Effects of cultivar and irrigation management on population growth of the twospotted spider mite Tetranychus urticae on greenhouse ivy geranium.

Greenhouse experiments were conducted to determine whether different ivy geranium cultivars and different irrigation frequencies influenced population growth of the twospotted spider mite (TSM), Tetranychus urticae. Results showed that mite numbers at harvest (about 4 weeks after plants were artificially infested) were very low on 'Sybil Holmes', but significantly higher on 'Amethyst 96'. Irrigation frequency of ivy geraniums had no measurable effect on TSM populations over a range in which water was applied when pot weight dropped 15, 30 or 45% from container capacity. However, the most frequently irrigated plants (15% treatment) produced as much or more growth than plants that received less water. These plants also had the lowest incidence of oedema. There was no significant interaction between water management and host plant resistance, implying the difference in host plant resistance to TSM between the two cultivars should remain constant, at least within the range of the irrigation frequencies studied. Our findings suggest that growers can take advantage of plant resistance to TSM on ivy geranium cultivars of current commercial importance, and that watering practices can be adjusted to optimize plant production without affecting pest populations.