Soybean Nitrogen Relations and Root Characteristics after Cerotoma trifurcata (Coleoptera: Chrysomelidae) Larval Feeding Injury

Abstract

Bean leaf beetle, Cerotoma trifurcata (Förster), is a pest of economic importance to soybean (Glycine max [L.] Merr.) production in the U.S. This greenhouse study was conducted to characterize larval feeding damage effects on nitrogen (N) assimilation and root system characteristics in soybean. Pots containing individual plants (n = 15) were exposed to 1 of 3 treatment levels that varied in infestation intensity: high intensity (19 larvae per pot), low intensity (5 larvae), and an uninfested control group. After 3 wk, the plants were dissected, and the fresh and dry weights of the roots and shoots were recorded. Also, the number of nodules, number of damaged nodules, and the volume of nodules were compared among the treatments. Leaf and pod chlorophyll, nitrate-N, ureide-N, amino-N, and total N concentrations were measured and compared among the treatments. Nodules were the only organs on the root system that were visibly damaged by larval feeding. External nodule surfaces were scarred, and frequently the internal matrices of the nodules were completely excavated by the larvae. Significantly more nodules per plant were found in the infested treatments, and damaged nodules were significantly more abundant on plants in the high infestation compared with those in the low infestation treatment. Also, plants from the highly infested treatment had significantly smaller nodules than the uninfested control, suggesting that the infested plants responded to larval damage by producing additional nodules. Leaf and pod ureide-N and total N concentrations were significantly lower in infested plants. Thus, feeding by C. trifurcata larvae affects the characteristics of nodules as well as the concentrations of N assimilates within soybean plants.