Larvicidal effects of grain sorghum (Sorghum bicolor) seedling extracts upon Culex pipiens larvae

A study of laboratory and field reared 2nd and 3rd instar Culex pipiens larvae suggests that extracts from 2 varieties of Sorghum bicolor seedlings are significant (P less than 0.05) larvicides under laboratory conditions. These plant extracts contain the organic cyanogen dhurrin and were calibrated to produce 90% mortality in 2nd instar Culex pipiens larvae at 0.82 ppm and 90% mortality in 3rd instar larvae at 1.12 ppm. A preliminary behavioral assessment of late 3rd instar larvae exposed to 1.42 ppm suggests that these plant extracts produce 80% mortality after only 4-5 h of contact. Plant extracts appear stable when stored at up to 32 degrees C in a closed container. Once the extracts are infused in water and exposed to air, however, they biodegrade after 24 h. These laboratory results emphasize the need for field tests against natural populations of Culex pipiens and nontarget organisms.