Study on the synergistic effect of Henna in enhancement of pathogenecity of Bacillus thuringiensis Berliner on third and fourth instars larvae of Colorado potato beetle, Leptinotarsa decemlineata (Say) (Col.: Chrysomelidae)

Abstract

In this study, the synergistic effects of Henna powder in the enhancement of pathogenicity of Bacillus thuringiensis var. tenebrionis and B. thuringiensis var. kurstaki on third and fourth instars larvae of Colorado potato beetle, Leptinotarsa decemlineata (Say) (Col.: Chrysomelidae) were evaluated. Combination of three concentrations of Henna powder (3000, 4000 and 5000 ppm) with two varieties of bacteria (500 and 1000 ppm for third and fourth instars larvae, respectively) was used against the third and fourth instars larvae in a completely randomised plot design in laboratory conditions. The results revealed that there was a remarkable synergistic effect of Henna powder on B. thuringiensis efficiency. The 3rd instar larval mortality for mixture of B. t. var. tenebrionis (500 ppm) with Henna (5000 ppm) was 72.5% after six days; however, the mortality of larvae treated with B. t. var. tenebrionis (500 ppm) alone was 17.5% for the same time period. The mixture of B. t. var. kurstaki (500 ppm) with Henna (5000 ppm) had caused 85% mortality in the 3rd instars larvae during 6 days. This concentration of B. t. var. kurstaki produced 22.5% mortality on these larvae. The mortality for fourth instars larvae treated with B. t. var. tenebrionis (1000 ppm) was 12.5% after six days; however, this rate for fourth instar larvae treated with mixture of B. t. var. tenebrionis (1000 ppm) and Henna (5000 ppm) was 47.5% for the same time period. The mortality of 4th instars larvae treated with mixture of B. t. var. kurstaki (1000 ppm) with Henna (5000 ppm) was 3.4-fold as much of the mortality of fourth instars larvae treated with B. thuringiensis var. kurstaki (1000 ppm). Henna powder used in combination with B. thuringiensis might potentially reduce overall insecticide use, thus delaying the development of insecticide resistance.