Greenhouse - Spider Mites and Biological Controls

Spider mites can be a problem pest in greenhouses and can explode quickly under the right conditions. Miticides can be effective; however, mites can build up resistance to these chemicals with overuse. Biological controls are available for mite control and can be quite effective. The following is an article from the University of Connecticut on spider mites and their control in greenhouses.

The two-spotted spider mite (TSSM), Tetranychus urticae, is a common pest in greenhouses. This mite has a wide host range including many different annuals, herbaceous perennials, greenhouse vegetables and herbs. Its short generation time, and potential for rapid increase has lead to many populations developing resistance to some commonly used miticides.

Adult female two-spotted spider mites can live for about one month. During this time, they may lay from 100 to 200 eggs. Mite eggs are small, spherical in shape and are laid on the underside of leaves. Eggs hatch in about three days and develop into pale green to light-yellow 6-legged larvae. The larvae develop into eight-legged nymphal stages with a feeding and resting stage. Optimum temperatures for mite development are between 85 to 95oF; with a lower threshold for development of 54°F and an upper threshold of 104° F. The life cycle from egg to adult varies depending upon greenhouse temperatures, relative humidity levels, and age and quality of the host plant. For example, on roses at temperatures of 77/95° F (Day/Night), spider mites developed from egg to adult at 8 days, and 50/68°F (Day/Night), mites developed at 28 days.
Female mites are three times more abundant than males. Fertilized adult females produce both males and females. Unfertilized adult females only produce males. Males have only one set of genes, so mutations such as pesticide resistance, are immediately expressed. Incorporating biological control strategies into your pest management program, can help slow down the development of resistance.

Biological control agents are best used preventatively, when pest populations are low. A regular monitoring program is needed for early detection of spider mites, and to insure the success of a biological control program. Weekly scouting and random plant inspections are needed to detect populations early before feeding damage occurs. Carefully inspect plants in hot, dry areas of a greenhouse or where there is no overhead irrigation that wets the foliage that may wash some of the mites off the leaves. Regularly inspect the most susceptible cultivars or species, and look for signs of plant damage. As spider mites insert their stylet-like mouthparts into plant tissue, they suck out plant juices removing the chlorophyll. At first, you see a slight flecking or stippling (chlorotic spot) on the leaves. Thin-leaved plants such as garden impatiens may show injury more quickly than thick-leaved plants such as ivy geraniums. Mite feeding damage on ivy geraniums is also often mistaken for oedema. As spider mite feeding continues, leaves turn yellow, bronzed and drop from the plant. When high mite populations develop, the fine webbing is extensive. Tag pest-infested plants as indicator plants to determine the effectiveness of biological control measures. A 10x to 16x hand lens is helpful to detect all stages of the mites. Because mites are easily carried on workers or their clothing, do routine greenhouse tasks and scout in mite-infested areas at the end of the day.

There are a number of biological control agents that may be incorporated into your pest management program for two-spotted spider mites. These include different species of predatory mites (Phytoseiulus persimilis, a specialist predator and Neoseilus californicus, a more generalist predator), and predatory midges (Feltiella acarisuga). No parasitoids are commercially available. Outdoors, natural enemies such as predatory mites, predatory thrips, predatory ladybeetles (Stethorus punctum), predatory midges, lacewings and pathogenic fungi may help to keep the spider mite populations low.

Reprinted from "Biological Control of Two-Spotted Spider Mites" by Leanne Pundt, Extension Educator, University of Connecticut. For the full article go to http://www.hort.uconn.edu/IPM/greenhs/htms/biocontrol_2spotmites.htm