The mirid bug, Cyrtorhinus lividipennis, is a plant-feeding and predatory insect, preferring plant- and leafhopper eggs and young nymphs (Shepard et al 1987). It is now one of the important predators of brown planthopper (BPH) Nilaparvata lugens and whitebacked planthopper (WBPH) Sogatella furcifera (Heong et al 1991). Its populations were highly correlated with planthoppers maintaining a high density in the field even in the absence of BPH, but Laba and Heong (1996) found no preference for BPH and WBPH eggs. Tritropic interactions among plants, herbivores, and natural enemies can be affected profoundly by host-plant quality, architecture, and distribution (Boethel and Eikenbary 1986). C. lividipennis population density increased significantly in the field with higher N fertilization through improved nutrient conditions in rice plants (Meerzainudeen and Kareem 1999). Through a series of laboratory experiments, we tried to quantify the influence of N on the predatory behavior of C. lividipennis in rice.
Rice plants (IR64) under four N regimes-200, 100, 50, and 0 kg ha-l (labeled 200N, 100N, 50N, and 0N, respectively)-were studied. Ammonium nitrate was applied at different rice growth stages. An electronic chlorophyll meter (SPAD 502, Minolta Camera Co., Osaka, Japan) was used to assess tis- sue N. By pooling the data from all plant growth stages, a relationship between leaf N content (N) and leaf SPAD readings was found to be N% = 0.1151 SPAD - 1.2772 (F = 162, P< 0.001). This linear model was used to predict the N content of plants for all the experiments. The insects employed in this experiment were successively mass-cultured on host plants with low (0N) and high (200N) N regimes and were labeled 0NGi and 200NGi for BPH and 0NFi for and 200NFi for mirid bug, respectively (i is the number of successive generations on rice plants with the same N regime).