Phytophagous Mites and Their Management on Ornamental Plants

Phytophagous or plant-feeding mites are among the most common pests in ornamental plant production and maintenance by greenhouse and nursery growers, landscape care professionals, arborists, grounds managers, and gardeners. Correct identification of mite species or life stages is important because many biological control agents and miticides target specific mite species and life stages. This article provides information on the identification, biology, and management of pestiferous mite species commonly encountered on ornamental plants grown in nurseries, greenhouses, and interior and exterior plantscapes.

Introduction

Mites are extremely difficult to detect because they are small (most are 0.15 to 0.3 mm or 6/1000 to 12/1000 of an inch) and prefer to hide on the underside or protected areas of leaves, shoots, buds, fruits, and bulbs. Mite populations grow very quickly because they have a short developmental time and high fecundity, and they can reproduce without mating when males are not available. They are also difficult to manage because of their high propensity to develop pesticide resistance. Mites can be introduced into nurseries, greenhouses, and landscapes through infested cuttings, liners, bulbs, or finished plants. They also can disperse from nearby infested weeds and crops by walking or being carried in the wind or on an animal.

Successful management of mites requires an integrated approach. A crop can be started “clean” by maintaining a weed-free growing area, quarantining incoming plant materials, and, when necessary, treating the incoming plant materials before moving them into the growing areas. When executed properly, preventive management using predatory mites and other biological control agents can be very successful in nurseries and greenhouses. Miticides are often required when curative treatment is needed to stop a mite population from doing further damage. An integrated pest management program that incorporates biological and chemical control tools will require careful consideration of the compatibility between biological control agents and pesticides.

Description, Life Cycle, and Damage of Common Species

Spider Mites: Family Tetranychidae

Members of the spider mite family are characterized by their ability to spin silk webs. Among them, the twospotted spider mite (Tetranychus urticae) is a major pest of a wide variety of ornamental plants. The twospotted spider mite is considered a “warm-season mite” because its population growth and damage are particularly severe during the warmer months of summer. Twospotted spider mites are highly variable in color but frequently have a dark spot on each side of the body (figure 1).

Several other spider mite species cause problems in particular plant groups. Spruce spider mite (Oligonychus ununguis), a “cool-season mite” species active in the fall and spring, is a common pest of conifers. Southern red mite (Oligonychus illicis) is another cool-season mite species that is particularly problematic on various woody plants. Lewis mite (Eotetranychus lewisi) is most problematic on poinsettias (Euphorbia pulcherrima) and strawberries (Fragaria × ananassa) and looks very similar to a twospotted spider mite. Lewis mite is smaller and has six or more small spots, whereas the twospotted spider mite has two large spots. Maple spider mite (Oligonychus aceris), bamboo spider mite (Schizotetranychus spp.), boxwood spider mite (Eutetranychus buxi), tumid mite (Tetranychus tumidus), and carmine spider mite (Tetranychus cinnabarinus) are pest problems on different crops in spring through fall.

Figure 1. All life stages of the twospotted spider mite can occur at the same time. Image credit: Matthew Brown, Clemson University.

All spider mite life stages can be found on the leaves (figure 1). Infestation starts on the underside of leaves but can spread to the upper side as the population grows. Adults of different spider mite species vary in coloration and size. A spider mite egg is typically spherical and clear when first produced but later takes on the color of the mite developing within. A hatchling is called a larva and has three pairs of legs. The next two instars (protonymph and deutonymph) are larger and have four pairs of legs, similar to the adults. Developmental times are similar among species and typically increase with temperature up to an upper developmental threshold, after which the development slows down or stops completely. A twospotted spider mite feeding on raspberry (Rubus idaeus) leaf completes development in seven days at 30 °C (86 °F) and twenty-five days at 15 °C (59 °F).¹ Each female lives for seventeen days and produces seven eggs per day at 30 °C.¹

Spider mites feed by puncturing cell walls and removing cell contents. Typical damages include stippling and bleaching or bronzing of leaves (figure 2); these symptoms can be used for detecting infestation. The spider mite population has already reached a very high density when webs begin to cover parts of plants (figure 3).

Figure 2. Typical damage of twospotted spider mites is stippling and chlorosis on the leaves. Image credit: Juang Chong, Clemson University.

Figure 3. Plants can be covered by webs spun by twospotted spider mites in a severe infestation. Image credit: Juang Chong, Clemson University.

Tarsonemid or Thread-Footed Mites: Family Tarsonemidae

The tarsonemid mite family derives its common name from two long thread-like hairs on the last pair of legs of adult females. Cyclamen mite (Phytonemus pallidus) and broad mite (Polyphagotarsonemus latus) are the most notorious members of this family. One way to tell them apart is that cyclamen mite eggs are elliptical and smooth, whereas broad mite eggs are elliptical and covered with tiny whitish bumps that make them look like tiny footballs. The football-like appearance of the broad mite eggs also helps distinguish this mite species from spider mites and other pests that may cause distortion to the growing terminals (such as thrips).

Cyclamen mite and broad mite attack many plant species, with begonia (Begonia spp.), African violet (Saintpaulia ionantha), geranium (Pelargonium), ivy (Hedera), (ornamental and vegetable) pepper (Capsicum annuum), and strawberry being the most frequently reported hosts. Reports of infestation of broadleaf trees by broad mites in outdoor nurseries have increased in recent years. Tarsonemid mites feed on growing buds and tender leaves, causing stunting, distortion, and death of tissues. Feeding damages often manifest as curled or distorted leaves, cracked and brittle leaves or fruits, unopened buds, and distorted or shriveled flowers. These symptoms often are misinterpreted as the result of disease infection or phytotoxicity, but the presence of tarsonemid mites or their eggs can usually be observed in damaged plants. Damage by cyclamen mite and broad mite is permanent; ridding the plants of mites does not restore the damaged parts but can protect new growth from damage. Therefore, management of cyclamen mite or broad mite should begin as soon as damage is detected.

Cyclamen mite and broad mite are sensitive to heat and humidity.² At 20 °C (68 °F) and 80% relative humidity, a cyclamen mite completes development in sixteen days when feeding on a strawberry leaf and produces fourteen eggs over an adult lifespan of twelve days.³ The developmental time of a broad mite feeding on an azalea (Rhododendron) leaf is thirteen days at 15 °C (59 °F) and 80% relative humidity but is reduced to 3.5 days at 30 °C (86 °F).⁴ Broad mite has the ability to hitchhike on another insect (a behavior termed “phoresy”), such as an adult whitefly, to disperse to another plant.

Eriophyid, Russet, Rust and Gall Mites: Family Eriophyidae

Eriophyid mites are microscopic (less than 0.25 mm or 1/100 of an inch), with two pairs of legs and elongated bodies, which give them an appearance of a banana or cigar. Eriophyidae is a large and diverse family with members often identified based on the damage they caused, such as blister, gall, bud, rust, stunt, and rosette. Compared with other mite groups, eriophyid mites are relatively host-specific, meaning that one species usually feeds on only one host plant species. Pear leaf blister mite (Eriophyes pyri) is common on pears (Pyrus spp.) in the Pacific Northwest. Maple bladder gall mites (Vasates aceriscrumena and V. quadripedes) are common across the country. Privet rust mite (Asculus ligustri) turn privet (Ligustrum spp.) leaves brown. Acaphylla steinwedeni and Phytoptus canestrinii are common on camellia (Camellia spp.) and boxwood (Buxus spp.), respectively, in Florida. Hibiscus erineum mite (Aceria hibisci) causes unsightly distortion and galls on tropical hibiscus (Hibiscus rosa-sinensis) in Hawaii and the Caribbean. Rose rosette mite (Phyllocoptes fructiphilus) is the vector of rose rosette virus that has ravaged roses (Rosa spp.) in the United States. Bermudagrass mite (Aceria cynodoniensis) and zoysiagrass mite (Aceria zoysiae) are increasingly problematic pests of bermudagrass (Cynodon spp.) and zoysiagrass (Zoysia spp.) turf, respectively.

All eriophyid mite life stages can be found on the same plant, often in secluded and hard-to-reach areas. For example, the majority of rose rosette mites are found within growing buds and under the sepals of roses. While some species can cause extensive distortion of growing terminals (such as the bermudagrass mite) or transmit viruses (such as rose rosette disease), the feeding damage of most eriophyid mite species is not typically detrimental to the overall health of mature plants grown in landscape. However, the damage can significantly reduce the quality, value, and appearance of ornamental plants grown in nurseries and greenhouses. Some species can cause significant changes in host plant appearance and physiology, which in turn leads to the decline of plant health. Eriophyid mites hide in distorted tissues; therefore, miticide solution often has difficulty contacting the mites, resulting in poor miticide efficacy. The extravagant galls and distortion caused by eriophyid mites also provide refuge to the tiny mites from unsuitable temperature and humidity.

Eriophyid mites generally have a very short development time and relatively low reproductive capacity. Rose rosette mite completes development on a multifloral rose (Rosa multiflora) leaf in eleven days at 23 °C (73 °F) and produces seven eggs over fourteen days of adult female life.⁵

False Spider Mite or Flat Mites: Family Tenupalpidae

Unlike the ‘true’ spider mites (Tetranychidae), false spider mites do not produce silk and are generally smaller and occasional pests of ornamental plants. False spider mites are usually flattened, which gives them the other common name—flat mites. Various Brevipalpus species are known to attack ornamental plants, including B. obovatus on privet and B. phoenicis on date palms in Florida. Red palm mite (Raoiella indica) is a pest of ornamental and fruit-producing palms in Florida. Phalaenopsis mite (Tenuipalpus pacificus) can cause significant damage to orchids in Florida greenhouses.

False spider mite species differ in their host range. Phalaenopsis mite can attack multiple species in the family Orchidaceae and Polypodiaceae. Red palm mite attacks mainly palm species but has also been found on species in the ginger and plantain families (Zingiberaceae and Plantaginaceae, respectively). Feeding damage also differs among species. Phalaenopsis mite first turns leaves silvery, then rusty brown. Red palm mite causes extensive yellowing of palm fronds. Some Brevipalpus species are vectors of viruses or virus-like diseases.

The life cycle of most false spider mite species is poorly understood. The developmental times of red palm mite on Manila palm (Adonidia merrillii) are thirteen days at 34 °C (93 °F) and thirty-six days at 20 °C (68 °F).⁶ Few eggs are produced by red palm mite at 30 °C (86 °F), and the number of eggs produced by each female is reduced from forty-seven at 27 °C (81 °F) to twelve at 20 °C.⁶

Bulb Mites: Family Acaridae

Bulb mites are traditionally pests of bulbs and corms; however, reports of bulb mites damaging cuttings and liners in tissue culture or propagation have increased in recent years. Infestations of bulb mites can be difficult to detect initially because the mites are often concealed under bulb scales and skins. Damage by bulb mites is often not apparent until the population has reached a very high density and decay has occurred. Bulb mites are often associated with the decomposition of bulbs and cuttings, but there are controversies on the role of bulb mites. Bulb mites may be secondary pests because they make use of the decaying plant tissues and organic material resulting from infection by pathogens, fungus gnats, or other pests. Bulb mites can also cause direct damage by feeding on plant tissues, which create wounds where pathogens (such as Fusarium, Pythium, Phytophthora, and Rhizoctonia) can enter and infect the plants. In many cases, both rot pathogens and bulb mites have to be reduced to achieve successful management.⁷ Currently, no miticide is registered for the management of bulb mites in greenhouses or tissue cultures. Fumigation and hot water dip may be used for post-harvest management of bulb mites on bulbs and corms.

Two main species of bulb mites attack ornamental plants–Rhizoglyphus robini and R. echinopus. Bulb mites are larger than other plant-feeding mite species previously discussed; adults are about 0.5 to 1 mm (1/50 to 1/25 of an inch) long, shiny, and creamy white. Bulb mites move and disperse very slowly. The nymphs have the ability to hitchhike on another insect (such as a fungus gnat) and, therefore, may be the life stage of dispersal. Infested bulbs, corms, and basal stems turn soft and then decay. Infested leaves are distorted. Infested plants also have difficulty rooting and establishing.

Rhizoglyphus bulb mites develop quickly, with development from egg to adult completed in ten to twelve days at 27 °C (81 °F).⁸ Each female produces between 100 to 600 eggs, varying greatly among plant species.⁸

Management

Start Clean, Stay Clean

The introduction of infested liners, cuttings, or seedlings is a major pathway for introducing mites into a production facility. In operations that produce their own cuttings and liners, stock plants should be maintained relatively free of mites with an integrated pest management approach. In operations that bring in propagated materials, incoming liners and seedlings should be quarantined and monitored before transplanting or moving into the growing areas. Pre-planting quarantine and treatment are to make sure no mite infestation is introduced.

Dipping cuttings and seedlings in 0.1% or 0.5% horticultural oil (such as SuffOil-X^®) before sticking or transplanting can reduce the number of spider mites and prevent severe infestation and damage during growth.⁹ Other options registered for pre-planting dip are insecticidal soap (M-Pede^®) and hexythiazox (Hexygon^® IQ). A lower initial population of spider mites not only allows for a longer period of time when miticide application is not needed but also allows biological control programs to be more effective. It is important, however, to test for phytotoxicity of the pre-plant dip on a small group of cuttings and seedlings before implementing the program on the entire crop.

Monitoring

The most important tool in mite management is the hand lens. Typically, one that magnifies 10 or 16 times is sufficient for examining plant tissues infested with spider mites, tarsonemid mites, and flat mites. Because of their small sizes, detection of eriophyid mites on their host plants is best performed under microscopes or with a high magnification hand lens (at least 20 times magnification).

Since each mite species typically has a list of preferred host plants and seasons/months when the population is most active, it is important for scouts and growers to familiarize themselves with the biology of problematic mite species. A more targeted scouting program can be developed based on the mite’s biological information and focused on the preferred hosts. This is more efficient than a general scouting program that sweeps up all potential problems.

Figure 4. Spider mite population can be sampled by shaking or beating a plant over a piece of white paper or paint palette. Image credit: Juang Chong, Clemson University.

Mites can be sampled in two ways. First, a scout may select specific plants from the block and observe for feeding damage, such as stippling, bronzing, and webbing (figures 2 and 3). Use a hand lens to examine the underside of the damaged leaves for mites. Alternatively, place a sheet of white paper (on a clipboard) or a white paint palette under the leaves or branches and then strike the leaves and branches sharply (figure 4). Examine the debris collected on the paper and look for mites moving on the surface. A hand lens can be used to identify the collected mites.

No treatment threshold has been established for any plant-feeding mite species on ornamental plants. Because of the potential for mite populations to cause permanent and severe damage quickly, growers and plant managers are encouraged to scout regularly and apply appropriate treatment or management tactics as soon as damage or mite population is found.

Cultural and Physical Control

The efficacy of various cultural and physical control tactics is rarely documented. Healthy and well-maintained plants are more tolerant of mite damage. Maintain an irrigation schedule that avoids water stress on plants. Spider mite outbreaks are associated with dusty conditions; therefore, regular sprays of water on pathways to keep down the dust may help. Even a forceful spray of water may dislodge spider mites from the foliage. Post-harvest hot water dip may reduce bulb mite populations on bulbs and corms.

Weed management can play an important role in managing mite populations on ornamental plants. Many species, particularly spider mite species, can feed on dozens if not hundreds of plant species. Some host plant species are also common weeds in greenhouses and nurseries, such as wood-sorrel (Oxalis spp.) and chickweed (Stellaria media). Mites can feed and reproduce on these weed species and disperse to the crops. Maintaining propagation and production areas weed-free can eliminate the refuges and potential sources of mites. Similar to weeds, pet plants and plants that are left from sales should not be allowed to remain in the growing areas. These plants can also serve as refuges for mite populations or sources of mite infestations.

Heavily infested plants, which show severe distortion or webbing, should be discarded immediately. Damage by mites is permanent, and miticides and other management approaches cannot reverse the damages. The most sensible and economical approach is to discard these heavily infested plants that could not be rescued. Additionally, the remaining plants in the production area should be treated to reduce the mite population and prevent further damage.

Biological Control

Various groups of natural enemies (predators and parasites) suppress mite populations in the landscapes and natural environment. In nurseries and greenhouses, natural enemies may have to be introduced to reduce the mite population. Several mite-feeding predatory insects and mites are commercially available; these are referred to as biological control agents, biocontrol agents, biocontrols, or beneficials (table 1).

Table 1. Commercially available biological control agents recommended for the management of various phytophagous mites.¹⁰

Note: Consult your biological control agent suppliers for availability. Although Amblyseius degenerans, Amblyseius swirskii, and Neoseiulus cucumeris are sometimes marketed for management of various mite species, they are generally not recommended for mite management.

In outdoor production areas and orchards, creating favorable conditions (such as planting cover crops) for existing natural enemy populations may be more cost-effective and efficient than releasing biological control agents purchased commercially. It is important to understand that predatory insects and mites are living creatures; hence, they have very specific environmental requirements in order to perform optimally. Biological control agents can be successful only when these environmental requirements are satisfied. Additionally, proper care of the biological control agents when a grower receives them also plays an important role in the survival and success of the biological control agents in the operation.¹⁰ Refer to the Land-Grant Press publication, “Biological Control Strategies in Integrated Pest Management (IPM) Programs,” and the guidelines by LeBeck and Leppla,¹¹ or consult with your biological control agent suppliers or local extension personnel to select the most suitable biological control agents or to develop a biological control program.

Biological control is most effective as a preventive management tool, meaning that a biological control program should be initiated before the mite population becomes too large or its damage becomes too severe. When plants are heavily infested, one to two applications of compatible miticides (table 2) may be needed to bring the mite population to a lower level before releasing the predatory insects and mites. It is important to recognize that insecticides and miticides can be harmful to biological control agents; therefore, one must select insecticides and miticides to be used in conjunction with a biological control program very carefully.

Table 2. Mobility property, effective life stage, and general compatibility with biological control of selected miticides.

Note: IRAC = Insecticide Resistance Action Committee. Mobility property: T = translaminar, C = contact. Compatibility rating: L = harmless or slightly harmful (< 25% reduction in population); M = moderately harmful (25-50% reduction in population); H = harmful or very harmful (> 50% reduction in population).¹²

If another pest (such as whiteflies) infests the same crop and the application of another pesticide is needed, carefully study the compatibility of the pesticide with the biological control agents of mites and other pests before application. Avoid using compounds, such as organophosphates, carbamates, and pyrethroids, which are known to be extremely harmful to biological control agents. Long residual insecticides, such as organophosphates and carbamates, can prevent the release and establishment of biological control agents for a long time after application. Often, safer or less disruptive alternatives are available. The compatibility of miticides presented in table 2 is an aggregate of compatibility for various biological control agent species within each functional group (i.e., predatory mites, predatory insects, and parasitoids). The compatibility of pesticides varies among biological control agent species. It is recommended to consult with your biological control agent suppliers for more detailed information on the compatibility of each miticide with a specific biological control agent species of interest.

Miticides

The most commonly used approach to managing mite populations in nurseries and greenhouses is the use of miticides. Because most damage caused by mites is permanent, it is prudent to employ treatments on the most susceptible host plants before damage becomes widespread. To achieve preventive control, operators should know the life cycle of the mites (so as to determine the best application timing) and select miticides that will provide the most effective acute and residual control. The residual longevity of miticides depends on the physical and chemical characteristics of the miticides and the environmental conditions (such as temperature, precipitation, and ultraviolet light). Always check labels for target species, application rates, application instructions, maximum number of applications within a crop or season, and sensitive plants.

A large number of pesticides are registered for use on ornamental plants, each with its specific target species (table 3 – link to download PDF file). However, it is recommended that growers only use miticides for mite management (table 2). Broad-spectrum insecticides, such as pyrethroids, are only marginally effective against mites but could cause significant harm to the natural enemy populations. Pest mite populations may grow more explosively if the natural enemy population is eliminated or reduced. Reserve these broad-spectrum insecticides for management of other insects.

Miticides generally have either contact or translaminar properties (table 2). Contact miticides, such as acequinocyl and bifenazate, kill mites when the active ingredients make direct contact with the mites or when the mites contact the residue. The reported maximum residual periods of most miticides are between twenty-eight to forty-five days, but re-applications may be needed in less than twenty-eight days because of environmental degradation (which reduces the residual toxicity of the treatment) and growth of new leaves or buds (which are not protected by the previous application). Contact miticides typically have a broad spectrum of activity against most life stages of spider mites. Because contact miticides only kill mites that are in contact with the solution or residue, it is critical to achieve complete coverage of all plant surfaces to successfully control mite populations when using contact miticides. In most cases, the addition of a surfactant (such as a spreader-sticker) to the miticide solution can improve treatment efficacy.

Some miticides, such as abamectin and spiromesifen, have both contact and translaminar properties (table 2). The active ingredients of translaminar miticides are capable of penetrating the leaf surfaces into the plant cells. This resident concentration of active ingredients provides extended residual activity against mites. Adult and immature mites die after they ingest the residue in the plant cells. The activity of these miticides against mite eggs is provided by the contact properties once immatures hatch, while the activity against immature and adult mites is provided by the translaminar and contact properties. Similar to contact miticides, it is a good idea to achieve thorough coverage when using translaminar miticides.

Each miticide has its own spectrum of efficacy against different life stages (table 2); therefore, it is important to determine the predominant life stages of the mite population before the application. When control of all life stages is desired, such as against a heavy infestation, choose miticides that can kill all mite life stages. Alternatively, a miticide with limited life stage activity may be tank-mixed with another miticide of a different mode of action to achieve the full spectrum of life stages. Combination products, such as Sirocco^® (abamectin + bifenazate), are developed to fit the need for controlling all life stages.

Miticide Rotation

The short development, high reproductive capability, and high mutability of their genomes make mites perfect candidates to develop miticide resistance. To delay the development of miticide resistance, practice integrated pest management (IPM). Only spray when needed, and always rotate among miticides of different modes of action (MOA). Refer to the Land-Grant Press publication “Insecticide Resistance: Overview and Management” for guidelines on developing a pesticide rotation program. Utilize tables 2 and 3 (table 3 PDF download) in developing a good miticide rotation program. Regardless of the maximum residual period, it is a good idea to mark some plants and check on the mite population density on these plants weekly to determine treatment longevity and effectiveness. If the mite population seems to be on the rebound, another application will be needed. The miticide used in the second application should be of a different MOA or IRAC group number from the first application.

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