Contact toxicity and proximate effect of fipronil on insect pest and predatory ant community structure in cocoa agro-ecosystem

Abstract

Although usage persists in some countries, fipronil is banned or restricted in many others. Prior to its ban on cocoa in Ghana, concerns about its effect on non-target insects and secondary outbreak of Anomis leona were conflicting. This study, which predates the ban, assessed the toxicity and the short-term effect of fipronil on specific insect community structure in the cocoa agro-ecosystem alongside bifenthrin and a non-insecticide control. Although the insecticides induced a high mortality (90-100%) on the target (mirid: Sahlbergella singularis, stink bug: Bathycoelia thalassina and coreid bug: Pseudotheraptus devastans) and non-target (ants: Oecophylla longinoda, Crematogaster africana, Pheidole megacephala and Camponotus consobrinus) insects, the knockdown to fipronil was very low compared to bifenthrin. On the field, fipronil was more detrimental to the ants. Insecticide-treated plots recorded relatively lower post-treatment pest diversity compared to the control, except the last sampling month while ant abundance, richness and diversity were lowest on the fipronil-treated plots at the end of the study period. This study demonstrates that although fipronil was effective against pests and did not result in acute secondary pest outbreak, it was harmful to the ants. This effect could potentially be replicated on these ant species in other cropping systems where the insecticide is used, adversely affecting ecosystem service delivery. Hence, research on its impact on non-target organisms in other cropping systems is needed to regulate and monitor its use.