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Mites of Floriculture Crops – Biology and ControlMites are not insects. They are arachnids, the same class as spiders. Several mites attack greenhouse and field floriculture crops. Economically important species include the two-spotted spider mite, Lewis mite, bulb mite, cyclamen mite, broad mite and false spider mites.
Two-Spotted Spider MiteThe two-spotted spider mite, Tetranychus urticae, is often called the “red spider mite” because of its red colour during the diapause or hibernation stage. Almost all broadleafed plants are susceptible to attack by two-spotted mites. They are known to attack over 300 plant species and thrive under greenhouse conditions. The adult reproductive stage is greenish with two dark feeding spots on the back and is about 0.5 mm long. Females lay an average of 100 eggs. The round, pearly white eggs are too small to be seen with the unaided eye. The newly hatched mites go through a larval and two nymphal stages before becoming adults. (See Figure 1.) The time from egg to adult is dependent mainly on temperature. Numbers build up quickly under hot, dry conditions when it can take as little as 8 days at temperatures of 25 to 35°C. Diapause is induced by decreasing temperatures and day length, causing the now reddish mites to seek out cracks and crevices to stay in until more favourable conditions in the spring. This stage cannot be controlled by pesticides.
Damage Scouting
Management
Biological Control Predatory mites are most effective at relative humidities greater than 70% and temperatures of 20 to 30°C. Very hot, bright conditions favour spider mites and can result in pest outbreaks. Ideally the plants should be in close contact, allowing the predators to move easily through the crop. They are not affected by day length and will remain on the crop as long as pest mites are present. Unlike some predacious mites, they only feed on mites and do not eat pollen. They will not survive between crops in an empty greenhouse and will disappear from the greenhouse if mites are not present on the crop. The most successful biological control programs are based on pre-releasing predators monthly or even twice a month on the assumption that the crop is likely to become infested. Introduction rates range from 2 to 20 per plant, depending on infestation levels and the crop involved. Suppliers can recommend more precise rates after inspecting the crop. The most effective and economical introduction system is to apply P. persimilis at the first sign of spider mites or their feeding (stippling on leaves) at a rate of two per infested plant and to every fifth plant in the rest of the crop. Subsequently treat any “hot-spots” that develop. Releasing some predatory mites into areas outside outbreaks encourages them to disperse to look for food. At very high spider mite levels it is often advisable to apply insecticidal soap or other non-residual miticide. For field crops, Neoseiulus fallacis, a predatory mite of temperate origin, is promising for two-spotted mite control but has not been fully evaluated. A naturally occurring small, black ladybug beetle, known as the spider mite destroyer (Stethorus spp.), is also being developed as a commercial biological mite control agent for both greenhouse and outdoor crops. When using beneficials in a crop, it is often necessary to apply pesticides for other insects or diseases. Select chemicals that will have the least negative impact on the biological agents. (Refer to Table 1.5, page 15 in the 2004 Floriculture Production Guide.) If possible, treat only hot-spots or spray only the part of the crop that is primarily under attack, such as the upper parts of roses for aphid control. Chemical Control Two-spotted spider mites breed rapidly, so the potential for resistance development is high. The primary method of minimizing the development of resistant mite populations is to avoid using the same miticide for more than three consecutive sprays. Tank-mixing pesticides with different modes of action is also thought to promote resistance. Because spider mites are very small, relatively immobile, and occur mainly on leaf undersurfaces, thorough spray coverage is essential for adequate control. Most pesticides will not kill the egg stage, so at least two applications are necessary. A spreader-sticker or wetting agent will improve the effectiveness of most miticides, particularly on waxy leafed plants. However, that may increase the possibility of phytotoxicity; do a test spray before doing any large-scale applications. Product labels have information on the use of adjuvants.
Table 1 lists currently registered products for mite control on commercial
flower crops. It also lists mode of action, residual activity, and the
target life cycle stage for each miticide, three factors to consider when deciding which
miticide to use. Check labels for
specific crop registrations and possible phytotoxicity. (Post this table in
your pesticide mixing area for easy reference.) Lewis Mite
Lewis mite is not a common pest in BC, but poinsettia growers should be vigilant for it. Plants that appear to loose colour or have bleached foliage should be closely inspected for Lewis mites. Scouting for Lewis mites is especially important if you have applied Impower 60 WP (imidacloprid) because while it controls aphids and white flies, it does not control spider mites. Scouting, management, and control methods for Lewis mites are similar to those for two-spotted mites. Bulb MitesBulb mites, Rhyzoglyphus spp., are slow moving and relatively large (0.5 to 0.9 mm long). They are pearly-white and smooth, with short reddish legs. From their origin in Europe, bulb mites have spread throughout the world presumably via shipments of infested bulbs. Bulb mites occur as large colonies, not as individuals. Females produce at least 100 eggs that are deposited near injured or decaying tissue. The time from egg to adult can be as short as 12 days at 25°C. Factors affecting development time are temperature, humidity, and host species. DamageBulb mites are usually considered secondary pests of bulbs, but can be responsible for serious loses. They invade bulbs at points of mechanical injury and, once established, they spread rapidly and destroy large areas of the bulb. In addition to feeding damage, the mites provide access to fungal root pathogens, such as Pythium, Rhizoctonia, and Fusarium. Infested bulbs may rot or produce stunted, distorted, or off-colour plants. Host crops include dahlia, freesia, gladiolus, hyacinth, iris, narcissus, orchid, and tulip. Vegetable bulbs can also be infested. Scouting Bulbs with apparent physical damage or rotted spots should be examined for bulb mites. They are easily seen with a low power hand lens. Management Careful handling of bulbs to avoid the damage that provides access to mites is important. In storage, maintaining low relative humidities will reduce the potential for mite infestations. Steam pasteurization will eliminate bulb mites from soil. Biological Control A predacious mite, Cosmolaelopss claviger, readily feeds on bulb mites, but is not commercially available. There is evidence that Hypoaspis mites, predatory soil-dwelling mites sold for fungus gnat control, may feed on bulb mites, but not to the extent that control is achieved. Chemical Control Consult with floriculture extension staff for current recommendations. Cyclamen MiteThe cyclamen mite, Stenotarsonemus pallidus, is tiny (0.13mm) and colourless. It does best at cooler temperatures and requires high humidity. Under optimal conditions the time from egg to adult is about 10 to 14 days. DamageSymptoms of cyclamen mite feeding often resemble disease symptoms. Cyclamen mites feed on all parts of the plant, but young foliage is most often affected. These mites avoid light, so they feed mainly on unopened leaflets and buds, resulting in wrinkled, deformed leaves and buds that do not open or produce distorted blooms. New growth is deformed and distorted. Leaves may be thickened, strap-like, and reduced in size. As leaves unfurl they may show signs of scarring or bronzing. Favoured host plants include African violet, azalea, begonia, chrysanthemum, cyclamen, dahlia, delphinium, exacum, fuchsia, gerbera, geranium, gloxinia, kalanchoe, New Guinea impatiens, and snapdragon. Scouting Recognition of the damage caused by these very small mites is the best way to determine the presence of an infestation. Especially characteristic are plants that have ceased blooming and have shortened internodes, distorted leaves and stems, and blasted buds. Infestations tend to be localized. Dissecting tissue under a microscope may be necessary to confirm the presence of cyclamen mites. Management If only a few plants are affected, the best way to deal with the problem is to discard them. Workers should be careful not to spread mites through-out the greenhouse on their hands or clothing. Cyclamen mites on nonblooming plants can be killed by immersion, pot and all, for 15 minutes in water heated to 43°C. Decreasing humidity can also reduce infestation levels. Biological Control No reliable biological controls have been identified for cyclamen mites, although the predatory mite Amblyseius cucumeris is known to feed on tarsonomid mites and may contribute to their control. Chemical Control Two or 3 applications at 5 to 7 day intervals of dicofol (Kelthane) or endosulfan (Thiodan) will control cyclamen mites. Use sufficient spray volume and pressure to thoroughly cover all surfaces of the plants. Several applications at weekly intervals are required because all stages are usually present and are generally well protected from sprays. Broad MitesBroad mites, Plyphagotarsonemus latus, are very small (0.1-0.2mm) and colorless. Under optimal conditions the time from egg to adult is about 5 days. Damage:Broad mites attack a wide range of commercially important plants, including gerbera, African violets, cyclamen, begonias, impatiens, verbena and gloxinia. Broad mite damage is usually expressed as distorted and downward curling leaves resulting from a toxin secreted by the feeding mites. As well, internodes and petioles of flowers become shortened, and flowers may fail to open. Severely infested plants become stunted and may die. Scouting: Broad mites are too small to be seen with a hand lens. A microscope is needed to confirm their presence. Broad mites prefer high relative humidity and therefore hide in growing points, flower buds, and overlapping surfaces of developing leaves. Although similar in appearance to cyclamen mites, broad mites can be differentiated by the whitish bumps on their eggs compared to the smooth eggs of cyclamen mites. Management: Control of this mite is difficult. Ideally infested plants should be identified quickly and carefully removed from the greenhouse before the mites spread. Imported cuttings should be examined to prevent introduction into a non-infested greenhouse. Growers with broad mite problems one year should be especially vigilant the next spring for the possibility of a carry-over population.
Biological control: Sprays applied to control spider mites will result in broad mite control. A power sprayer is necessary to achieve adequate penetration. Repeated applications at five day intervals may be required in warm weather. False Spider MiteFalse spider mites or flat mites (Family Tenuipalpidae) are flattened, very small, reddish, and slow moving. The main pest species are in the genus Brevipalpus. Adult false spider mites are about 0.3 mm long. The eggs are red, somewhat flattened, and take up to 3 weeks to hatch. The immature stages feed and develop for 5 to 6 weeks before becoming adults. The life cycle is about 5 times longer than the two-spotted mite life cycle. Brevipalpus species attack a wide variety of greenhouse plants, but are most commonly found on orchids. DamageFalse spider mites are not common in BC greenhouses. When present, they feed first along the midribs of leaves and then disperse outwards. They puncture the epidermis of the leaf and suck out the plant juices. This results in the leaves having first a mottled and later a silvery appearance. Under conditions of severe infestations, plant tissue turns brown and dies. Stems are also attacked. False spider mites do not produce webbing.
Biological Control Pesticides that control two-spotted mites will also control false spider mites. Because they feed on the undersides of the leaves, the pesticide must be directed upwards for best control. Table 1. Miticides for Ornamental Crops*
Ministry of Agriculture and Lands Revised October 2006 View this page in printable PDF format Return to Pest Management Home
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The Lewis mite, Eotetranychus lewisi, is primarily a pest of poinsettias in
BC. Lewis mites are similar in appearance to two-spotted mites, but are
slightly smaller and have several small spots rather than two large spots
when viewed under a microscope. (See Figure 2.) The damage they inflict on
crops is similar to that of two-spotted spider mites. They feed on the undersurfaces of leaves, piercing the epidermis and removing cell contents.
This results in a stippled appearance on the upper leaf surface. Eventually
the entire leaf becomes bleached and falls off. Heavy infestations will
produce webbing, but not as extensive as that of two-spotted spider mites.
Lewis mites do not enter a diapause or hibernation stage. The developmental time from egg to adult is 12 to 14 days at 21oC.
Females oviposit 2 to 3 eggs per day for about 30 days.