Evaluating the role of predation in population fluctuations of the soybean aphid Aphis glycines in farmers' fields in Indonesia.
The seasonal phenology of A. glycines was studied in soyabeans at 15 sites on Sumatra. A. glycines densities peaked in the vegetative plant stage and declined rapidly afterwards or towards the flowering stage. A clear statistical dependency of coccinellid predator densities on aphid densities was found. Age-specific survival and reproduction of A. glycines were determined in clip-cages on 3-, 5- and 7-week-old plants. The growth rate was highest on 3-week-old plants and declined linearly with plant age. Predation rates were measured through direct observations of individual coccinellid and staphylinid predators in the field at different times during daylight hours. Predator observations were made at various field densities of A. glycines, and thus a functional response of predation rate to prey density was obtained for each predator type. A simple deterministic model was developed to calculate population changes of A. glycines based on field-measured diurnal predation functions and aphid growth rates. The model was adopted to evaluate whether aphid population declines observed in the field could be ascribed to predation. In young soyabean crops (<40 days), steep population declines of A. glycines that occurred after brief peak densities could not be ascribed to predation with the model. In older soyabean crops (> 40 days), the observed population declines to low levels were attributable to predation. This pattern was observed on subsequent occasions in the same crop, and was observed concurrently in crops of three different ages planted in randomized blocks. The influence of other mortality factors (competition and migration of A. glycines, rainfall, plant age) on A. glycines dynamics is discussed. Coccinellid predators play a role in suppressing A. glycines in soyabeans. Therefore it is important that predators are conserved early in the season by avoiding unnecessary insecticide applications.