Impact of the English Grain Aphid, Sitobion avenae (F.) (Homoptera: Aphididae), on the Yield of Wheat Plants Subjected to Water Deficits

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Abstract
The effects of two population levels of Sitobion avenae (F.) and two water stress levels on the yield of winter wheat were examined. Wheat, cv. Talento grown in 7.6-liter pots under greenhouse conditions was subjected to three water regimes for establishing three plant water potential levels. Aphids were caged on wheat heads, and mean survival, mean fecundity (first 10 d), mean total progeny per adult, and the intrinsic rate of increase (rm) were calculated for each of the treatments. Water deficits appeared to reduce reproductive rates, although differences were not significant at P = 0.05. Water stress was the single most significant factor reducing plant growth and yield. Leaf stomatal conductance decreased in the water-stressed plants. This was associated with stomatal control of transpiration, and consequent yield reductions were observed. A density of six adult aphids for 32 d on the head significantly reduced most of the yield components when plants were grown under nonstress conditions (leaf water potential [Ψ] between −0.90 and −1.29 MPa [megapascals]), but there was no significant decrease in yield at a density of two adult aphids per head. No significant differences between treatments were observed for the moderate stress regime (Ψ −1.42 to −2.28 MPa). However, under severe stress conditions (Ψ −1.84 to −2.60 MPa), plants infested with six adult aphids per head suffered a substantial yield reduction (up to a 39.3% loss in kernel weight per head) when compared with the noninfested plants. A significantly higher percentage of yield reduction for a given aphid density was obtained when plants were grown under severe water stress versus nonstress conditions. This result suggests the existence of a synergistic effect on wheat yield between aphids and water stress. Therefore, the water status of the plant should be considered to determine an economic injury level for S. avenae infesting winter wheat.