With the current hot and dry weather conditions in Ohio, we expect to hear reports of spider mite outbreaks on specialty crops. Because mites are tiny, they are often overlooked or misdiagnosed as a disease. Infested leaves have fine webbing on the leaf undersides. Tomato leaves damaged by spider mites usually have yellow blotches, while bean leaves show white stipples or pin-prick markings from mite feeding. Pumpkins can tolerate moderate levels of mites, but watermelons are more sensitive to injury from mite feeding. A simple method of diagnosing spider mites is to shake leaves over a piece of paper and look for moving specks that are visible to the naked eye. A closer look with a magnifier can show the tiny mites that are white, marked with two large dark spots on the middle of the body.
Mites have many natural enemies that kill them, such as specialized predatory mites or generalist lacewings, ladybugs, and pirate bugs, but these helpful predators are often killed by pesticides. Mites can be suppressed by periodic overhead irrigation.
Chemical intervention can be needed to keep the crop alive if spider mites are abundant. In some fields, the mite infestation is worst on a field edge by a dusty road. When a mite infestation is limited to field edges, infested fields should be scouted, and a miticide applied as a spot treatment to isolated infestations. Mite control is better when higher volumes of water are used; 30 to 50 gallons of water per acre is better than 10 gal/A.
Several pesticides are registered for spider mite control; some are restricted use, and most are for general use. Some of these products kill only the motile mites (immatures and adults), while some kill eggs. Most do not have systemic activity but some do. These details are summarized in three attached tables. One table shows details about target life stages and mite species affected, as well as any insect target pests. Another table shows details about which products are registered for use on key vegetable crops, and another table for show similar registrations for hops and fruit crops.
At some locations, the old organophosphate Dimethoate is still effective for mite control. Dimethoate is an option for melons but is not allowed on squash or cucumbers; it has been a preferred product for mite control on soybeans. Dimethoate is prohibited from use on ornamental crops in high tunnels and greenhouses but is not prohibited from vegetable crops in high tunnels and greenhouses. Where Dimethoate is not effective, Agri-Mek (abamectin) is generally the most effective product for mite control but it is a restricted-use product, while Acramite (bifenazate) and Oberon (spiromesifen) are nearly as good but are not restricted-use products. Other options for some crops are Portal, Envidor, Zeal, Nealta, Onager, Savey, Apollo, and Kanemite, as well as a new product called Magister. Although Brigade (bifenthrin) and Danitol (fenpropathrin) are labeled for spider mite control when used at the high end of the rate range, they are generally not as effective as the true miticides. Vydate (oxamyl) is a Restricted Use product that is registered for use on eggplant for mite control. Several broad-spectrum products are available for use on organic farms to control mites as well as various insect pests: Grandevo, PFR-97, Sil-Matrix, SucraShield, as well as sulfur, oils, and insecticidal soap (such as M-Pede or Des-X). Soaps and oils can be used for mite control, but thorough coverage of the undersides of leaves is needed for good control because the action is by smothering of the mites. Soap can cause phytotoxicity if applied under sunny hot conditions. Soap is a good alternative in conventional fields that are too close to harvest to use a true miticide; insecticidal soap has a 12-hour re-entry interval and a 0-day pre-harvest interval.
-Celeste Welty, Extension Entomologist