The possible effect of insecticide drift from citrus orchards, and acute toxicity of insecticides on the biocontrol agents of Pontederia crassipes (Mart.) Solms-Laub (Pontederiaceae) established along citrus orchards in the Lowveld region of Mpumalanga Province, South Africa

Abstract

This study investigated the possible effect of insecticide drift on naturalized biological control agents of Pontederia crassipes (Mart.) Solms-Laub (Pontederiaceae), in the Lowveld region of Mpumalanga Province of South Africa. Occurrence and abundance of biocontrol agents were recorded at three sites on the Crocodile River and at three dams adjacent to citrus orchards. Leaves of P. crassipes and water samples were collected for insecticide residues and also nutrient status of the water and plants. Eccritotarsus catarinensis Carvalho (Hemiptera: Miridae), Neochetina spp. (combined) (Coleoptera: Curculionidae), and Orthogalumna terebrantis Wallwork (Sarcoptiformes: Galumnidae) were recorded with notable variation in abundance between the river and dams across regions. Insecticide residues were not detected on all leaves sampled across study regions, however, nutrients were detected with nitrate ranging between oligotrophic and mesotrophic. Phosphorus was also detected, but, neither of the nutrients correlated with the occurrence and abundance of naturalized biological control agents of P. crassipes. Bioassays were conducted to measure the effect of commonly used insecticides (viz. Methomyl and Chlorpyrifos) on the survival and feeding damage of biological control agents of P. crassipes. Survival of individual insects was recorded between 0.5 and 120 hours for Megamelus scutellaris and Neochetina eichhorniae Warner (Coleoptera: Curculionidae) adults for treatments where insecticides were topically applied onto the insects or leaves were dipped into the pesticides. Concentrations below field rates, recommended and above field rates of Methomyl and Chlorpyrifos on either exposure techniques significantly reduced survival and feeding of biocontrol agents. Methomyl was more toxic compared to Chlorpyrifos and it significantly reduced the survival of M. scutellaris and N. eichhorniae. In conclusion, in this study, population abundance of biocontrol agents of P. crassipes at the Lowveld region of Mpumalanga was not influenced by pesticide drift, but, insecticides commonly used in the citrus orchards has the potential to negatively impact naturalized biological control of P. crassipes as demonstrated by the bioassays.