin-binding protein gene expression was in the 2362 strain. The consortium of L. sphaericus III(3)7 and 2362 showed the highest expression of both genes being with field-collected and Rockefeller larvae. The results suggest that hemolysin D and the chitin-binding protein can be two novel toxic elements involved in the entomopathogenic activity of L. sphaericus. These proteins, along with the other L. sphaericus toxins, make this bacterium a suitable alternative to replace the chemical insecticides used for the control of A. aegypti populations.

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The increased incidence of vector‐borne diseases and insecticide resistance in mosquitos constitute public health concerns in the tropics. Biological control is an effective alternative in the management of Aedes aegypti Linnaeus populations. Lysinibacillus sphaericus a bacterium proved to be harmless for non‐target organisms has shown promising entomopathogenic activity. In Colombia, the control of A. aegypti using L. sphaericus has not been contemplated as part of vector control programmes. To assess the susceptibility of three A. aegypti populations to a L. sphaericus formulation consisting of vegetative cells of 2362 and III(3)7 strains, simulated‐field bioassays were conducted in the municipalities of Ricaurte in Cundinamarca and Tauramena in Casanare, as well as in the municipal inspection of San Joaquin, Cundinamarca. Sixty larvae were deposited in a test device, which contained nine litres of chlorine‐free tap water and 100 ml of the bacterial formulation (109 CFU/ml). Six replicates were set‐up for treatment with bacteria and for the control. Larvae mortality was recorded at 24 and 48 hr. The three A. aegypti populations were susceptible to L. sphaericus formulation with a mortality rate higher than 90% at 48 hr. The formulation of L. sphaericus comprised of vegetative cells of 2362 and III(3)7 strains showed promising results that could guarantee the implementation of this formulation to control of A. aegypti populations.