Abstract
The effect of water-stress on population growth of the European Red Mite (ERM) Panonychus ulmi (Koch) was tested in a plastic shelter using young potted apple trees under three water treatment. Mite abundance was significantly greater with higher water supply. The relationship between plant water status and mite number was non-linear, both treatments with leaf water potential from-30 to about -5 bars bearing the lower mite populations. Over the threshold value of-5 bars, mite number sharply increased four times. A non-linear regression analysis showed that the stomatal conductance explained 49.7% of the variance in mite number. Low leaf surface humidity resulting from reduced leaf transpiration of water-stressed trees may increase mite egg and larval mortalities. Daily leaf surface temperatures on the water-stressed trees averaged 1.5°C higher than on the other trees, but did not result in an increase in mite populations, because of the negative influence of low leaf humidity.
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