Toxicity of a mosquitocidal metabolite of Pseudomonas fluorescens on larvae & pupae of the house fly, Musca domestica.

ABSTRACT

BACKGROUND & OBJECTIVES: Biological control through the use of parasitoids and pathogens is one of the alternatives to the use of chemical pesticides for control of insects of public health importance. At the Vector Control Research Centre, a liquid formulation developed using the metabolite of a Pseudomonas fluorescens strain was found to be lethal to larvae as well as pupae of vector mosquitoes. The lethal fraction of the metabolite is a protein with a molecular mass of 44 kDa and toxicity studies showed that it is safe to mammals. In the present study, this formulation was evaluated against immatures of the common house fly, Musca domestica, to find out whether it could be developed into a potential biocontrol tool. METHODS: Early second instar larvae of house fly were introduced into rearing medium incorporated with the formulation at concentrations of 1, 5, 10, 15, 20 and 25 per cent, which were equivalent to respectively 1.13, 5.63, 11.25, 16.88, 22.50 and 28.13 microg of the toxic protein/ g of rearing medium. Mortality was monitored until the emergence of adult house fly. Net mortality of larvae and pupae were calculated and the LC50 and LC90 values were determined through probit regression analysis. RESULTS: Larval mortality was obtained from day 3 to 6 post-treatment. Net mortality of larvae was higher at the concentration of 20 than at 25 per cent. However, it was higher at 25 per cent on day 5 and continued to day 6 when there was no larval mortality at other concentrations. The net mortality of pupae was higher than that of larvae at all the concentrations except at 20 per cent. The LC50 and LC90 values calculated from the net mortality of larvae and pupae together, from day 1 to 12 post-treatment, were respectively, 8.25 and 51.79 microg protein/g of the fly rearing medium. INTERPRETATION & CONCLUSION: The formulation prepared from the exotoxin of P. fluorescens was toxic to the house fly. Pupae were more susceptible than larvae and the activity of the toxin might have been through cuticular absorption. The results are indicative of the possibility of development of the mosquitocidal metabolite for house fly control through appropriate field evaluations.