Long-term effects of the treated municipal wastewater of the city of Riyadh on the Mosquito culex pipiens L.

Abstract

Studies of the effect of the secondary treated municipal wastewater (SW) on the longivity of the larval stage of the mosquito Culex pipiens L. were started in the year 1987, and continued for more than three years. Two separate aquamedia were used for rearing a susceptible strain of the mosquito. In the first, SW was the sole liquid for the developmental stages of the first group of the insect, and fresh water (FW) was that used for the second. Susceptibility to chlorpyrifos insecticide, of the larvae of the two groups was evaluated periodically, in the form of percent mortalities, LC50 values and slope of toxicity curves. The results exhibited that longivity of the larval stage of SW-reared mosquitoes was less than those of FW-reared ones with 3.7 and 8.3 days in the years 1987 and 1989, respectively. An increase in the sus-ceptibility of the SW-reared larva to chlorpyrifos was observed. Values of LD50 for the first group of insects ranged from 1.6 to 4.2 ppb; and those for the second were 2.25 to 4.10 ppb. Slopes ofthe toxicity curves ranged from 1.79 to 2.4 and from 1.7 to 1.9 for the first and second groups, respectively.