Mite outbreaks in hot weather

Posted: July 11, 2012

Hot dry weather has been favorable for mite outbreaks. Generation turnover increases dramatically.
Two spotted spider mites.  Photo credits Eric Day, Virginia Tech

Two spotted spider mites. Photo credits Eric Day, Virginia Tech

Shelby Fleischer and Tianna Dupont, Penn State Extension

Recent hot dry weather has been favorable for mite infestations.  Generation turnover increases dramatically.  Mites go from egg to adult in about 37 days at 60-degrees.  At 70-degrees, it only takes 17 days.  By 86-degrees, it only requires 7 or 8 days.  A community of predatory species feeding on slow-moving mites tends to keep populations in check, but when temperatures rise, ‘out-run’ their predators by rapid reproduction.

In the last two weeks we have seen mites in watermelons, soybeans, tomatoes, eggplant, cucumbers, strawberries, and raspberries. Severe outbreaks have been reported in watermelons.  In field settings, it is almost always the two-spotted spider mite, which is distinguished by the two lateral oblong “spots” which are accumulated waste products that show through the body wall.  In greenhouses/high-tunnels, and sometimes in peppers, we also see eriophyid or cyclamen mites, which are much smaller that lack the two spots.

Mites damage plants by rupturing plant cells on the undersides of leaves and ingesting the cell fluids. Damaged areas appear as yellowish white spots (stippling) from destroyed chlorophyll in the plant cells. As populations increase, the whole leaf will eventually yellow. Heavily infested plants will also have webbing present.  This problem is best caught early.  Suggested spray thresholds vary with the crop and setting.  For watermelons, begin treatment when 10 percent of the crown leaves are infested early in the season, or 50 percent of terminal leaves late in the season.

Plant inspection is the only reliable method to assess the presence of mites.  Search undersides of leaves along leaf veins.  A good technique is to tap the leaves over a sheet of white paper, dislodging mites and making them easier to identify. Using a good hand lens is essential to viewing mites in egg, nymph and adult stages.  In field settings, infestations typically start near field margins, or grassy areas, often in hot-spots in the field. 

Mites move off drying vegetation by ballooning on silk threads.  During hot, dry spells, avoid mowing reservoirs of weedy vegetation near your fields, as this will send mites immigrating into the irrigated vegetable crop. Reducing moisture stress is another cultural control that helps by allowing the crop to tolerate the reduction in chlorophyll.  

Mites are not insects – they are more closely related to spiders.  While some insecticides also act as miticides, that’s not typically the case.  In field crops, systemic options exist that are not allowed for vegetable production.  However, miticide options have increased in just the last couple of years.  Check the current 2012 Commercial Vegetable Production Guide for labeled options for your crop, and READ THE LABEL.  Examples of selective miticides allowable in watermelon include Agri-Mek, Acramite, Zeal, Portal, and Oberon.  All of these have modes-of-action that are very different than those for pyrethroids, neonicotinoids, phosphates, or carbamates.  If you are dealing with a difficult mite outbreak, consider using one of the selective miticides, even though they are more expensive.  Be aware that some of these options severely limit the number of applications per season.

If mites are not causing widespread problems, try to limit application to hotspots.  Spot-treating will tend to conserve the beneficial predators, slowing the spread of the pest mites. Pesticide applications can flare mite problems in two ways: by killing off predators, and by increasing the reproductive rate of mites at sublethal doses.  Entomologists have documented reproductive stimulation for DDT, guthion, dicofol, malathion, carbaryl (Sevin), and imidacloprid. 

Mites like it dry and warm. In greenhouses, they are often found near vents, doors and heaters. Release of predatory mites can be effective for two-spotted mites in greenhouse/high-tunnel settings.  Predatory mites will be moving quickly, feeding on the relatively slow-moving plant-feeding mites.  Neoseiulus californicus is effective both preventatively and to clean up hot spots of spider mites. Phytoseilus persimilis is also effective on a curative basis. Neoseilus is supplied in tubes of 1,000 to 2,000 adults mixed in vermiculite or corn grits. To increase effectiveness:

·         Concentrate predator introductions at spider mite hot spots.

·         Gently roll the tube to mix the predatory mites equally in the carrier before application.

·         Place predators directly onto infested plants.

·         Store in refrigerator at 55 F for up to 5 days.

However, release of biocontrols are rarely effective on cyclamen or eriophyid mites.  Probably the best way to manage these mites in greenhouses is to remove and destroy infested plants as soon as they are detected.  Also the new miticides described above may be helpful.

For more details on biocontrols in the greenhouse, see the Greenhouse IPM with an Emphasis on Biocontrols handbook.


2012 Commercial Vegetable Production Guide:

Thomas, C. and Rajotte, E. 2005 Greenhouse IPM with an Emphasis on Biocontrols. Pennsylvania Integrated Pest Management Program.