Twospotted Spider Mite (Acari: Tetranychidae) Population Dynamics and Growth of Euonymus alata ‘Compacta’ in Response to Irrigation Rate

Potted dwarf burning bush, Euonymus alata ‘Compacta’, plants were maintained at trickle irrigation rates of 500, 800, 1,100, or 1,400 ml water per day during most of the 1987 and 1988 growing seasons. Burning bush plants obtained from a nursery arrived in May 1987 infested with small populations of twospotted spider mites, Tetranychus urticae Koch. All plants were treated with carbaryl to suppress predators. Spider mite populations and leaf abscission were monitored weekly in July and August. Plants tested in 1987 were held over the winter and given the same irrigation treatments again in 1988. Water stress to plants held at low irrigation rates in 1987 was manifested as reduced stem diameter of new growth in spring 1988. Burning bush leaf abscission was correlated with T. urticae populations in 1987 (T2 = 0.50-0.73) and 1988 (T2 = 0.15-0.75) but not with irrigation rates (T2 = 0.03-0.09). In 1987, mite population growth was similar on plants at all irrigation rates. However, mite populations in 1988 developed more rapidly on plants receiving 500 and 800 ml of water per day than on plants receiving 1,100 and 1,400 ml of water per day.