Response of soybean to attack by stemfly Melanagromyza sojae in farmers' fields in Indonesia.
To determine the effect of M. sojae, a pith-boring agromyzid fly, on soyabean under field conditions in East Java Province, Indonesia, physiologically mature plants from 70 field sites were uprooted in 1996. Plant parameters and exit holes in the stem (created by M. sojae prior to pupation) were recorded. M. sojae infested 84% of plants examined. Exit holes in the hypocotyl are indicative of early attack and were associated with decreases in seed number per plant, stem diameter and plant height. This suggests that early attack adversely affects plant development. Exit holes above the hypocotyl are indicative of attack later in the season and were associated with an increase in plant parameters. It is proposed that late attack occurred in response to plant size or vigour. 87% of all exit holes occurred above the hypocotyl. Applications of insecticides had no influence on M. sojae infestation level. Multiple regression was used to analyse infestation effects on seed production. Infestation had no effect on seed weight. Early attack caused <2% yield loss on a per-plant basis. Later attack did not reduce yield. It is concluded that measures to control M. sojae would have been unwarranted.