Spider mites can be a serious problem in the landscape. They are often brought in from external sources on purchased plant material, however they can be blown in with the help of wind currents. They feed on many different herbaceous ornamental and woody plants, vegetable plants, fruit bearing plants, and even broadleaf and conifer trees. The name “spider mite” comes from the silk webbing they spin similar to spiders.
Spider mites are most effectively managed when Integrated Pest Management (I.P.M.) is practiced. In short, IPM is the use of multiple control strategies in a comprehensive and preventative approach to reduce pest populations, maintaining plant health, and minimizing the use and impact of pesticides in the environment. These management strategies involve mechanical, physical, cultural, biological, and chemical controls.
Scouting & Identification
Spider mites feed on the leaves through their piercing-sucking mouthparts. They remove contents from individual plant cells, leaving behind the cell wall, which makes the emptied cells appear silvery. The most noticeable damage of symptom of infestation is white stippling on the leaves. Heavily infested plants take on a faded, yellowish or greyish cast. Severely infested plants are covered by a thin layer of webbing created by the large numbers of spider mites.
When examining plants, it is best to take a white sheet of paper and tap the leaf several times with the hand, examine the paper for creatures that are dislodged. To the naked eye, all mites will look like little dots running around on the paper. Using a magnification aid can help. A simple 10X hand lens (jewelers loupe), or even a pair of high magnification reading glasses, can aid in seeing and identifying different mite species on the paper.
There are hundreds of species of spider mites in South Carolina, but not all cause damage to desirable plants in landscapes and gardens. Three species are most commonly encountered and damaging to ornamental plants – the twospotted spider mite (Tetranychus urticae), the spruce spider mite (Oligonychus ununguis), and the southern red mite (Oligonychus ilicis, McGregor), The twospotted spider mite (Tetranychus urticae) are easily identifiable by two spots on their backs that look like saddle bags. Twospotted spider mites are typically the most common mite seen on many plant species in the home garden and landscape, particularly in the warmer summer months.
Another serious mite that can cause problems is the Spruce spider mite (Oligonychus ununguis). Infested plants often take on a brownish appearance. Host plants include spruce, arborvitae, juniper, hemlock, pine, and other conifers. Dwarf Alberta spruce are the preferred host, and are even known to “have free mites with every purchase”. Spruce spider mites prefer cooler temperatures and are more problematic in spring and fall. Southern red mite (Oligonychus ilicis, McGregor) is another cool temperature mite pest, seen in spring and fall, particularly on Rhododendron and Camelia species. These mites are easily seen due to their red body color and can affect many other species of plants.
Cultural controls for spider mites mainly involve reducing plant stress and ensuring the plant’s environment is not conducive for spider mite development. Garden areas that remain dry, such as along pathways and landscape bed edges, are among the first place spider mites will be found. It is best to ensure plants are watered adequately, especially during periods of drought, as spider mites will often be worse on drought stressed plants first. In greenhouses or indoor settings, higher humidity levels can reduce spider mite populations and damage.
Similar to aphids, spider mites can be rinsed off of plant leaves. Rinsing treatments must be done frequently enough to ensure the mites will not climb back up the plants. Mid-season washing of the leaves can help reduce the potential for spider mite population booms. Unfortunately, rinsing is not effective on high populations.
One of the best preventative methods for managing spider mite control is using and encouraging the presence of natural enemies. Most of the predators that feed upon plant-damaging mites, are beneficial mites. They can be purchased from online sources, and are becoming more readily available. Not all predatory mites are created equal. Be sure to read all specifications, technical information, and ensure that the pest mites have been positively identified. Take a sample of infested leaves to the local Cooperative Extension Service for a positive identification. For a nominal fee the sample will be sent to the Plant and Pest Diagnostic Clinic for a positive identification.
Predatory mites that are readily available come in several product forms. The simplest is loose packed in cardboard tubes. In this package, the mites are packed on a carrier, such as saw dust, oat bran, vermiculite, or other material, that can be sprinkled over plants. These products are excellent for making both general broadcast applications over an area, or on individual plants. Another delivery method includes sachets. Sachets contain a similar material as in the tubes, but in a small packet with a tiny hole. They are designed to be placed on or near plants and then they slowly release mites over a period of time. In a long term setting such as in landscape, garden, or house plant, these sachets work very well. Replenishment times can even be set to a smart phone for reminders of when to replace the sachet.
The number one predator for twospotted spider mites is the Persimilis mite (Phytoseiulus persimilis). This mite works best at temperatures between 55 °F and 85 °F. However, above 85 °F, they cannot keep up with the twospotted spider mite populations. They are best released in the spring and fall. In the garden they can be applied at rate of 1 mite per square foot, but individual plants can also be treated.
During hot summer months, the Swirski mite (Amblyseius swirskii) works best on twospotted spider mites. Swirskii works best at temperatures above 68 °F. They are very aggressive and can be applied generally at a rate of 5 to 10 mites per square foot, or put onto individual plants. A side benefit is that they will also feed on some species of thrips, and whiteflies.
The Californicus mite (Amblyseius californicus) is another choice for twospotted spider mite. These mites work best at temperatures between 55 °F and 110 °F. They don’t do well for large pest populations, so they are best used as a preventative measure. Typical rates for broadcast applications are 1 to 5 mites per square foot, and individual plants can be treated as well.
For indoor or greenhouse settings, Mesoseiulus longipes can be used similarly to the Persimilis mite. These mites work better at low humidity, and can be used on indoor plants where other mites cannot. Release rates are 1 to 3 mites per leaf, and they attack similar prey as Persmilis mites.
When all other control measures have failed to keep the populations under control, a chemical miticide may be needed. The primary goal with miticide use is to choose the one with minimal impact to pollinators and natural enemies, but still be effective on the insect causing the problem. Before using a miticide, be sure to read and follow ALL label directions. The label is the law! The product label is the final authority on what plants and in what areas the product can be applied and at what rate. When purchasing a miticide, be sure to look on the package for the active ingredient and choose the product with the proper active ingredient to control the pest.
The first effective choice to spray would be either an insecticidal soap or horticultural oil. These miticidal products are designed to coat the mite’s exoskeleton and cause it suffocate. These products also can kill beneficial mites and insects upon contact, but they do not have residual activity. So only beneficial mites, insects, and pollinators that were directly hit by the application will be harmed. Note that these products can be phytotoxic (damaging to foliage) to drought stressed plants, or when temperatures of 90 °F or higher. Applications should be made when temperatures are cooler, such as during the mid- to late evening to avoid any potential phytotoxic effects. Other miticidal oils include those derived from botanicals such as rosemary, clove, and cinnamon oil. These oils can be effective, but use the same precautions as with soaps and other oils.
There are stronger miticides available to control spider mites. However, most are listed as “professional use only”, “licensed applicators”, or labeled as “restricted use pesticides”, which means only individuals with a commercial pesticide license can handle and use them. If all other treatments, including oils have not provided adequate control, it is best to contact a licensed company, such as a lawn care or landscape management company, to make the application.