Economic Injury Level and Demography-Based Control Timing Projection of Spodoptera litura (Lepidoptera: Noctuidae) at Different Growth Stages of Arachis hypogaea.

Abstract

Spodoptera litura (F.), one of the most devastating pests in many Asian countries, is normally controlled by relying on chemical insecticides. To encourage an integrated pest management approach, we determined the economic injury level (EIL) for S. litura on peanut, Arachis hypogaea L., by larval infestation with late instars at different crop growth stages. The cumulative consumption rate of the fifth- and sixth-instars was used as the relative unit for the "Spodoptera injury equivalent" (SIE). The yield of marketable pods significantly decreased from 6.19 to 1.63 g.plant-1 as larval infestation intensity increased throughout the entire cropping season. When supplemented with timely applications of the insecticide, indoxacarb, an oxadiazine insecticide, the EIL values obtained in the larval infestation trial ranged from 3.26 to 13.47 SIE per 20 plants depending on the timing of initial infestation. The economic threshold (ET) for late instars, i.e., multiplying the EIL by 0.75, could not be utilized as a control timing index for the outbreak of injurious larvae population because of the time-lag. When the occurrence of natural mortality in the egg to pupal stage was considered, the ETs were adjusted to reflect the average survivorship. ETs of 27.3, 55.9, 51.3, and 112.6 eggs.m-2 were recommended at the early vegetative growth, blooming/pegging, pod-setting, and pod-filling stages, respectively, for initiating control measures. By simulating the pest population with the program, Timing-MSChart, we integrated the stage-specific EILs and ETs with the life-table data of S. litura on peanut and then proposed a demography-based control timing.