Toxicity of insecticides to predators of rice planthoppers: Spiders, the mirid bug and the dryinid wasp

The toxicity of nine insecticides to predators of rice planthoppers was examined with first instars of four spider species, i.e. Pardosa pseudoannulata, Tetragnatha maxillosa, Ummeliata insecticeps and Gnathonarium exsiccatum, and females of the mirid bug Cyrtorhinus lividipennis and the dryinid wasp Naplogonatopus apicalis, by dipping the test arthropods into insecticide solution. Deltamethrin was the most toxic to the spiders, with LC50 at 0.033 to 1.1 ppm, followed by ethofenprox. The results indicate that the spiders are susceptible to synthetic pyrethroids. Insecticide susceptibilities of the spiders varied among species. T. maxillosa was more susceptible to seven insecticides, particularly to diazinon, than the other spiders, but P. pseudoannulata was more susceptible to phenthoate and carbaryl than T. maxillosa. Many insecticides, particularly phenthoate, imidacloprid and deltamethrin, were toxic to C. lividipennis. All insecticides tested were toxic to H. apicalis. We also evaluated the effect of insecticides on the spiders and C. lividipennis in paddy fields. Deltamethrin had a destructive effect on the spider populations and may have induced a resurgence of the Nilaparvata lugens population. Phenthoate reduced the abundance of lycosid spiders, and ethofenprox reduced the abundance of Tetragnatha. The C. lividipennis abundance decreased to a low level in all insecticide-treated plots except those treated by buprofezin.