Efficacy of entomopathogenic nematodes combined with insecticides against Bradysia odoriphaga larvae

Summary Entomopathogenic nematodes (EPN) as an environmentally-friendly biocontrol agent in combination with low toxic insecticides can increase control efficacy against insect pests. In this study, Steinernema carpocapsae All (Sc-All) combined with four common insecticides was used to evaluate the control efficacy against chive root gnat (Bradysia odoriphaga), an important pest of vegetables, e.g., chive, onion or garlic. The compatibility of nematodes with insecticides and host-seeking behaviour were also evaluated by the laboratory bioassay. The results showed three insecticides (matrine, imidacloprid and chlorpyrifos) at the recommended concentrations (RC), 10% RC or 2% RC and insecticide phoxim at 10% RC or 2% RC had no effect on nematodes survival. Sc-All at 50 infective juveniles (IJ) per insect larva in the presence of the four insecticides at 10% RC demonstrated a potentiated, additive or a synergistic effect on the corrected mortality rates of insect up to 100% (imidacloprid) when compared with the corresponding insecticide and Sc-All alone. A synergistic effect resulting in lethal effect was found as early as at 24 h when 200 IJ of Sc-All per insect larva were combined with 10% RC imidacloprid, whilst only 9.4% and 0 corrected mortality were detected, respectively, when exposed to the same amount of imidacloprid and Sc-All alone. For the first time a Pluronic gel system assay revealed that the presence of insecticides significantly improved Sc-All host-seeking ability as early as 30 min post exposure. The results indicated that low doses of Sc-All-imidacloprid combination would be an effective strategy to control chive root gnat.