Know Your Friends

Phytoseiulus persimilis

This predatory phytoseiid mite was accidentally brought from Chile to Germany on orchid roots and was subsequently sent to other parts of the world. The 1/32 inch long adults are shiny orange-red and pear-shaped, with long front legs and no spots. Each female produces about 50 football-shaped eggs that are twice the size of twospotted spider mite eggs. The eggs hatch in about three days into non-feeding, six-legged larvae. A day later they molt into eight-legged nymphs that consume 10-12 spider mite eggs or small mites during their development. The immatures are pale salmon and oval in shape. Both adults and immatures move rapidly when disturbed or exposed to bright light. Under optimal conditions, these predators consume 30 eggs or 24 immature spider mites per day. At favorable temperatures and humidity the predatory mites can develop twice as fast as their prey.

Both pesticide-susceptible and organophosphate-resistant strains of this predatory mite are available commercially. The live mites are usually shipped mixed in vermiculite, bran or a similar material to cushion them in transit. The carrier-mite mixture can be sprinkled directly onto the foliage of infested plants and the mites will disperse on their own. Additional releases may be necessary every two to four weeks to achieve good control within four to six weeks. Predatory mites have been used successfully for many years for biological control of spider mites in European greenhouse vegetable production and in some outdoor crops (especially strawberries in California).

Predatory mites have effectively controlled spider mites on chrysanthemum, rose, and other flower crops in greenhouses under experimental conditions. However, the low tolerance for cosmetic damage on floral or foliage crops may make biological control of mites difficult, especially when pesticides that kill mite predators must be used to suppress other pests and diseases.

Phytoseiulus persimilis is the most commonly available and most commonly released predatory mite in greenhouses. Other predatory mite species also provide good control of spider mites; the species most suited to your environmental conditions should be selected.

Release rates for predatory mites vary considerably depending on the species of spider mite, plant species, and conditions such as temperature and humidity that influence the growth rate of both predator and prey.

Predatory mites should be introduced at or before the first sign of spider mite damage. Mite damage and reproduction can continue for one to three weeks before the predators can destroy the mites. Most control failures occur when the predator was released too late. Predators are most effective when introduced while spider mite populations are low.

To enhance control by predatory mites in greenhouses, temperature and relative humidity should be adjusted to favor the predators. P. persimilis is most effective at 68-82F (although more heat tolerant strains are being investigated in Australia and Israel) and 60-90% relative humidity. Development ceases and oviposition and longevity decline sharply at relative humidities below 60%. Because they avoid areas of high temperature, poor control may result on parts of plants in bright hot sunlight that favor spider mites. Misting the plants once the predators have been released will aid in spider mite suppression without dislodging predators like hosing down the foliage would. P. persimilis will also benefit from the increased humidity from misting.

- Susan Mahr, University of Wisconsin- Madison

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