Pesticide-mediated trophic cascade and an ecological trap for mosquitoes

Broad-spectrum pesticides can have immediate toxic effects both on target pest species and on non-target species. They may also have positive residual effects on mosquitoes after pesticide degradation, by altering the community structure, that is, by reducing abundances of mosquito competitors and predators, and via a trophic cascade, which may increase food resources for mosquito larvae. Alternatively, if a pesticide-mediated trophic cascade results in toxic or inedible algae, the pesticide can act as an ecological trap for some taxa by attracting oviposition in sites where algae are abundant but unsuitable. The present study assessed mosquito oviposition habitat selection, mosquito larval performance, and community structure alterations after applications of various pesticides. The experiment was conducted in outdoor mesocosms assigned to one of four treatments: (1) control, no pesticides; (2) Bacillus thuringiensis var. israelensis (Bti), a narrow-spectrum bacterium well known for its larvicidal activity on mosquitoes and other dipterans; (3) temephos, an organophosphate mosquito larvicide with community-wide spectrum effects; and (4) pyriproxyfen, a pyridine-based insect growth regulator (IGR) class with wide-spectrum effects. Soon after pesticide application, Culex pipiens oviposition was highest in the control pools. Invertebrate species richness and abundance were strongly reduced in the broad-spectrum pesticide treatments (temephos and pyriproxyfen) when compared to control. One month after pesticide application, Cx. pipiens oviposition was highest in the pyriproxyfen-treated pools, although larval survival remained lowest in the pyriproxyfen-treated pools. Our results suggest that pyriproxyfen causes a chemically mediated trophic cascade and provides an ecological trap, that is, attracting mosquito oviposition due to an altered community structure, but causing high mosquito larval mortality.