Effects of sublethal doses of selected insecticides on reproduction in Culex quinquefasciatus Say : (Diptera : Culicidae)

Abstract

Separate groups of Culex quinquefasciatus Say, UTMB strain, were exposed for 24 hrs as fourth instar larvae to sublethal concentrations of malathion, methoprene, propoxur and resmethrin. Effects on the insecticide-exposed and the untreated F1 progeny varied with the insecticide used. The LC₅₀ of methoprene significantly decreased female wing length. Both concentrations of methoprene reduced female longevity. Malathion and methoprene affected egg production and egg raft size. The LC₅₀ of malathion and methoprene reduced egg production by 50% and 39%, respectively. In contrast, the 0.1 LC₅₀s slightly stimulated egg production, and egg raft size. Also, the LC₅₀ of methoprene reduced the number of eggs/raft by 30% and egg hatchability by 36%. In addition, the eggs in the rafts were not aligned in straight rows or as tightly packed as in the control rafts, and some eggs floated singly on the water's surface. The 0.1 LC₅₀ and LC₅₀ of malathion, methoprene and resmethrin decreased egg length and F1 first instar larval head capsule width. Sex ratio of the adult populations of the exposed generations was affected by the LC₅₀s of methoprene, propoxur and resmethrin. Only methoprene affected the sex ratio of the F1 offspring. Propoxur and resmethrin reduced the number of females, and methoprene increased the number of females. Most insecticidal treatments increased the time to pupation and emergence of the adult populations, but none of these effects was observed in the F1 generation. Previtellogenic and vitellogenic follicle growth was delayed in the adult population by malathion, methoprene and resmethrin. Previtellogenic follicles of treated mosquitoes grew nearly as large as those of controls but required a longer development time. The length of vitellogenic follicles was not significantly different before the bloodmeal. However, those from treated females developed slower and resulted in smaller follicles than controls. This resulted in the production of shorter eggs. These data indicate that a single, sublethal exposure to certain insecticides can have a significant effect on mosquito reproduction. Many effects were manifested in the parental and F1 generations, but the F2 generation was unaffected. Methoprene produced the widest range and most severe effects.