VARIATION IN YIELD LOSS PER APHID-DAY DUE TO SITOBION AVENAE-INFESTATION IN HIGH-YIELDING WINTER-WHEAT

In 1995, two experiments were performed to examine the relationship be tween different infestation trends of Sitobion avenae and yield loss i n winter wheat grown under optimal conditions in climate chambers with out nitrogen or water deprivation to obtain high yield levels (experim ent I: 1.9 g per ear, experiment II: 2.5 g per ear). Each trial includ ed a comparison of the following three treatments: A) early aphid infe station under the influence of coccinellids and C) untreated control. In experiment I, the early aphid population (A) caused the same yield loss as the later undisturbed aphid population (B), while the aphid in dices differed (A: 1055, B: 2307 aphid-days per tiller). The yield los s per aphid-day and tiller in treatment A (0.52 mg) was significantly higher than that in treatment B (0.30 mg). In experiment II, no signif icant yield loss (A: 0.01 mg B: 0.1 mg per aphid-days per tiller) coul d be determined despite the high cumulative aphid indices (A: 708, B: 2511 aphid-days per tiller). This phenomenon indicates enormous tolera nce responses of wheat plants. The treated plants tolerated the low ap hid density during the early milky ripe and the high density during th e dough stage. Optimal growing conditions led to the extremely high at tainable yield level, which seems to enhance the tolerance responses. The results of two experiments cannot be applied to field conditions. However, they show the enormous deviation in cereal aphid infestation - yield loss relationship. Moreover, they contribute to the understand ing of physiological responses of the wheat plant to aphid infestation .