ARS | Publication request: Studies on larvicidal and pupicidal activity of Leucas aspera Willd. (Lamiaceae) and bacterial insecticide, Bacillus sphaericus, against malarial vector, Anopheles stephensi Liston (Diptera: Culicidae)

Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 17, 2011
Publication Date: September 20, 2011
Citation: Kovendan, K., Murugan, K., Vincent, S., Barnard, D.R. 2011. Studies on larvicidal and pupicidal activity of Leucas aspera Willd. (Lamiaceae) and bacterial insecticide, Bacillus sphaericus, against malarial vector, Anopheles stephensi Liston (Diptera: Culicidae). Proceedings of the National Academy of Sciences. 10:1-6.

Interpretive Summary: The primary threat to effective mosquito vector control is resistance to conventional synthetic insecticides in the vector population. Botanical insecticides may provide an alternative to synthetic insecticides. Botanicals are considered comparatively safe, are biodegradable, and often can be obtained from local sources. In this study, Bharathiar University (India) and ARS scientists evaluated phytochemicals extracted from a type of mint plant for toxicity to the larvae and pupae of an important malaria vector in India. The plant extract killed 65-75% of the mosquito larvae and pupae in tests in the laboratory but failed to measurably increase mortality in immature mosquitoes when used in conjunction with commercial bacterial insecticides. The results of this study indicate that combinations of L. aspera extract and bacterial insecticide do not significantly enhance biological activity of the latter but that plant extract kills mosquito larvae and pupae and warrants further study for use as a botanical insecticide against A. stephensi.

Technical Abstract: The efficacy of whole plant ethanolic extracts of Leucas aspera and of Bacillus sphaericus was determined for larvae and pupae of Anopheles stephensi. When larvae were exposed to one of five concentrations of plant extract (6%, 8%, 10%, 12%, and 14%) for 24 h, mortality in 4th instars ranged from 10% (8% in pupae) at the 6% concentration to 75% (60% in pupae) at the 10% concentration. The median lethal concentrations (LD50) for first to fourth instars and pupae were 9.69%, 10.27%, 10.82%, 11.30%, and 12.73%, respectively. Bacillus sphaericus tested at 0.02, 0.04, 0.06, 0.08, and 0.10 ug commercial product per ml induced 14 to 80% mortality (at the low and high concentrations, respectively) in 4th instars after 24 h and similarly in pupae, 12 to 69% mortality; respective LD50 values were 0.051, 0.057, 0.062, 0.066, and 0.073 ug commercial product per ml. When 2% L. aspera whole plant ethanolic extract was combined with B. sphaericus at each of the five concentrations noted above, the LD50 for 4th instars and pupae was, respectively, 0.058 and 0.074 ug commercial product per ml. The results of this study indicate that whole plant ethanolic extracts of L. aspera have potential for use as a botanical insecticide against A. stephensi but that combinations of L. aspera extract and bacterial insecticide do not significantly enhance biological activity of the latter.