Response of soybean to attack by stemfly Melanagromyza sojae in farmers' fields in Indonesia

1. To determine the effect of Melanagromyza sojae, a common pith-boring agr omyzid fly, on soybean under field conditions in Indonesia, physiologically mature plants from 70 farmers' field sites were uprooted. Plant parameters and exit holes in the stem (created by M. sojae prior to pupation) were re corded. 2. Melanagromyza sojae was present at all sites and infested 84% of the pla nts. Farmers' applications of insecticides had no influence on the infestat ion level by M. sojae. Most exit holes occurred above the cotyledons. 3. Exit holes in the hypocotyl (i.e. the stern section below the cotyledons ), indicative of early attack, were associated with a decrease in the numbe r of seeds per plant, stem diameter and plant height. This suggests that ea rly attack adversely affects plant development. Exit holes above the hypoco tyl, indicative of attack later in the season, were associated with an incr ease ill plant parameters, suggesting that late attack occurred in response to the plant's size or vigour. 4. Multiple regression was conducted to evaluate the influence of infestati on on seed production. Separate models were used to analyse early and late attack. For late attack, the plant variables 'stem diameter' and 'plant hei ght' were added to the regression. 5. Early attack caused less than 2% yield loss on a per-plant basis. Possib le compensation by neighbouring plants was not considered. Later attack did not reduce yield. Therefore measures to control M. sojae would have been u nwarranted. 6. Infestation had no effect on the 100-seed weight, confirming that the nu mber of seeds per plant reliably predicted yield.