Effects of nitrogen on egg-laying inhibition and ovicidal response in planthopper-resistant rice varieties

Abstract

A series of experiments was set up to examine the effects of nitrogen on rice (<i>Oryza sativa</i> L.) resistance against <i>Nilaparvata lugens</i> (Stål) and <i>Sogatella furcifera</i> (Horváth). Egg laying by <i>N. lugens</i> was reduced on the <i>indica</i> variety IR60. Nymph biomass (<i>N. lugens</i> and <i>S. furcifera</i>) was also lower on IR60: this was associated with low honeydew production and a high proportion of xylem-derived honeydew in <i>N. lugens</i> but not in <i>S. furcifera</i>. Nitrogen increased egg-laying by <i>S. furcifera</i> and increased <i>N. lugens</i> nymph biomass on all varieties tested. Oviposition and egg mortality in both planthopper species were examined on plants at 15, 30 and 45 days after sowing (DAS). <i>Sogatella furcifera</i> laid more eggs on plants at 15 DAS, but laid few eggs during darkness; <i>N. lugens</i> continued to lay eggs on older rice plants (30 DAS) and during darkness. Egg mortality was high on cv. Asiminori, highest at 45 DAS, and higher for <i>S. furcifera</i> than for <i>N. lugens</i>. Mortality of <i>S. furcifera</i> eggs was associated with lesions around the egg clusters. These were more common around clusters laid during the day and suggested induction by Asiminori of an ovicidal response. Egg mortality declined under higher soil nitrogen levels. Results are discussed in the light of improving rice resistance against planthoppers and reducing rates of planthopper adaptation to resistance genes.