Seasonal occurrence pattern and within-plant egg distribution of bean bug, Riptortus pedestris (Fabricius) (Hemiptera: Alydidae), and its egg parasitoids in soybean fields

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A study on the seasonal occurrence and within-plant distribution of Riptortus pedestris (Fabricius) (Hemiptera: Alydidae) and its two egg parasitoids was conducted to evaluate the effectiveness of parasitoids as a biological control agent in soybean fields. We sampled soybean plants during the planting seasons in 2007 and 2008 in Andong, Korea. Adult R. pedestris occurrence and egg deposition began at the R2–R4 stage of soybean. Nymphs started to appear during the R5 stage of soybean. Peak population densities of both eggs and adults were recorded in the R5 stage of soybean, whereas that of nymphs started in the R6 stage. Seasonal total parasitism by Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae) was significantly higher than Gryon japonicum (Ashmead) (Hymenoptera: Scelionidae). For the within-plant distribution of R. pedestris eggs, 90% were found on leaves and 70% on the under-leaf surface. Egg parasitism by O. nezarae was higher on the leaf surface than on pods. Results suggest that R. pedestris starts to occur at the R2–R4 stages of field soybeans regardless of seed sowing time, and population density peaks after the R5 stage. Sampling eggs on leaves is an alternative method to measure R. pedestris populations in the field since most eggs are found on leaves. Since O. nezarae was synchronous with the bugs occurrence, it would be a more efficient biological control agent of R. pedestris than G. japonicum.