The two-spotted mite (TSSM), Tetranychus urticae, is a common pest in greenhouses. It has a wide host range including many ornamentals, herbs, herbaceous perennials and greenhouse vegetables. This mite can be very difficult to control because of its short generation time, potential for rapid increase and the development of resistance to many commonly used miticides. Growers often overlook spider mites, due to their small size and tendency to feed on the underside of leaves, until outbreaks occur.
Spider mites are more closely related to spiders and ticks than insects. They have eight legs rather than six legs and only two body regions (the head and thorax are fused together). Adult females small, (less than 1/50 of an inch long), soft bodied and oval in shape. Their color varies from greenish or yellowish to slightly orange. Bright orange forms occur when mites are overwintering. Do not confuse this form with the faster moving predatory mite species that are released for biological control. When mature, two-spotted spider mites have two dark spots on either side of their body. Spider mites have a pair of silk glands near their mouthparts enabling them to spin webbing or silk threads.
See photos of two-spotted mite damage
As spider mites insert their stylet-like mouthparts into plant tissue, they suck out plant juices removing chlorophyll. At first, you can see a slight flecking or stippling (chlorotic spot) on the leaves. As mite feeding continues, leaves can turn yellow, bronzed and drop from the plant. When high populations develop, fine webbing is extensive. Spider mites migrate to the young, new growth where they can become easily airborne and blown to new hosts.
Two-spotted spider mites can feed on over 300 different species of plants. Many greenhouse ornamentals are favored hosts including dracaena spikes, ivy geraniums, New Guinea impatiens, garden impatiens, hydrangea, marigolds, poinsettias, vinca vine, roses, verbena and viola. Many herbaceous perennials including hollyhock, monarda, columbine, daylily, butterfly bush, primula, scabiosa, verbena, and salvia are also prone to mites. Herbs such as lemon balm, lemon verbena, lemon grass, oregano and mints can also be favored hosts. Greenhouse vegetables including cucumbers, tomatoes and beans are susceptible to mites. Weeds such as chickweed, oxalis, pigweed and henbit found in and around greenhouses can be sources of continuing infestations.
Biology and Life Cycle
Adult females can live for about one month and lay from 100 to 200 eggs during this period. Mite eggs are small, spherical in shape and are laid singly on the underside of leaves. Eggs hatch in about three days and develop into pale green to light yellow 6-legged larvae. The eight-legged nymphal stages have both a feeding and resting stage. The life cycle from egg to adult can vary from 7 to 21 days depending upon greenhouse temperatures. Mite outbreaks are most severe during hot, dry conditions as optimum temperatures for development are between 85 to 95oF. At these temperatures, two-spotted mites can complete their development in as little as 8 to 12 days. Several generations can develop in a growing season.
Weekly scouting and random plant inspection is needed to detect populations early before feeding damage occurs. Inspect at least 10 plants per 1000 square feet. Monitoring is especially important during the summer months when hot, dry conditions prevail. In the greenhouse, focus on scouting hot and dry locations near furnaces. Check near the doorways and vents, where mites may be blown in from weedy areas outside. Tag pest infested plants as indicator plants to determine the effectiveness of control measures.
Plant inspection is needed to detect all stages of mites. First, look for slight discoloration or flecking. Stippling does not occur on ivy geraniums. Mite injury on ivy geraniums resembles oedema, but the damage spreads to the youngest leaves. Then, turn over leaves, especially the older, more mature leaves and look for the mites that tend to be found along the leaf vein. A 10x hand lens or hands-free magnifier will be needed to detect the eggs and all stages of mites. Look on the underside of the leaves for cast skins and empty eggshells, too. Because mites are easily carried on workers or their clothing, do routine greenhouse tasks and scout in mite-infested areas last.
Do not over-fertilizer plants, as lush growth is attractive to mites. Use resistant cultivars whenever possible. Keep records of which cultivars are more susceptible to mites. Misting will also help to reduce mite populations. However, misting may also lead to the development of foliar diseases.
Outdoors, natural enemies such as predatory mites, thrips, ladybeetles, predatory midges and pathogenic fungi help to keep the spider mite populations low. Several different predatory mites are commercially available. They can drive down an outbreak, but can only prevent leaf damage if they are released when spider mite populations are low. Frequently, a mixture of different species is used, so the natural enemies can work under changing environmental conditions or if prey densities change. Predatory mites are sensitive to many different pesticides. Koppert Biological Systems has compiled a list of pesticides and their effects on natural enemies (see the web site www.koppert.com and click onto "side effects".)
Phytoseiulus persimilis feeds on all stages of the two-spotted mites and is the most commonly used predatory mite in greenhouses. It has been successfully used for a number of years in greenhouse vegetable production. This voracious predator performs best under humid conditions and moderate temperatures. Mesoseiulus longipes tolerates drier conditions than P. persimilis. Neoseilus californicus is lower acting than P. persimilis, but can survive longer in the absence of prey. It is useful for keeping low mite populations under control. A tiny predatory midge larvae, Feltiella acarisuga, also feeds on two-spotted mites. This midge prefers humid conditions and is able to forage on hairy leaves. It is commercially shipped in the pupal stage and hatches into midge adults on arrival. These adults lay eggs near spider mite infestations.
Many miticides work by contact with no systemic activity. When treating with contact materials, thorough coverage is needed to the underside of the leaves where the spider mites are feeding. Resting stages and eggs can be more tolerant to many mitcides so repeated applications are often needed.
Spider mites develop resistance to miticides very rapidly. If possible, spot treat locally infested plants instead of the entire greenhouse. The miticides used in your rotation schedule should have different modes of actions (i.e. come from different pesticide classes and work differently). Follow long-term rotations and all label restrictions in terms of amount and frequency of use. Many of the newer miticides are more selective toward a particular life stage or are more effective when populations are low. Regular monitoring, careful timing and application are needed to get the desired results with more selective miticides. Consult the most recent edition of New England Floricultural Crop Pest Management and Growth Regulation Guide: A Management Guide for Insects, Diseases, Weeds and Growth Regulations for more specific up-to-date recommendations.
Leanne Pundt, Extension Educator, University of Connecticut
Casey, C. 2000. Integrated Pest Management for Bedding Plants. A Scouting and Pest Management Guide. Cornell Cooperative Extension Publication No. 407. 117 pp.
Gill, S. and J. Sanderson. 1998. Ball Identification Guide to Greenhouse Pests and Beneficials. Ball Publishing. Batavia, Ill. 244 pp.
Gilrein, D. 2001 New Miticides for Greenhouse, Nursery and Professional Landscape Use. Plugged In. The Newsletter of the Connecticut Greenhouse Growers Association. Issue 1. Pp. 4-7.
Oetting, R. and J. Chong. 2002. Spider Mites. In Proceedings of the Society of American Florists' 18th Annual Conference on Insect and Disease Management on Ornamentals. San Diego, CA.
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