The economic impact of sugarcane moth borers in Indonesia
Two years of field experiments were conducted in Indonesia (East Java) to investigate damage levels and economic impact of three major sugarcane moth borers Chilo sacchariphagus, C. auricilius and Scirpophaga excerptalis. To collect such information, we created different levels of borer damage by comparing treated (biocontrol vs. insecticide applications) and untreated plots with natural infestation. In the plant cane experiment, damage levels in untreated plots were the highest with an average of 14.5 % internodes bored by Chilo spp. and 15.8 % stalks damaged by S. excerptalis. In comparison, the best treatment obtained with insecticides had only 4.6 and 3.8 % damage level respectively, followed by plots treated with Trichogramma releases. The untreated plots had lower sucrose yield, higher fibre content and lower cane yield than all treated plots, which represents a loss in cane yield of 45.4 t/ha (?34 %). In the ratoon crop, both yields and the percent of damaged stalks were lower (9.5 % by top borer and 40 % by stem borers) and the yield reduction in biomass was estimated at 12.5 t/ha (19.2 %). In the situation of high infestation as observed in the plant crop, stalk height and other parameters were affected, particularly when damaged by S. excerptalis. For this borer, the weight loss per stalk was estimated at 40.8 % compared to 15 % from stemborer damage (2.7 times less). Using these figures, the overall economic impact was calculated, knowing the relationship between weighted borer damage and sucrose yield in plant cane, the cost of applications and the net value per tonne of extra sugar. Therefore, a damage threshold can be applied to trigger control strategies.