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Container of Neoseiulus californicus,a predatory mite.

Phytoseiulus macropilis. This mite has been investigated for greenhouse use on dieffenbachia,dracena, parlor palm (Chamaedorea elegans), and schefflera (Brassaia actinophylla).The globose, light- to deep-red females 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 during its development and an average of eight eggs per day as an adult.P.macropilis has a very 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.

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 V25-inch (1 mm) adults are a 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 3 days into nonfeeding, 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 in color and oval in shape.The optimum temperature range for development is 70°-81°F. Development ceases and oviposition and longevity decline sharply at relative humidities below 60%. Under warm and humid conditions, the adults of these predators consume 30 eggs or 24 immature spider mites per day. At favorable temperatures and humidity this predatory mite can develop twice as fast as its prey. Both pesticide-susceptible and organophos-phate-resistant strains of this predatory mite are available commercially.

Other predators Feltiella acarisuga (=Therodiplosis persicae). This predatory gall midge occurs throughout Eurasia and North America. It feeds on all stages of spider mites, but generally prefers the eggs or larvae. Adult gall midges are small, delicate, pink-brown flies with long legs. They do not feed and only live 3-4 days after emergence from the cocoon. Each female lays an average of 30 yellow eggs near high densities of mites, usually where webbing occurs.The yellow- or orange-brown midge larvae grow to about 1/16 inch (2 mm) long.They can consume over 300 mite eggs as they complete their development in about a week in the greenhouse.They then spin fluffy white cocoons on the underside of leaves, usually along a leaf vein.This midge is commercially available.There are several closely related species that are also good mite predators, including F.minuta,a common predator in British Columbia.

Macrolophus nubilis. This mirid bug is a voracious predator of spider mites in Europe. It was a promising candidate in trials in greenhouses in Uzbekistan, but it is not commercially available. A related species,M.caliginosus, is offered commercially as a predator of whiteflies.

Scolothrips sexmaculatus—sixspotted thrips. Both the adult and larval stages of this predatory thrips feed on spider mites.The tiny, slender, pale amber adults have three dark spots on each forewing. Females deposit eggs in soft plant tissue.The larvae that hatch are white or yellow, without dark markings. The robust, yellow pupae occur on the leaf surface in the mite colonies. S.sex-maculatus prefers spider mite eggs, but adult females will consume other mite stages.The rate of predation depends on temperature. Adult females consume 50 spider mite eggs per day at 86°F but only 21 eggs per day at 68°F. It is commercially available.

Phytoseiulus persimilis is the most commonly released predatory mite used in greenhouses.

Stethorus spp. These spider mite predators are small black lady beetles. Both the larval and adult stages are preda-ceous on all mite stages, but their use in greenhouses has not been evaluated. S. punctillum,a predator of European red mite—a pest of fruit trees—is commercially available.

Possibilities for effective biological control

In Europe, predatory mites have been used successfully for several decades to manage spider mites in greenhouse vegetable production. Predatory mites have 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. Phytoseiulus per-similis is the most commonly available and most commonly released predatory mite in greenhouses. However, 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 also does not do well on tomatoes because the mites become trapped on glandular hairs on the leaf petioles and stems, and they are affected by toxic compounds in the leaf. Other predatory mite species listed in the "Natural Enemies"section also provide good control. Galendromus occi-dentalis and Neoseiulus californicus,for example, may be better suited for use on semi-permanent greenhouse crops such as rose or gardenia than on short-term vegetable crops.These predatory mites can often be used in conjunction with other predators as well.

Long-term control may result from a single inoculative release of these predators, especially if nondiapausing, insecticide-resistant strains are used. However, chemical control with selective miticides may be required during establishment and occasionally thereafter. If there is an average of more than one spider mite per leaf, a chemical spray with low toxicity to predatory mites, such as insec-ticidal soap or horticultural oil, should be applied to reduce the spider mites to less than 10% leaf infestation. Do not use residual pesticides within a month of releasing predatory mites, including sulfur-containing fungicides, because they are highly toxic to mites.

Predatory mites are most effective when introduced while spider mite populations are low, ideally at or before the first sign of spider mite damage. In greenhouses with a history of spider mite problems, the first releases should be made 1 week after plant emergence. If you are using indicator plants, release the predatory mites as soon as those plants show the first visible signs of damage. Proper timing is essential to achieve economic control, especially for carmine spider mites, which cause extensive plant damage at low populations. Mite damage and reproduction can continue for 1-3 weeks before the predators can destroy the mites. Most failures of biological control occur when the predator is released too late.

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. P.macropilis performs better than P.per-similis on ornamental plants under warm, humid conditions. Mesoseiulus longipes is frequently used to control spider mites in hot, dry greenhouses on taller plants. It tolerates lower humidity than does P.persimilis. N.californicus does well on most potted plants in greenhouses with moderate temperatures and average humidity. 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 cannot 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 that leaves touch may improve biological control on some plant species.

On average, one predator for every five twospotted spider mites, or a rate of 10 predators per ft2,should be more than adequate for control. Other predator-to-pest ratios that have been successful under specific circumstances are 1:6, 1:10, and 1:25, but specific ratios can only be determined by experience. In greenhouses a rate of two predators per ft2 (or two per plant if the plants are small) is suggested. On vegetables, a predator-to-prey ratio of 1:50 may be sufficient, but on ornamental floral or foliage plants that cannot sustain any aesthetic injury the ratio may have to be reduced to 1:10 or 1:5. In the United Kingdom, one predator for every 10 chrysanthemum cuttings released 2-3 weeks after planting, before damage is apparent, has given satisfactory control. For the carmine spider mite, a rate of 50 or more predators per plant is suggested because of the potential for rapid plant damage at low pest populations.

Predatory mites are usually released according to one of three distributional patterns: uniformly, in infested patches, or from banker plants. 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. 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. (Be sure to shake the container occasionally to distribute the mites throughout the container evenly.) However, treating infested plants only requires regular inspection of all plants until predators are well established, which may be commercially impractical. If indicator plants are used to detect the first infestation, you can release the predators on the indicator plants and allow them to move out into the crop.

Inundation of a crop with predators at extremely high rates can also reduce pest populations—effectively and quickly—but this method may be prohibitively expensive. Another successful technique uses "banker"cucumber plants at either end of the greenhouse. The plants are intentionally infested with spider mites and predators are released on them when damage begins to show.Within a month thousands of predators are available for release on the main crop. On tomatoes, predators can be moved to hot spots by workers during harvesting operations.This may not be practical in large operations.

The predatory gall midge Feltiella can also be used for spider mite control in conjunction with P.persimilis.The predatory mite has low dispersal ability, so it may fail to find patches of high pest density.Feltiella is good at finding hot spots, so the two predators are complementary. Feltiella is particularly useful on hairy-leaved plants (such as tomatoes) that P.persimilis does not work well on, and it can also effectively control carmine spider mite. Even though Feltiella larvae eat at least five times as many spider mites per day as does Phytoseiulus, the midge alone is usually not able to control spider mites.

Spider mites

Enemies

Predatory mites

nymph -

adult nymph -

Feltiella

Sixspotted thrips

Stethorus adult egg

The midges are shipped as cocoons on leaves or the carton.The carton should be opened and placed on the soil surface of plants in the greenhouse, protected from direct sunlight, for at least 1 week. Adults emerging from the cocoons in the container will fly to spider mite colonies. High humidity improves midge emergence. Optimal conditions for Feltiella are 68-81°F and relative humidity greater than 60%, although larvae can tolerate a wider range of conditions than can the adults. Weekly releases of one per 10 ft2 of plant material are recommended, but because of the high cost of these insects many growers use only one per 40 ft2 and allow the population to build up over time.

Sixspotted thrips appears to be a good predator for use in greenhouses. Unlike predatory mites, thrips can fly away to find new prey. Release rates will vary depending on the crop and mite density. Contact the supplier for recommended rates.

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.

Alternative control methods

Sanitation

Remove any weeds or old plants that may harbor spider mites. Avoid spreading mites on workers'clothing or implements by restricting movement through mite-infested areas. Always visit mite-infested areas last.

Irrigation and fertilization

Maintaining proper soil moisture, and misting or hosing off plants helps prevent mite populations from developing and dispersing to other plants. High nitrogen levels are often associated with severe mite infestations.

Manipulation of the greenhouse environment

High humidity produced by misting suppresses spider mite populations and favors P.persimilis and Feltiella.

Chemical controls

Localized infestations, or high populations that need to be reduced before predator introduction, should be spot-treated with selective chemicals. Integrating chemical control on the upper portion of the canopy with biological control on the lower part of the plant has worked well for commercial rose production.

There is good potential for the integration of predatory mites and insecti-cidal soap applications to control spider mites on ornamental foliage plants. Insecticidal soap controls spider mites, but when wet it also kills predatory mites and other natural enemies. It has a slight effect on spider mite eggs but does not affect predator eggs. Once insecticidal soap dries, it is nontoxic.Thorough coverage of infested surfaces and several sequential applications are necessary to maintain spider mite populations at low levels.

Horticultural oils, both refined petroleum distillate products and those made from vegetable oils, can kill mites.These horticultural oils may be toxic to some plants.Various brands are registered for use on vegetables and ornamentals in greenhouses to control many pests, including spider mites.

Abamectin is an insecticide/miticide derived from the microorganism Streptomyces avermitilis.It is registered for control of spider mites and leafminers on ornamental plants in greenhouses.

Spinosad is an insecticide based on natural metabolites produced under fermentation conditions by the actin-omycete Saccharopolyspora spinosa. The product is fast acting and works both on contact and when ingested. It is very effective against thrips; control of spider mites is inconsistent, but is improved with the addition of certain adjuvants. It is registered for control of many pests on landscape ornamentals; check with your chemical supplier on availability for use on greenhouse ornamentals.

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