Determination of an action threshold for tobacco caterpillar, Spodoptera litura (F.) based on pheromone trap catches in castor (Ricinus communis L.)

Abstract

Field experiments were conducted for four years to study the relationship between tobacco caterpillar, Spodoptera litura (F.) (Lepidoptera: Noctuidae) moths captured on synthetic sex pheromone trap and associated field population and damage to castor for determining action threshold for this insect, based on number of moth catches in pheromone trap. Peak trap catches, oviposition and larval incidence of S. litura was recorded during September to October. Economic damage of more than 25% defoliation by S. litura was observed during September to November. Pooled analysis of four years data revealed that current week trap catches of moths was found to be significant strong positive correlation with oviposition (r = 0.852 to 0.855), while two and three week after moth catches in trap recorded significant strong positive correlation with larval incidence (r = 0.814 to 0.873) and per cent defoliation (r = 0.868 to 0.892) of S. litura in the field, respectively. Regression analysis revealed that the per cent defoliation in the field was a linear function of three week after moth catches in pheromone trap, indicating the possibility of assessing the level of damage from trap catches. From this relationship, the pheromone trap catches corresponding to the economic threshold level of 25% defoliation of S. litura on castor was estimated to be 81.4 moths/trap/week. Thus, pheromone trap based action threshold identified can be used to forecast the seasonal status of S. litura.