SIMULATION OF DAMAGE IN WINTER-WHEAT CAUSED BY THE GRAIN APHID SITOBION-AVENAE .1. QUANTIFICATION OF THE EFFECTS OF HONEYDEW ON GAS-EXCHANGE OF LEAVES AND APHID POPULATIONS OF DIFFERENT SIZE ON CROP GROWTH

Damage in winter wheat caused by the grain aphid Sitobion avenae F., was studied in the laboratory and at field level. Honeydew substitute solution was applied to flag leaves of winter wheat plants. One and fifteen days after application, the rates of net carbon dioxide assimilation and transpiration were measured. The rate of dark respiration was significantly higher one day after application of honeydew substitute as compared to the control. Other parameters describing the carbon dioxide - light response were not significantly affected. Fifteen days after honeydew substitute application, the two experimental years yielded different results. Under hot and dry conditions, the rate of dark respiration was higher while the rate of carbon dioxide assimilation at high irradiance, the mesophyll conductance and the leaf nitrogen content were lower than for the untreated control. In addition, chlorotic symptoms were visible. At moderate temperature and relative humidity, the initial light use efficiency was significantly lower fifteen days after treatment with honeydew substitute whereas other parameters were not significantly affected. In field experiments at two locations natural aphid populations consisting of mainly S. avenae were treated weekly with an aphicide from crop development stages DC 71, 73, 75 and 77, respectively, to create infestations of different size and duration. In the unsprayed controls densities reached 15.8 and 44.4 aphids per tiller, amounting to 182 and 544 aphid-days, respectively. Crop samples were analysed weekly, starting at flowering. At the smaller natural infestation, no significant effect on any yield component nor on yield was found at any treatment. The larger infestation caused a significant reduction of leaf area index, leaf weight, amount of water-soluble carbohydrates and grain weight on various sample dates when the infestation was not sprayed before DC 75. No effects were found on tiller and grain density nor on nitrogen content of plant parts at harvest.