Deterrent Effects of Glyphosate on Oviposition and Larval Development of Culex quinquefasciatus.

Glyphosate is one of the most common herbicides used to control weeds in the USA. This herbicide can impact the mosquito life cycle through being carried to mosquito habitats by runoff. This study investigates the effects of glyphosate on the larval development and oviposition of a laboratory colony of Culex quinquefasciatus. Our experiments indicated that a concentration of 1 g/liter glyphosate was lethal to larval instars or the larvae impacted were either unable to molt to pupae or never emerged as adults. Larval instars exposed to 0.5 g/liter glyphosate experienced a similar impact; however, the larval stage was extended and pupation was considerably delayed. Mosquitoes oviposited in 0.5 g/liter glyphosate laid the same number of egg rafts as in water, but a considerably lower number of eggs exposed to glyphosate hatched as first instars. If gravid females laid their eggs in 1 g/liter glyphosate versus water, the difference between the 2 groups would be statistically significant and a very low number of eggs exposed to glyphosate could ever hatch into larvae. In an oviposition assay carried out in a cage with 3 choices of water, 0.5 g/liter, and 1 g/liter glyphosate, a higher number of egg rafts were laid in water, followed by 0.5 g/liter and 1 g/liter glyphosate concentrations, respectively. Our study indicated such a severe effect of glyphosate on all immature stages of Cx. quinquefasciatus that no adult could ever emerge.