Spider Mites Fact Sheet No. 5.507 Insect Series|Home and Garden by W.S. Cranshaw and D.C. Sclar* Spider mites are common pest problems Quick Facts on many plants around yards and gardens in Colorado. Injury is caused as they feed, • Spider mites are common plant bruising the cells with their small, whiplike pests. Symptoms of injury mouthparts and ingesting the sap. Damaged include flecking, discoloration areas typically appear marked with many (bronzing) and scorching of small, light flecks, giving the plant a leaves. Injury can lead to leaf somewhat speckled appearance. loss and even plant death. Following severe infestations, leaves become discolored, producing an unthrifty • Natural enemies include small gray or bronze look to the plant. Leaves and lady beetles, predatory mites, needles may ultimately become scorched and minute pirate bugs, big-eyed drop prematurely. Spider mites frequently kill bugs and predatory thrips. plants or cause serious stress to them. Spider mites are classed as a type of • One reason that spider arachnid, relatives of insects that also includes mites become a problem spiders, ticks, daddy-longlegs and scorpions. is insecticides that kill their Spider mites are small and often difficult to Figure 1: Honeylocust spider mites, with eggs. natural predators. see with the unaided eye. Their colors range • Irrigation and moisture from red and brown to yellow and green, management can be depending on the species of spider mite and seasonal changes in their appearance. important cultural controls for Many spider mites produce webbing, spider mites. particularly when they occur in high populations. This webbing gives the mites and their eggs some protection from natural enemies and environmental fluctuations. Webbing produced by spiders, as well as fluff produced by cottonwoods, often is confused with the webbing of spider mites. The most important spider mite is the twospotted spider mite (Tetranychus urticae). This mite attacks a wide range of garden plants, including many vegetables (e.g., beans, eggplant), fruits (e.g., raspberries, currants, pear) and flowers. The twospotted spider mite is also the most important species on house plants. It is a prolific producer of webbing. Evergreens tend to host other mites, notably the spruce spider mite (Oligonychus ununguis) on spruce and juniper, Oligonychus © Colorado State University Figure 2: Twospotted spider mites, with eggs. Extension. 12/98. Revised 7/14. www.ext.colostate.edu *W.S. Cranshaw, Colorado State University Extension entomologist and professor, and D.C. Sclar, former research assistant; bioagricultural sciences and pest management. 7/2014 Table 1: Pesticides useful to control spider mites in yards and gardens.
Active Ingredient Trade Name(s) Comments acephate Orthene, certain Insecticide with some effectiveness against spider mites. Systemic. Isotox formulations abamectin Avid For commercial use only on ornamental plants. Primarily effective against twospotted spider mite; less effective against mites on conifers. Limited systemic movement. bifenthrin Talstar, others Insecticide with good miticide activity. hexythiazox Hexygon For commercial use only on ornamental plants. Selective miticide that affects developing stages and eggs only. One application per season label restriction. horticultural oils Sunspray, others Used at the “summer oil” rate (2 percent), oils are perhaps the most effective miticide available for home use. insecticidal soap several Marginally effective against twospotted spider mite and where webbing prevents penetration. Broadly labeled. spiromesifan Forbid For commercial use only on ornamental plants. Selective against mites and conserves natural enemies. sulfur various Generally sold in dust formulation for control of various fungal diseases and some mites on some ornamental and vegetable crops. subnudus on pines, and Platytetranychus rapidly during this time, becoming full- libocedri on arborvitae and juniper. grown in as little as a week after eggs hatch. Honeylocust, particularly those in drier After mating, mature females may produce sites, are almost invariably infested with the a dozen eggs daily for a couple of weeks. honeylocust spider mite (Platytetranychus The fast development rate and high egg multidigituli). Other mites may affect shade production can lead to extremely rapid trees such as elm, mountain ash and oak. increases in mite populations. Another complex of mites is associated Other species of spider mites are most with turfgrass, including the clover mite active during the cooler periods of the and Banks grass mite. These are discussed growing season, in spring and fall. This separately in fact sheet 5.505, Clover and includes the spruce spider mite and most Figure 3: Twospotted spider mite injury to eggplant. Other Mites of Turfgrass. Clover mites of the mites that can damage turfgrass. also are the common mite that enters These cool-season spider mites may cease homes in fall and spring, sometimes development and produce dormant eggs to creating significant nuisance problems in survive hot summer weather. the process. Dry conditions greatly favor all spider mites, an important reason why they are so important in the more arid areas of Life History and Habits the country. They feed more under dry Spider mites develop from eggs, which conditions, as the lower humidity allows usually are laid near the veins of leaves them to evaporate excess water they excrete. during the growing season. Most spider At the same time, most of their natural mite eggs are round and extremely large enemies require more humid conditions in proportion to the size of the mother. and are stressed by arid conditions. After egg hatch, the old egg shells remain Furthermore, plants stressed by drought and can be useful in diagnosing spider can produce changes in their chemistry that mite problems. make them more nutritious to spider mites. Figure 4: “Spider mite destroyer” lady beetle. There is some variation in the habits of the different mites that attack garden plants, trees and shrubs. Outdoors, the Control twospotted spider mite and honeylocust Biological Controls spider mite survive winter as adults hidden Various insects and predatory mites in protected areas such as bark cracks, bud feed on spider mites and provide a high scales or under debris around the garden. level of natural control. One group of Other mites survive the cool season in small, dark-colored lady beetles known the egg stage. As winter approaches, most as the “spider mite destroyers” (Stethorus mites change color, often turning more red species) are specialized predators of spider or orange. This habit may be why they are mites. Minute pirate bugs, big-eyed bugs sometimes called “red spiders.” (Geocoris species) and predatory thrips can Most spider mite activity peaks during be important natural enemies. the warmer months. They can develop Figure 5: Minute pirate bug. A great many mites in the family may delay egg laying until new webbing is Phytoseiidae are predators of spider mites. produced. Sometimes, small changes where In addition to those that occur naturally, mite-susceptible plants are located or how some of these are produced in commercial they are watered can greatly influence their insectaries for release as biological controls. susceptibility to spider mite damage. Among those most commonly sold via mail order are Galendromus occidentalis, Chemical Controls Phytoseiulus persimilis, Mesoseiulus longipes Chemical control of spider mites and Neoseiulus californicus. Although generally involves pesticides that are these have been successful in control of specifically developed for spider mite spider mites on interior plants, effective control (miticides or acaricides). Few use outdoors has not been demonstrated in insecticides are effective for spider mites Colorado. Predatory mites often have fairly and many even aggravate problems. high requirements for humidity, which Furthermore, strains of spider mites can be limiting. Most suppliers provide resistant to pesticides frequently develop, information regarding use of the predator making control difficult. Because most mites that they carry. miticides do not affect eggs, a repeat One reason that spider mites become application at an approximately 10- to problems in yards and gardens is the use 14-day interval is usually needed for of insecticides that destroy their natural control. Table 1 includes a summary of enemies. For example, carbaryl (Sevin) pesticides that may be useful for managing devastates most spider mite natural spider mites. enemies and can greatly contribute to spider mite outbreaks. Malathion can Control of Spider Mites aggravate some spider mite problems, on House Plants despite being advertised frequently as Control on house plants can be effective for mite control. Soil applications particularly frustrating. There generally of the systemic insecticide imidacloprid are no biological controls and few effective (Merit, Marathon) have also contributed to chemical controls (primarily soaps and some spider mite outbreaks. horticultural oils). Water Management When attempting control, treat all susceptible house plants at the same time. Adequate watering of plants during Trim, bag and remove heavily infested dry conditions can limit the importance of leaves and discard severely infested plants. drought stress on spider mite outbreaks. Periodically hose small plants in the sink or Periodic hosing of plants with a forceful shower. Wipe leaves of larger plants with a jet of water can physically remove and kill soft, damp cloth. Reapply these treatments many mites, as well as remove the dust that at one- to two-week intervals as long as collects on foliage and interferes with mite populations persist. predators. Disruption of the webbing also
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