Economic impact of sugarcane moth borers in Indonesia
Field experiments were conducted over two years in Indonesia (East Java) to investigate damage levels, yield decline and economic impact of three major sugarcane moth borers Chilo sacchariphagus and C. auricilius (referred to as stem borers) and Scirpophaga excerptalis (referred to as top borer). Different levels of borer damage were created to allow comparison between plots treated, i.e. biocontrol and insecticide applications and untreated plots that were naturally infested. 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 level obtained with insecticides was only 4.6% and 3.8% damage respectively, followed by plots treated with Trichogramma releases at the rate of 100 000/ha. The untreated plots had lower sucrose yield, higher fibre content and lower cane yield compared with all treated plots, which represented a loss in biomass of 45.4 tonnes/ha or 34% lower yield compared with the treated plots. In the ratoon crop, both yield and percent damaged stalks were lower i.e. 9.5% and 40% by top and stem borers, respectively, and the biomass yield reduction was 12.5 t/ha or 19.2% lower than the treated plots. Where high infestation was recorded in the plant crop, stalk height and other stalk quality parameters were adversely affected, particularly by damage due to S. excerptalis. The latter borer caused a 40.8% reduction in stalk mass compared to 15% due to stem borer; a 2.7 magnitude difference. The economic impact of these borers was calculated using the values obtained from this loss study, in addition to: information known about the relationship between weighted borer damage and sucrose yield in plant cane; the cost of applications, and the net value per tonne of sugar yield gain. It was possible thereafter to derive a damage threshold that can be consulted to determine when to apply control strategies.