Two-spotted spider mites (TSSM) can develop quickly during hot dry weather like we have been having recently. Although we have not seen outbreaks yet this season, it is important to monitor for mites and treat when populations are low if using biological controls. Also, it is important to prevent outbreaks of two-spotted mite infestations prior to fall to minimize infestations next spring.
Two-spotted spider mites are common on greenhouse tomatoes, herbaceous perennials, left-over spring crops, 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. Tap leaves over a sheet of white paper, dislodging the mites and providing easier identification. When inspecting plants, 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.
When removing infested plant material from greenhouses, 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. Diapausing mites are less susceptible to chemical pesticides and biological control so it is important to manage them early. 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.
Biological control can be used on low populations of mites.
- Phytoseiulus persimilis is a fast acting, bright red color, predatory mite that only feeds upon spider mites, and will disperse or starve with no prey. 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.
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)
Mite Pests in Greenhouse Crops: Description, Biology and Management, Ontario Ministry of Agriculture, Food and Rural Affairs
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