Spider Mites On Vegetable Crops

Mites are arthropods, along with spiders, ticks, daddy-longlegs,and scorpions. Adult mites have two main body parts and four pairs of legs. Spider mites live on plants, mostly on the undersides of leaves, and feed by puncturing plant cells. There are over 1,200 known species of spider mites, including the carmine spider mite, brown wheat mite, desert spider mite, pacific spider mite, tomato russet mite, and the twospotted spider mite.^1,2,3,4 Of these, the two-spotted spider mite (Tetranychus urticae) is the most common and economically damaging to plants, affecting over 300 plant species worldwide. The two-spotted spider mite (TSSM) is the most common mite species affecting a number of vegetable crops, including tomato, eggplant, potato, melon, and cucumber.^1,5

TSSM is most commonly a problem on beans, tomatoes, and cucurbits (cucumbers, melons, squash, and watermelons), but can also damage celery, eggplant, pepper, sweet corn, and several other vegetable crop species. Several tree fruits, smallfruits, agronomic crops, ornamentals, and weed species are also hosts of TSSM and other spider mite species.^5,7,8

The TSSM overwinters as female adults in plant debris, under ground cover, and under bark, as females can tolerate low winter temperatures. All stages of TSSM can also overwinter on plants growing in greenhouses and high tunnels. Adult spider mites are spread by wind and on clothing and tools.^1,5,8 A female can lay about 100 eggs over her three-to-four-week life span. Unfertilized eggs hatch into males, while fertilized eggs develop into females. The cycle from egg to adult can take from five to nineteen days depending on the temperature.

Development and population increases are promoted by high temperatures and low humidity.^7,8 Spider mites can be washed off the plants by heavy rainfall and overhead irrigation.⁴ Dust (e.g., dust from adjacent field roads) on the plants can increase the severity of spider mite infestations.²

Successful management of spider mite infestations depends on early detection and treatment.⁴ Inspect vegetable seedlings for spider mites in the greenhouse before transplanting into the field. Look for stippling on the upper leaf surfaces and webbing on lower surfaces. Look for mites using a hand lens or tap symptomatic leaves over a white sheet of paper and inspect for crawling “dots”.^5,7 Scout fields frequently, especially during periods of hot, dry conditions. Initially, focus scouting efforts on field borders where mite infestations are most likely to begin. If mites are detected on the borders, then expand scouting to ten random locations within the field interior. Yellow sticky cards can also be used for detecting the presence of mites.^3,5,7

Cultural Practices: Protect populations of natural enemies (predators and parasites) by limiting the use of broadspectrum insecticides, especially early in the season. Reduce plant stress by maintaining appropriate levels of fertilization and irrigation, but avoid excessive levels of nitrogen.^2,5,7,8 Manage weeds, such as field bindweed, in and around the field. However, avoid mowing weeds that are already infested with spider mites, as mowing can cause the mites to migrateonto the crop plants. Try to minimize the accumulation of dust on the plants. Row covers can be used to exclude mite colonization early in the season.^2,8

Biological Control: Natural enemies (beneficial insects and mites) are usually able to keep spider mite populations in check on field-grown plants.^1,7 The western predatory mite (Galendromus occidentalis), sixspotted thrips (Scolothrips sexmaculatus), western flower thrips (Frankliniella occidentalis), lady beetles (Stethorus sp.), minute pirate bug (Orius tristicolor), and lacewing larvae (Chrysoperla carnea) all help keep spider mite populations low. Releasing commercially available predatory mites, such as Phytoseiulus persimilis, may providesome protection for field and greenhouse-grown tomatoes,and the predatory mite Amblyseius fallicis is sometimes used tohelp manage spider mites on greenhouse-grown plants.⁵

Chemical Control: Several miticides are available to help manage spider mites on vegetable crops, but labeled productsvary with crop species. Consult regional vegetable production and pest management guides and product labels for information and restrictions on application on specific crops. Applications should be initiated when spider mite infestations are first detected and symptoms on plants are observed. If mites are detected on field borders, treat a buffer zone at least 100 feet beyond the mite-infested area. Applications needto reach both the upper and lower leaf surfaces to provideadequate management of spider mites. Spot treat when population buildups are detected, preferably before webbingoccus.^1,2,7

Miticide active ingredients listed in regional vegetable production guides for use against spider mites on some vegetables include abamectin (group 6), acequinocyl(group 20B), bifenazate (group UN), bifenthrin, (group 3A), cyflumetofen (group 25), fenbutain oxide (group 12B), fenpyroximate (group 21A), spiromesifen (group 23), and sulfur (group UN). Insecticideal soaps and various horticultural oils can also be used to help manage spider mites, especially in organic production systems^{. 1,2,3,4,5,8} Always read and followpesticide label directions.

Some miticides can be harmful to bees and other beneficial insects, so care should be taken in selecting and applying products for mite management. Whenever possible, apply insecticides and fungicides late in the day when pollinators are less active in the field.^4,7 If making multiple applications, alternate among products with different modes of action to help manage the development of resistant mite populations.^5,7