Soybean aphid population dynamics, soybean yield loss, and development of stage-specific economic injury levels
Stage-specific economic injury levels (EILs) form the basis of integrated pest management for soybean aphid (Aphis glycines Matsumura) in soybean [Glycine max (L.) Merr.]. Experimental objectives were to develop a procedure for calculating EILs of the soybean aphid specific to the R2 (full bloom), R4 (full pod), and R5 (beginning seed) soybean development stages using the law of the diminishing increment regression model. Soybean aphid population growth over time appeared to follow the symmetrical bell-shaped and logistic growth curve models. Peak soybean aphid population levels and rates of increase occurred at the R5 development stage and then declined sharply thereafter. Highest peak soybean aphid populations were 21,626 aphids plant–1 for infestations starting at V5, and 6446 aphids plant–1 for infestations starting at R2. Highest maximum aphid-days plant–1 recorded were 537,217 for V5-introduced soybean aphids and 148,609 aphid-days plant–1 for R2-introduced soybean aphids. On average, the calculated maximum possible yield loss was 75% for soybean aphid infestations starting at the V5 (five node) stage and 48% for soybean aphid infestations starting at the R2 stage. Interrelationships among the current or predicted market value of soybean, cost of soybean aphid control, and the yield potential of the soybean field were considered in the calculations of the stage-specific EILs. Practical examples for calculating stage-specific EILs are presented. Economic injury levels were calculated both as soybean aphids plant–1 and soybean aphid-days plant–1. Use of these stage-specific EILs may enable growers to manage soybean aphids more accurately.