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Greenhouse Updates: Aug 18, 2015

Two-spotted Spider Mites Cleanup
August 18, 2015

It is important to clean up two-spotted mite infestations prior to fall to minimize infestations next spring.

Two-spotted spider mites (TSSM) multiply quickly during the hot, dry days of summer and are common on greenhouse tomatoes, herbaceous perennials, weeds and many other hosts.  It takes about 28 days to develop from egg to adult at cool temperatures (50-68°F) but only about 8 days at 77-95°F.

To monitor, look on the underside of the oldest leaves along the mid-veins for the mobile mites, their eggs and white empty eggshells. You may see faint stippling, or chlorotic growth that resembles a nitrogen deficiency. During outbreaks, webbing can be seen or leaves may become bronzed. Often infestations begin on a few isolated plants.

Do not allow infested weeds, unsold plants and greenhouse crops to remain in the greenhouse to harbor spider mites and other pests. When removing infested plant material from greenhouses (at the end of the day, so you do not spread the mites to healthy crops), be sure that cull piles are located far away from the greenhouses.

Two-spotted spider mites enter hibernation (diapause) in the fall when day-length shortens and evening temperatures drop. During diapause, the spider mites change color, turning orange to orange-red and walk off plants to hide in cracks and crevices in the greenhouse.  (Do not confuse this resting stage with the beneficial predatory mite, Phytoseiulus persimilis, which is a bright orange color without the two dark spots.)  Diapausing mites are less susceptible to chemical pesticides and feed very little before leaving the host plant in search of hibernation or resting sites. They tend to move downwards towards the floor and away from light. Diapausing mites are more difficult for predatory mites to find because they hide in concealed places and since they do not feed, they do not release attractants that predatory mites use to find spider mites.  As soon as temperatures are favorable in the spring, they slowly come out of diapause and move to the nearest plants. This is why you may have a mite problem in the same location each year.

Contact or translaminar miticides can be used to manage two-spotted mites. Translaminar miticides penetrate leaf tissues and form a reservoir of active ingredient within the leaf which provides extended residual activity. Miticides with translaminar activity include abamectin (Avid, and other generics), etoxazole (TetraSan), chlorfenapyr (Pylon), and spiromesifen (Judo). After treatment, mark several plants and use a 10 to 20X 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 materials with contact activity. Contacts such as bifenazate (Floramite), cyflumetofen (Sultan), SuffOil-X (other horticultural oils), neem oil (Triact 70) and insecticidal soaps can also be used.  Consult label for information on plant safety.  If it is too hot to spray, for a few isolated problem plants or areas, a forceful jet of water can help dislodge the spider mites.

Check mite infested plants within a few days to see how well the treatment worked.  Look for dead and dying mites, and mite eggs. Continue to monitor and repeat treatments as needed.

Note that biological control should be released early when mite populations are low.   

Phytoseiulus persimilis is a fast acting predatory mite that 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 localized hot spots of mite activity. Relative humidity should be greater than 75% and temperatures above 68F for some hours of the day.  At relative humidity less than 60%, eggs shrivel and do not hatch. At temperatures above 86°F, P. persimilis can’t keep up with the reproduction of two spotted spider mites.   

The spider mite predator Neoseilus californicus is slower acting than P. persimilis, but can survive longer at low pest densities by surviving on other mites, thrips, molds and nectar. N. californicus can also be introduced preventively and is compatible with P. persimilis.

Amblyseius andersonii is another generalist predatory mite that feeds upon mites (two-spotted, broad and cyclamen) and may survive on thrips and fungal spores in the absence of mites. It also can be released in the presence of low populations of spider mites. If hot spots develop, P. persimilis can also be used with this species. A. andersonii can be applied to both greenhouse and outdoor crops and is active at temperatures between 42°F and 104°F.

The predatory midge, Feltiella acarisuga, larvae feed on all stages of spider mites. Adults can fly so are helpful in locating spider mites throughout a greenhouse. This midge prefers temperatures between 68° and 80°F and 80% relative humidity. F. acarisuga is also more effective than P. persimilis on greenhouse tomatoes.

Fact Sheets
Two-Spotted Spider Mites (UMass Extension)
Managing Two-Spotted Spider Mites in the Greenhouse (UConn Extension)
Biological Control of Two-Spotted Spider Mites (UConn Extension)

Photos
Close up of mites and eggs, mite damage on various crops (scroll down a little)

Resources
New England Greenhouse Floriculture Guide. The guide can be ordered from the University of Connecticut or University of Massachusetts.
New England Vegetable Guide: Greenhouse Tomatoes – Insect Section: http://nevegetable.org/crops/insect-control-25

Ferguson G. Impact of Fall Conditions on Spider Mites and Implications, Ontario Ministry of Agriculture, Food and Rural Affairs

Tina Smith, UMass Extension and Leanne Pundt, UConn Extension