Synergistic activity of Bacillus thuringiensis -endotoxin and TMOF against Culex pipiens and Spodoptera littoralis larvae

Trypsin Modulating Oostatic Factor (TMOF) is a decapeptide hormone that inhibits the biosynthesis of digestive enzymes in the mosquito midgut. The hormone inhibits food digestion and ultimately leads to starvation and death. It has been used as a biological insecticide to control mosquitoes. In an attempt to increase the insecticidal activity of TMOF, a combination of CryIC (-endotoxin from Bacillus thuringiensis) and TMOF was determined. Eight recombinant proteins fused with GST (glutathione-S-transferase) were expressed in Escherichia coli cells. Their insecticidal activities were determined against Culex pipiens and Spodoptera littoralis larvae. Purified GST-TMOF and its analogue GST-YDPAS exhibited a moderate toxicity on C. pipiens larvae with LC50 of 145.9 and 339.9g/mL, respectively. Unexpectedly, no mortality was observed in first instar larvae of S. littoralis. Puirified GST-TMOF and GST-YDPAS together with Bt toxin showed a synergistic toxic effect on both Culex and Spodoptera larvae. In the presence of 100g/mL GST-TMOF and GST-YDPAS, the median lethal concentration of entomocidus on culex larvae decreased from 52.1 to 16.7 and 31.9g/mL, respectively. Likewise, GST-TMOF and GST-YDPAS incorporated with 0.07g/cm(2) of enotmocidus showed insecticidal activity against S. littoralis with LC50 of 16.4 and 21.9g/cm(2). The E. coli lysates containing GST-CryIC and its 3-truncated version showed low toxicity against the lepidopteran insect (10.8 and 16.6g/cm(2)) compared to 0.15g/cm(2) of the native crystalline form of CryIC. Similarly, the mosquitocidal activity of the recombinant Bt toxins was low.