Assessment of Damage Caused by Two-Striped Leaf Beetle (Medythia nigrobilineata Motschulsky) Larval Feeding of Root Nodules in Soybean and Its Control during Furrow Cultivation at Seeding Time

Abstract

Two-striped leaf beetle (TSLB) larvae are known to damage soybean root nodules but the effect they have on yield has not been quantitatively assessed. This study was aimed to evaluate the impact of TSLB and to establish a technique to control the larvae. We surveyed 106 farmers’ fields in Aichi prefecture for two years to quantify the changes in TSLB adult population, nodule damage by larvae, and nitrogen (N) content of seeds and stems, growth and yields of soybean plants treated with two different pesticides that were applied to furrow at the time of seeding. The results showed that damage caused by TSLB larval feeding of root nodules was observed particularly in soybeans grown in N-poor soil, and that ureide N content was decreased in plants with damage caused by larval feeding of root nodule, and such damage caused reduction of yield consistently in both years. Furthermore, we also showed the effectiveness of applying insecticides to the sowing furrow at the time of seeding. Specifically, the application of disulfoton to the sowing furrow at a rate of 4 g m–2 as a simple method of pest control limited insect occurrences until the beginning of the blooming stage, curbed the feeding damage to root nodules caused by larvae, and increased the yield of soybean by 20%. This method has been adopted widely in Japan because it can be readily implemented using the fertilization application equipment installed on seeding machines and the insecticide is relatively inexpensive.