Spider mites are one of the most damaging pests to strawberry crops throughout the U.S., according to entomologist Frank Zalom from the University of California-Davis. They favor hot and dry conditions and plants that are stressed due to insufficient irrigation.
“A number of pesticides also favor the development of spider mites, and they can become important induced pests as a result of broad spectrum pesticides that eliminate their natural enemies,” Zalom says.
The most common is the two-spotted spider mite (Tetranychus urticae), but depending on the crop and location, others, such as the carmine spider mite (Tetranychus cinnabarinus), can be important, as well.
Spider mites are usually found on the undersides of leaves and immature pests look the same as adults, only smaller. It’s easy to confuse female two-spotted spider mites with carmine spider mites because they turn a reddish color in colder areas of the country, Zalom says; however, this does not occur in warmer regions. Two-spotted spider mites live on a variety of crops and weeds, and are typically carried on the wind, causing new strawberry plants to be invaded quickly. The highest densities occur in June, then drop off naturally and populations rebuild in late summer, peaking in the fall on summer plantings, according to Zalom.
“Spider mites tend to cause more damage early season than later, although they can be damaging at high enough levels at any part of the season,” Zalom says. “In California, first-year plantings are most susceptible from the time fruiting buds are differentiating through perhaps a month after harvest begins, and at this time one spider mite per midtier leaflet can result in 1% yield loss.”
Spider mite damage results in yield reduction, with strawberry plants producing fewer fruit but not affecting fruit size. The effect of early season feeding will continue throughout the season, according to Zalom. Signs of damage include yellow spots on upper leaves, red to purple leaves, and webbing, but by the time these symptoms are observed, Zalom says, yield loss has already occurred.
While there are currently no spider mite-resistant strawberry varieties on the market, Zalom says short-day varieties tend to be more tolerant of spider mite feeding than day-neutral varieties. Recent varieties introduced by the University of California, Diamante and Aromas, tend to be less susceptible than earlier varieties like Selva, he says.
To monitor spider mites, determine the number of mites and predators per midtier leaflet, Zalom says. A midtier leaflet is a mature leaflet from the middle tier of the strawberry plant, one that is dull green, not shiny, but not old or dirty. In fall plantings, sample every other week starting when the first leaf is fully expanded, and begin weekly sampling when daytime temperatures reach 68°F to 70°F consistently or when mites begin to increase. In summer plantings, begin monitoring as soon as two or three leaves are present on plants in the fall. Sample every other week until February, then begin weekly sampling.
Presence/absence sampling in the field is fast for early season but inaccurate when densities exceed 10 per leaflet or following acaricide sprays, according to Zalom. He says it is best to sample the oldest fully expanded leaves early in the season; and later in the season, select mature leaves from the middle tier of the plant. Sample at least 50 and preferably 100 midtier leaflets at random from each block of 5 to 10 acres. In the field, the same method can be used to detect mites using a 10x hand lens, Zalom suggests.
Based on studies of Selva strawberries, a common threshold for early season are less than five mites per midtier leaf; for later season, 15 to 20 mites per midtier leaflet is the threshold.
Products used in the past had extended preharvest intervals or worker re-entry intervals that often dictated when they had to be applied, leading to preventative use. Many new products representing an assortment of modes of action are registered. These provide flexibility for preharvest intervals, impacts upon beneficials, and resistance management, according to Zalom.
The Insecticide Resistance Action Committee (IRAC) Mode of Action classification scheme’s premise states it is best to rotate materials with different modes of action. This became a problem for strawberry growers when they lost use of Omite, with no alternatives
available, Zalom says.
“For many years after the loss of Omite, growers only had Agri-Mek (abamectin, Syngenta Crop Protection) registered as an effective acaricide, and for many of those years it was only registered on an emergency basis,” Zalom says. “Growers were really up against a wall in terms of spider mite control.”
A number of predatory mites feed upon spider mites, some of which are endemic, occurring naturally, and others must be released by growers annually, Zalom says. The most commonly released predatory mite in California strawberries is Phytoseius persimilis.
“Its effectiveness is limited by the intensity of monitoring and management that occurs in the specific fields in which it is released,” Zalom says. “In general, it is most important to manage spider mites in the early season, regardless of the presence of predatory mites, since the spider mites are damaging at such low densities. However, predatory mites can regulate spider mite populations after this period if they are not limited by broad-spectrum pesticide
applications for control of spider mites or other arthropods.”
Zalom recommends growers review the University of California Integrated Pest Management Guidelines for Strawberries at www.ipm.ucdavis.edu/PMG/r734400111.html. The reference is updated regularly and provides an overview of spider mite damage and control.