Bastiaan M. Drees, Professor and Extension Entomologist
Department of Entomology
Texas A&M University System
Four groups of mites are important in the greenhouse (Table 1): 1) spider mites (two-spotted spider mites); 2) false spider mites or flat mites; 3)broad and cyclamen mites; and 4) bud, gall, rust or eriophyid mites. Mites are small and are thus difficult to identify. However, knowledge of the species and its biology is the first step in designing a strategy to best manage them:
1) Two-spotted spider mites have round, pale yellow to reddish eggs deposited on the leaves; nymphs and adults have two prominent greenish to brownish dots on their bodies; these mites spin webs for protection and dispersal; damage includes stippling or bronzing of leaves.
2) False spider mites or flat mites have a longer life cycle than two-spotted spider mites; eggs are bright red and oval-shaped and are laid in clusters on both surfaces of leaves; adults are reddish with black patterns on their backs and produce no webbing; damage includes silvering of leaves, but stems are also attacked.
3) Broad mites produce eggs with many tubercles or facets which appear like "jewels" under the microscope; nymphs remain in cracks and crevices of leaves and plants; after some development, nymphs enter a quiescent of sessile stage; females are clear after emergence but soon turn straw colored and develop a prominent white stripe down their backs; damage appears on the younger foliage and on the undersides of leaves, giving leaves scalloped edges resembling physiological damage.
Cyclamen mites produce clear, pearlescent oblong eggs; nymphal stage bodies are constricted behind the third pair of legs; females are clear after emergence but turn amber with age; damage includes deformed buds, and these mites are not found on open leaf surfaces; they do not attack chrysanthemums.
4) Bud and rust or eriophyid mite bodies are elongate and cone-shaped, tapering toward their backends and the legs are very short; damage includes galls, deformed or discolored plant tissue.
Monitoring methods. Mites are small and difficult to see with the naked eye. Using a 10x hand lens will enhance your ability to see mites and their eggs. Some mites (eriophyid mites, broad and cyclamen mites) are so tiny that a dissecting scope may be necessary.
Spider mites can be detected by looking for the typical mite-caused damage, mites or symptoms of mite infestations such as cast skins and webbing. Spider mite damage is easy to see. Their mouthparts are small toothpick-like structures (chelicerae) that they use to poke holes in cells before they suck out the cell contents using their other mouthparts (palpi). The result is small clusters of empty cells that appear from a distance like stipples. These stipples turn brown or bronze after a while. Injured leaves take on a bronze appearance, with most of the damage occurring around major leaf veins.
The mites, eggs and cast skins can best be seen by examining the under surfaces of randomly-selected leaves. Mites can also be sampled using the "beat method" whereby plant parts are beaten onto an off-white piece of paper or card. The dislodged mites can then readily be seen crawling on the paper. This method works particularly well for evergreens and small-leaved plants.
Cultural Control. A number of greenhouse production practices can affect mite outbreaks. Use of clean, pest free plants and cuttings is essential. Knowledge of mite prone species/ varieties can enable the grower to avoid these plants or to monitor these most closely as "indicator"plants. Watering practices affect spider mite populations. Drought-stressed plants are most prone to mite outbreaks while overhead sprinkler systems are less favorable for mite outbreaks.
Biological Control. A number of predatory mite species are available for spider miter control in the greenhouse: Phytoseiulus persimilis, Mesoseiulus longipes (=Phytoseiulus longipes), Metaseiulus occidentalis (=Galendromus occidentalis), and Neoseilus californicus (=Amblyseius californicus) have been marketed for released into greenhouses, occasionally under products names such as Spidex® Shaker Bottle, Spidex-Plus® Paper Sachets. These mite predators have unique properties. For instance, M. occidentalis tolerates a wide range of humidities and some strains are tolerant of azinphosmethyl (Guthion®) and carbaryl (Sevin®). M. longipes better tolerates high temperatures and drier conditions. N. californicus is better able to survive at lower spider mite densities and take longer to suppress populations. Frequently, combinations of two or more species are released. They are best used as preventive releases made periodically at or before the first detection of spider mites or their damage. Occasionally they are applied as "biotic insecticides" using an "inundative release" to try to bring down an existing population. The price, availability of obtaining and releasing these predaceous mites should be considered and may be prohibitive. Furthermore, pesticide use before and after making releases may impact the success of augmentive releases.
Physical Control. High-volume, high pressure water sprays, as achieved through some application devices such as the Water Wand and Jet-All Water Wand, can dislodge many mites from foliage and temporarily suppress mite populations. Rose growers in east Texas are reportedly using these devices successfully for spider mite suppression.
Chemical Control. The Control Options Database presents a partial listing of currently registered products available for control of ornamental plant insect/mite pests. The objective in developing this list was to list specific examples of products containing various active ingredients. There are many other products marketed that contain the same active ingredients that may or may not be similarly formulated.
Many insecticides also have miticidal activity, while some insecticides affect only some types of mite species or have no effect on mites at all. In fact, over-use of some insecticides (ie. carbaryl) can actually lead to spider mite outbreaks later on. Insecticide/ miticide products are best used when a complex of mite and insect pests are present. Before releasing predatory mites, insecticidal soap can be applied to reduce the number of spider mites without leaving a harmful residue after it dries that will affect these natural enemies. Whenever possible, use the most target-specific, least toxic available, and use in strict accordance to directions provided on the product label.
Different miticides have different performance characteristics. For example, Avid® is somewhat systemic in that it penetrates into treated foliar plant cells where it remains active while the material that remains on the leaf surface rapidly breaks down; has a unique mode of action on the nervous system; and has few phytotoxicity problems. Pentac® is relatively slow acting; is U.V. light sensitive; has few phytotoxicity problems; has no reported resistance; has some ovicidal activity; and usually requires two applications. Thiodan® is phytotoxic to some varieties of chrysanthemums and geraniums. Insecticidal soap is a contact miticide/insecticide and has no residual activity. Sulfur, registered as a miticide on some vegetable crops, is highly phytotoxic; forms residues on treated plants; has some fungicidal activity; and is inexpensive. Some products have a greater negative impact on natural enemies than others and some strains of predaceous mites have been selected to be able to toleratesome pesticides.
Adjuvants: Tank additives (spreaders, stickers, buffers, etc.) can affect the performance of miticides applied. A good rule of thumb to follow is: use an adjuvant only when increased performance has been demonstrated. Use of a spreading or wetting agent will improve the efficacy of most miticides, particularly on waxy-leaved plants. Insecticidal soap has been used to increase penetration of miticides into the mites. Conversely, use
of some types of adjuvants may actually decrease effectiveness of some miticides (ie. Nu-Film-P plus Avid®) or lead to phytotoxic responses treated plants (ie. spreader-sticker materials or agricultural oils plus Joust®). Check with product labels or manufacturers for specific information regarding tank mixtures and use of adjuvants. Results of evaluations to determine if the addition of the mite pheromone product, Stirrup-M®, increases performance of miticides have been inconclusive.
Resistance management: There is heightened concern about the proper use of available products to extend their effectiveness because of 1)the continuing loss of insecticide/miticide products available to the commercial nurseryman (ie. Kelthane®, Plictran®, Temik® 10G, Vendex®,Vydate®), and 2) the ability of certain mite species to become resistant to pesticides (ie. spider mite resistance to Kelthane®, pyrethroids). Spider mites breed rapidly and can therefore resistant/ tolerant strains can develop quickly. Use of products with any single mode of action (particularly on the nervous system) can provide such selection pressure. Theoretical methods used to avoid intensive selection pressure include: 1) Use insecticide/miticides only when mites or plant injury they cause is first detected; 2) Use the lowest effective miticide rates initially, reserving use of increased rates as pest densities increases; 3) Use "long" rotation of miticides with different modes of activity. Use of tank mixtures containing two or more products with different modes of action on the mite's nervous system or use of "short" rotation (use of a product with different mode of activity for every treatment) is believed to select for "multiple resistance."
The twospotted spider mite develops in roughly two weeks (5 to 20 days, depending on temperature and conditions). As an example of a "long" rotation program, could include: 1) a series of applications using one miticide such as Avid® (with or without insecticidal soap) for a two week period (approximate length of one generation) followed by 2) if mites are still present, apply a series of treatments using a miticide with a different mode of action such as Pentac® (with or without insecticidal soap) for another two weeks. Thereafter, a series of treatments using a miticide with yet a third mode of action or reverting back to using Avid could be initiated if needed. Frequency of applications and rates should be based on the presence and density of spider mites, avoiding treatments when mites are undetectable.
| hortIPM Publications | hortIPM Index |