Greenhouse and Nursery - Managing Spider Mites

The following is a good article from the New England Greenhouse Update on managing spider mites in greenhouses. It would also apply to perennial nursery production under protection.

Watch for two-spotted spider mites on ipomoea, New Guinea impatiens and other crops. Mites may come in on incoming plants or have over-wintered in your greenhouse. Look for dull stippled foliage on plants particularly in warm, dry locations in your greenhouses such as near steam pipes, furnaces, heaters or overhead hangers. Use a 10x handlens and look on the underside of mature leaves, especially along the midvein for eggs, immature stages and adults. Note that young nymphs do not have the dark two spots.

Contact or translaminar miticides can be used to manage two-spotted mites. Translaminar means that the material penetrates leaf tissues and forms a reservoir of active ingredient within the leaf which provides extended residual activity. Miticides with translaminar activity include abamectin (Avid), etoxazole (TetraSan), chlorfenapyr (Pylon) and spiromesifen (Judo). After treatment, mark several plants and use a 10X hand lens to look for live and dead mites and eggs. Most miticides are not effective against the egg stage, so repeat applications may be needed in 5-7 days. Thorough coverage is important for contact activity. Insecticidal soaps and horticultural oils are also effective. Consult label for plant safety. Go back and check plants within a few days to see how effective the treatment worked. You will hopefully see dead and dying mites, but you may also see eggs. Continue to monitor and repeat treatments as needed.

Biological ControlA fast acting predatory mite that is commercially available is Phytoseiulus persimilis. This predatory mite only feeds upon spider mites, and will disperse or starve with no prey. The adult P. persimilis is bright red in color, pear shaped, long-legged and slightly larger and more active than spider mites.

P. persimilis is best released when mite populations are first noticed, in hot spots of mite activity and adjacent areas. Relative humidity should be greater than 75% and temperatures above 68F for some hours of the day. (At low relative humidity, less than 60%, eggs shrivel and do not hatch.). According to Raymond Cloyd, University of Kansas, P.persimilis is suitable for short-term crops such as bedding plants at release rates of 1-4 mites per ft² per week. Two applications, one week apart may be required. Spider mite colonies should be reduced in two to three weeks. To scout, shake plants over white paper and observe mites. Pest control materials that have been shown to be compatible with P. persimilus include spinosad (Conserve), pymetrozine (Endeavor) and clofentezine (Ovation). Bifenazate (Floramite), spromesifen (Judo) and chlorfenapyr (Pylon) may be harmful.

The spider mite predator Neoseilus californicus is slower acting than P. persimilis, but can survive longer in the absence of prey. It is useful for keeping low spider mite populations under control. In certain situations where high temperature or relative humidity variations can occur, N. californicus may be an option. N. californicus is active at temperatures between 46°F to 95°F, 40-80% relative humidity. At low pest densities, it declines less than P. persmilis, for N. californicus can survive on other mites, thrips, molds and nectar. N. californicus can also be introduced preventively and is compatible with P. persimilis. Some suppliers offer a mix of different species of predatory mites.

Reprinted from the New England Greenhouse Update http://www.negreenhouseupdate.info/greenhouse_update/index.php