There are effective biological controls for spider mites in greenhouses. The following is a short article on the subject.
Spider mites may be introduced into your greenhouses on incoming plant material, or may move from weeds onto your crops. If hot spots of mite activity are detected early by regular scouting, biological control may be an option.
A 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.
It is best released when mite populations are first noticed, in hot spots of mite activity. 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.) Spider mite colonies should be reduced in two to three weeks.
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. Additional species are available that are adapted to outdoor use.
Article by Leanne Pundt, Extension Educator, University of Connecticut in the May 1 edition of the New England Greenhouse Update http://www.negreenhouseupdate.info/greenhouse_update/index.php
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