Spider mites might require a hand lens to see, but their damage can be highly visible. Although related to insects, mites belong to the arachnid class. Numerous species exist, including many spider mite pests. Of the pests, the two-spotted spider mite is generally the most troublesome. This mite, Tetranychus urticae, attacks fruiting and podded vegetables, plus small fruits, tree fruits, tree nuts and other crops.
Spider mites live in colonies, and a single colony can contain hundreds of them. They prefer leaf undersurfaces. Less than 0.05 inch in size, spider mites resemble tiny moving dots to the naked eye. Shake an infested leaf over a piece of paper and you’ll see specks scurrying on the paper. They often produce webs similar to those of spiders. Adults have four pairs of legs and two reddish eye spots near the head; their bodies are greenish, pink or cream-colored, often with black spots.
These mites develop in four stages: translucent oval eggs, a six-legged immature, an eight-legged immature, and the adult. In the heat of midsummer, a new generation may emerge in less than a week. In warm climates, spider mites may reproduce all winter. In cool areas, they overwinter in ground litter, bark scales and debris.
Spider mites insert their stylet mouthparts into leaf cells and suck out the contents. This depletes the chlorophyll, resulting in spotted, stippled or pale leaves. The leaves turn bronze or brown and eventually die. A plant can recover from a light infestation, but if mite populations are high, the plant can die. Vegetables such as squash and tomatoes can suffer sunburned fruit and yield losses. The pods of beans and peas may be unmarketable. Well-established fruit trees can often withstand an outbreak, but the fruit quality can be adversely affected the following season.
Unfortunately, by the time spider mite damage becomes apparent, every spider mite life stage may be present. Scouting is imperative – at least weekly for most crops. Many insecticides control only specific stages, and as with most pest management, early application is most effective.
Squash and tomato plants can suffer sunburned fruit and yield losses if mite populations are high.
PHOTO BY DGPHOTOGRAPHY/ISTOCKPHOTO
Spider mites prefer hot, dry conditions. Adequate irrigation produces healthier plants that are better able to cope with pests. Also, drought-stressed plants appear more attractive to spider mites. If an infested area is limited, employing jets of water, particularly directed to the undersides of leaves, can physically remove and kill many mites. In addition, disrupting the webs may delay egg laying.
Insecticides applied to manage other pests may produce or worsen a spider mite problem. Spider mite outbreaks result from the insecticide’s lethal action against the spider mite’s natural enemies, particularly its predators. According to Colorado State University’s “Spider Mites” fact sheet, Sevin (carbaryl) can contribute greatly to outbreaks. Malathion, Merit and Marathon have also played a part in mite outbreaks.
Moreover, carbaryl, some pyrethroids and some organophosphates can actually favor spider mites by increasing the nitrogen level in leaves. “Pest Notes,” an integrated pest management guide published by the University of California, Davis, reports that carbaryl can even stimulate spider mite reproduction.
Pesticides developed especially for mites, i.e., miticides, provide the best control. Tom Ford, Penn State Extension educator, notes that spider mites are typically a problem on farms where producers are heavy pyrethroid users. While pyrethroids can kill mites at high doses, they also injure beneficial insects and mites. Ford says, “If growers use registered miticides in lieu of pyrethroids in the field for mite problems, they should have no problem in controlling them.”
Acramite, Agri-Mek, Oberon, Portal, Zeal and others are often recommended as controls in many vegetable grower guides. Some tree fruit miticides include Acramite, Apollo, Dicofol, Envidor, FujiMite, Kanemite, Mesa, NeXter, Omite, Savey, Thionex, Vendex and Zeal. Growers should always check with their local extension office for advice about their region and for information regarding changes. For instance, some of the pesticides are labeled for specific crops only, and some are not compatible with adjuvants. Also, many perform best when targeting specific mites.
Greenhouse recommendations for spider mites include Akari, Ecotec, Floramite SC, Kontos, M-Pede, Shuttle O and TetraSan. M-Pede is an insecticidal soap that has no residual action, and Kontos is for transplants, not vegetable production. Growers should also note that the label is always the law.
Miticides are also known as acaricides.
Various predator mites can offer good control. Jim Walgenbach, professor and extension entomologist at North Carolina State University, has been working with the predatory mite Phytoseiulus persimilis. This predator has shown promise in many areas. Although it is native to the Mediterranean region, it has successfully overwintered in central North Carolina. Strawberry growers in California use P. persimilis.
The Walgenbach Lab is also focusing on acaricide resistance in the two-spotted spider mite, especially with tomatoes and cucurbits. Using a molecular approach to better understand the gene flow, more effective miticide resistance management practices will result. The economic threshold for acaricide applications is low – only two to four mites per leaflet. This low threshold can easily lead to excessive acaricide applications, which ultimately leads to acaricide resistance.
Walgenbach advises, “Try not to rely on one particular control. Also, never wait to apply necessary controls; the larger the mite population, the more difficult it is to control. If multiple applications are needed, rotate the chemistries.”
Several companies, including Biotactics, Koppert Biological Systems and IPM Laboratories, Inc., provide the predator P. persimilis and several other predatory mites.
Two new biopesticides derived from naturally occurring soil fungi are effective against numerous small fruit and vegetable pests, including spider mites. Novozymes’ Met52 functions as a contact insecticide. PFR-97 from Certis USA infects both foliar and soil-dwelling pests. In addition, Marrone Bio Innovations’ new microbial biopesticide, Grandevo, with a complex mode of action, functions as a stomach poison.
Dorothy Noble is a writer-researcher specializing in agriculture.