Greenhouse - Overhead Irrigation Reduces Mite Populations

The following is a report of research on the effect of overhead irrigation on mite populations in greenhouses.

Irrigation and Twospotted Spider Mites.

Research update by George Opit, USDA-ARS GMPRC, Manhattan, KS

For many years the literature has stated that overhead irrigation of greenhouse crops can reduce insect/mite pest populations. Unfortunately, it also can be labor intensive if done by hand, may increase disease incidence, and typically increases water usage. Therefore, does the increased switch away from overhead irrigation by production greenhouse growers also result in potentially higher insect/mite pest pressures? The USDA, in cooperation with the Kansas State Cooperative Research and Extension, recently conducted a study to determine impacts of overhead irrigation vs. systems that do not wet foliage (e.g. drip tubes, flood floor, & ebb & flow benches) on insect/mite pests. Their research specifically compared hand watering vs. drip tube irrigation methods on twospotted spider mite (TSSM) pest populations and beneficial predacious mites (P. persimilis). The crop selected for this study was impatiens ‘Impulse Orange’.

Two studies were conducted to test the effect of the different irrigation strategies, hand-watering and drip-tube, on the populations of TSSM and predacious mites (P. persimilis). The first study the crop was watered 2 times every 3 days, and the second study tested two frequencies – ‘wet’ (the crop was watered 2 times every 2 days) and ‘dry’ (the crop was watered 3 times every 4 days). The environmental conditions were controlled within narrow ranges – the temperatures were maintained at 23-26? C, and the relative humidity was maintained between 34-46%. The number and release of adult female twospotted spider mites and adult female P. persimilis predacious mites on the crop was also tightly controlled.

The first study, in which the crop was watered 2 times every 3 days, indicated that the hand watered plants had far fewer pest mite populations than those plants that were irrigated with drip tubes. In fact, the experiments showed that the hand-watered plants had a 98% reduction in pest mite populations compared to the drip tube irrigated plants. The impact of the irrigation types on the beneficial mite (P. persimilis) populations was also very dramatic. The predatory mite numbers were much lower on the handwatered plants compared to those irrigated with drip tubes. Similar to the twospotted mites, their populations were reduced by greater than 97%. The different irrigation frequencies, ‘wet’ and ‘dry’, applied in the second study had no effect on either the TSSM populations or the P. persimilis mite populations. Each of the frequencies applied provided similar reductions.

The use of both hand watering and the release of P. persimilis mites significantly lowered TSSM pest numbers. The P. persimilis mite was capable of multiplying rapidly under drip tube irrigation. However, hand watering was even more effective than the predatory mites at reducing TSSM. Furthermore, hand watering reduced spider mite related crop damage more successfully than P. persimilis on drip tube irrigated plants. Although the role of P. persimilis as a control for spider mites in hand watered crops is small, they do provide a major contribution for reductions with drip tube irrigated plants. In general, however, greenhouse production areas not using overhead irrigation will typically be more susceptible to spider mite infestation problems.

Greenhouse production sites that normally use drip tube, flood floor, or ebb & flow bench irrigation methods could incorporate overhead watering to suppress local hot spots of twospotted spider mites preceding the release of P. persimilis. Such a strategy would epitomize the true meaning of integrated crop management (ICM).

Reprinted from the Winter 2006 edition of Greenhouse IPM Notes a joint newletter from the Cooperative Extension Services of Rutger and Cornell.