Spider Mites: The Summer Scourge

Spider Mite feeding damage on mandevilla.
Mandevilla showing the effects of spider mite feeding. Photo courtesy of Griffin Greenhouse Supplies.

There’s a lot to like about summer vacations: cookouts, frisbees and all kinds of outdoor activities. Unfortunately, spider mites find a lot to like about summer, as well.

Abundant foliage, combined with hot, dry weather over much of the country shifts spider mites reproductive capacity into overdrive. They have flexed their muscles in recent years by demonstrating their ability to fend off most of the registered miticides through pesticide resistance. Growers have reported “super mites” that survive aggressive spray rotations. Pesticide resistance has become a major issue for the greenhouse industry and it’s forcing a change in pest-control strategies.

Scouting For Spider Mites

As the temperature goes up, the length of the spider mites’ life cycle goes down, leading to population explosions. Early detection is critical so control can be implemented before serious crop damage is observed; this also makes it easier to use biological control agents (BCAs) or softer pesticides.

Concentrate scouting on those crops known to be particularly vulnerable to spider mites. Some growers have planted a few bush-bean plants to act as trap crops. Spider mites thrive in lower humidity, so pay special attention to areas in the greenhouse with lots of air movement, such as near intake louvers and doors. Spider mites are small enough that magnification is needed for effective scouting. Tapping plant leaves and stems onto white paper is one way to scout for spider mites.

Spider mites feed on lower leaf surfaces, removing chlorophyll from a cluster of cells that causes small tan or bronze spots to appear on upper leaf surfaces. Heavy feeding can cause a bronzing of the entire leaf and may progress to defoliation. Webbing is sometimes seen but by the time that occurs, it’s generally too late to save affected plants.

Crop nutrition also plays a role in spider mite development. Entomologist Dr. Raymond Cloyd from Kansas State University has mentioned in various publications a link between high nitrogen levels and increased spider mite populations. Most plants will tolerate more nitrogen than they actually need, so it’s important to fertilize only as much as is needed for optimum growth.

Biocontrols’ Role In Spider Mite Control

Several biological controls have proven effective in controlling spider mites, and BCAs are valuable tools as our industry adapts to increasing pesticide resistance. Each BCA option has attributes that may make it the best choice based on several variables, including the crop to be protected, time of year and other pests that need to be controlled at the same time. Pesticides already applied to the crop may leave behind harmful residues that can damage predators for weeks or even months. Unless you’re experienced in using BCAs, we suggest consulting with your supplier or other biocontrol experts to help you start and maintain a biocontrol program.

Horticultural oils like Suffoil-X or Ultra-Pure Oil can be important allies in combating spider mites. There are some challenges associated with using the oils. Oils work by suffocating mites, so thorough coverage is essential for success; that can be difficult in dense crop canopies, even with the best sprayers. Plant safety can also be a concern.

To minimize this issue, apply horticultural oils when rapid drying will occur. Control is achieved quickly after spraying, so prolonged foliage wetting is not needed for good results. Microbial insecticides like Preferal and Met 52 EC are labeled for spider mites, though grower feedback on these products is not yet widely available.

Miticides Provide Relief

Many different miticides are labeled for controlling spider mites in greenhouses, representing several modes of action. While that sounds like good news, resistance is widespread and can vary from one population of spider mites to the next.

Listed below are examples of labeled miticides organized by mode of action. Of course, always read and follow the entire pesticide label. Not all pesticides are labeled in every state. Pesticides other than those listed may be safe and effective. All materials listed, except for Kontos, are applied as foliar applications.

Contact activity:
• MOA UN- Floramite*- Anecdotally, this product appears to perform best with spray-water pH in the range of 5.5 to 6.5.
• MOA 20B- Shuttle* has label cautions regarding use on impatiens and roses.
• MOA 21A- Sanmite, Akari*
• NC- Horticultural oils- Ultra-Pure* and Suffoil-X* Tips for successful applications are listed above.
• 10A- Hexygon- Long residual product that works by life-cycle interruption.

Translaminar systemics:
• MOA 13- Pylon*- also available as a total-release aerosol, Pylon TR*.
• MOA 23- Judo, Kontos* (drench only) – Judo damages several popular crops; contact your supplier for details. Kontos cannot be used on geraniums, dracaena and a few other crops.
• MOA 10B- Tetrasan, Beethoven TR- Mite growth regulator, long residual.
• MOA 6- Avid
• MOA UN + 6- Sirocco – combination of active ingredients found in Avid and Floramite.

* Indicates use is approved for at least one edible crop, check label for details.

Sultan is a new miticide that brings a new mode of action class to greenhouse miticides (MOA 25). Sultan is registered for spider mites and several other mites on ornamentals. Good plant safety and a 12-hour REI will make it a good fit for many spider mite rotations.

 

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