Know Your Friends

Predator Mites of Spider Mites in Greenhouses

Phytoseiulus persimilis is the most commonly used predatory mite in greenhouses, but many other species of predatory mites in the family Phytoseiidae feed on spider mites and can provide good control of spider mites in greenhouses.

The globose, light- to deep-red females of P. macropilis lay oval orange eggs that hatch into six-legged larvae. Both larvae and nymphs have a similar white to light orange color. Males are identical to females in shape and color but are smaller. These mites have a strong preference for immature spider mites over adults. Each predator consumes four to six spider mite eggs or larvae daily during its development and an average of eight eggs per day as an adult. P. macropilis has a short life cycle in comparison to many spider mite species, allowing it to build up quickly to suppress pest populations. In the absence of spider mites they will prey on their own immatures. P. macropilis occurs naturally in Florida and is available commercially.

The western predatory mite, Galendromus occidentalis, is smaller than P. persimilis and develops best at cooler temperatures, but it tolerates a wide range of relative humidities (40-80%). It has the capability of regulating spider mite populations at lower densities and for longer periods of time than P. persimilis, although it will not control spider mite populations as quickly as P. persimilis can. It can also survive long periods without prey. Several different strains are commercially available, including non-diapausing strains that allow control of spider mites through the winter, when days are short, and strains resistant to a number of organophosphate insecticides.

The commercially-available Mesoseiulus longipes is similar to P. persimilis in activity, but can tolerate warmer temperatures-up to 70-90F-and relative humidity as low as 40%.

Neoseiulus (=Amblyseius) californicus is smaller, pale, and does not suppress spider mite populations as quickly as P. persimilis. However, it is useful for keeping low populations under control because it can survive longer periods without prey. Some other species of Neoseiulus, such as N. fallacis, feed on a variety of tetranychid mites and are commercially available, but little is known of their utility in greenhouses.

Several phytoseiids in the genus Euseius are commercially available, but these predatory mites have not been evaluated for use on greenhouse crops.

Predatory mites have been used for years to manage spider mites in European vegetable greenhouses. They have also effectively controlled spider mites on chrysanthemum, rose, and other ornamental crops under experimental conditions. However, the need to prevent cosmetic damage on floral or foliage crops may make biological control of mites difficult, especially when pesticides that kill predatory mites are used to suppress other pests and/or diseases. Your spider mite control strategy may depend on the crop you raise and conditions in your greenhouse, especially temperature and humidity.

Species selection and release rates vary considerably depending on the plant species and the environmental conditions such as temperature and humidity which influence the growth rate of both predator and prey. P. persimilis is an excellent predator of spider mites on low growing plants in humid greenhouses with moderate temperatures. There are a few crops on which P. persimilis cannot be used. For example, P. persimilis slips off the stems and leaves of carnations. It does not do well on tomato because the mites become trapped on glandular hairs on the leaf petioles and stems, and are also affected by toxic compounds in the tomato leaf. P. macropilis performs better than P. persimilis on ornamental plants, such as dieffenbachia, dracena, parlor palm, and schefflera, under warm, humid conditions. M. longipes is frequently used to control spider mites in hot, dry greenhouses on taller plants because it tolerates lower humidity than does P. persimilis. N. californicus does well on most potted plants in greenhouses with moderate temperatures and average humidity. G. occidentalis and N. californicus may be better suited for use on semi-permanent greenhouse crops such as rose or gardenia than on short-term vegetable crops. A combination of predators released at regular intervals works best in greenhouses or interior plantscapes with a variety of plants and growing conditions.

Plant density and plant architecture influence the distribution of spider mites on a plant species and the ease with which the predators can find patches of prey. For example, P. persimilis is very efficient on cucumbers that have large leaves and vines that intermingle, but less so on peppers with smaller leaves that don't touch. P. persimilis is also less effective on cut rose varieties with fewer leaves because the mites can't move around as easily on these plants. N. californicus is a better choice for control of spider mites on roses, if introduced early. Arranging plants so their leaves are touching may improve biological control on some plant species.

Predatory mites are most effective when introduced while spider mite populations are low. In greenhouses with a history of spider mite problems, the first releases should be made one week after plant emergence. Most failures of biological control occur when the predator is released too late. Spider mite populations should be monitored by observing foliage of susceptible plants at least weekly. Additional predator releases may be necessary every 2-4 weeks to achieve good control within 6 weeks.

Live predator 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.

Predator mites can be released uniformly throughout the greenhouse, or concentrated in infested patches. Uniform distribution of predators throughout the greenhouse is the most common method of introduction. It provides predictable levels of control. Introduction in patches of mite damage will often result in better control than uniform distribution.

- Susan Mahr, University of Wisconsin - Madison

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