Effect of Nitrogen Fertilizer Levels on the Incidence of Insect Pests in Four Rice Cultivars

Aim: To determine the effect of local rice cultivars and inorganic nitrogen fertilizer on the incidence of stem borer, Scirpophaga incertulas (Walker); leaffolder, Cnephalocrocis medinalis (Guenee); and gall midge, Orseolia oryzae (Wood Mason) and on grain yield of rice.

Study Design: A field experiment for two consecutive years was carried out in a split-plot arrangement in randomized block design with three replications.

Place and duration of the study: Department of Entomology, Agricultural Research Station, Nellore, Andhra Pradesh, India during Kharif, 2013 and 2014.

Methodology: The rice cultivars (NLR 20104, NLR 20106, NLR 3135 and NLR 33892) were attributed to experimental main plots and nitrogen levels (40, 80, 120, 160 and 200 kg ha-1) to the sub plots. Observations on incidence of stem borer, gall midge and leaf folder was recorded in randomly selected 10 hills per subplot in replication and percent incidence was calculated.

Results: The pest incidence for the treatments with high N rate as 160 and 200 kg/ha was higher (31.43 & 43.86% stem borer, 9.23 & 13.59% leaffolder and 2.75 & 3.00% gall midge, respectively) than that for lower N rate i.e. 40 and 80 kg ha-1 (19.24 & 20.04% stem borer, 7.37 & 7.95% leaffolder and 1.42 & 1.93% gall midge, respectively). Different rice cultivars significantly influenced the leaffolder incidence but not the stem borer and gall midge incidence. The Highest leaffolder incidence (13.44%) was noticed in NLR 20104 cultivar which was followed by NLR 3135 cultivar with 9.64% leaffolder incidence. Gall midge and leaffolder incidence was not significantly influenced by the combined effect of rice cultivar and nitrogen levels; in contrast, interaction effect has a significant influence on stem borer incidence. Significantly lowest stem borer incidence (18.21%) was noticed with NLR 3135 rice cultivar treated with 40 N ha-1, which was statistically on par with the NLR 20106 rice cultivar treated with 40 kg N ha-1 (18.23%) and NLR 20104 rice cultivar treated with 40 kg N ha-1 (19.76%). Among the different cultivarscultivar, NLR 33892 cultivarhad produced significantly highest grain yield (5955 kg ha-1) may be for having higher physiological indices. The highest/optimum grain yield (5726 kg ha-1) produced by plots that received 120 kg N ha-1. Further increase in nitrogen level beyond 120 kg ha-1 did not significantly improve the grain yield. Optimal N rate was revealed to be 120 kg ha-1 for four rice cultivars for reducing the stem borer/leaffolder/gall midge incidence so as to minimize the cost of inputs for pest control measures and for achieving high grain yield.

Conclusions: The incremental doses of nitrogen fertilizer significantly enhanced stem borer, leaffolder and gall midge infestation on rice, which further affects the grain yields.