Effects of fenitrothion on Gryon japonicum (Hymenoptera: Platygastridae) parasitizing non-viable, refrigerated eggs of Riptortus pedestris (Hemiptera: Alydidae)

• Non-viable host eggs can be released in a soybean field to enhance natural parasitism.
• Acute and sublethal toxicity of fenitrothion on a parasitoid of bean bug was assessed.
• Fenitrothion caused high acute toxicity on the parasitoid within 24 h.
• Effect of fenitrothion did not differ between refrigerated and unrefrigerated eggs.

Gryon japonicum (Ashmead) is an egg parasitoid of Riptortus pedestris (Fabricius), an economically important pest of soybean in Korea and Japan. One conservation biological control tactic for this pest is to release refrigerated, non-viable eggs of R. pedestris in a soybean field to enhance natural parasitism by G. japonicum. Since fenitrothion is commonly used against R. pedestris, in this study we investigated its potential effects on the parasitization of refrigerated host eggs by G. japonicum. The acute toxicity of fenitrothion was compared with that of eight other pesticides by direct spray, exposure to residues, and oral ingestion. Fenitrothion, spinosad, cyfluthrin, etofenprox, carbosulfan, and thiamethoxam all caused 100% mortality of the parasitoid adults within 24 h following direct spray or exposure to residues. Fenitrothion was also highly toxic to G. japonicum if ingested orally. There were no significant sublethal effects of fenitrothion on G. japonicum emerging between refrigerated and unrefrigerated host eggs although adult emergence of G. japonicum was reduced by 13 and 35%, respectively, compared to the control when fenitrothion was applied to eggs on the 8th day after parasitization. Parasitism of refrigerated and unrefrigerated eggs was not statistically different, but decreased significantly when the parasitoids were provided with fenitrothion-sprayed eggs 1 h after application. In conclusion, while fenitrothion is detrimental to the parasitoid in the acute toxicity assay, the negative effects of fenitrothion on parasitization by G. japonicum did not differ between refrigerated and unrefrigerated eggs, suggesting that refrigerated eggs can be used in soybean fields to enhance parasitism.

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