Got mites on your orchid?
Yellow speckles or browning of leaves on your orchids? Webbing of silk on various plant parts and no spiders to be seen? Consider mites as possible culprits…
Mites are tiny creatures related to spiders and ticks, and are not insects. Plant-feeding mites can be thought of as plant parasites and are often thought of as the most serious pests of cultivated orchids. Common orchid cultural conditions in homes and hobby greenhouses can favour mites, and the use of pesticides removes natural predators and allows development of resistant populations.
Mite species that are pests on cultivated orchids generally fall into two main categories, spider mites and flat mites. The latter are also called false spider mites, but the name flat mite is preferred as it is accurately descriptive and avoids confusion with spider mites.
Spider mites are a pest that affects many crops worldwide. There are well over 1200 species of spider mite, of which more than a hundred can be considered as a pest, and about ten of those as major pests. The most well-known and problematic spider mite is Tetranychus urticae (common names include red spider mite and two-spotted spider mite). Their ability to reproduce extremely rapidly enables them to cause enormous damage in a short period of time.
The spider mites are a yellowish-green and usually with two large dark areas on either side of the body at about mid length. They are an active species that can easily be seen wandering the plants. Spider mites received their name because of the silk webbing that they produce, not because they may appear like small spiders. The two-spotted is also known by other common names, including the “red spider mite” because of an orange-red over-wintering form.
Flat mites recognized as pests on orchids are the orchid mite Tenuipalpus orchidarum, the phalaenopsis mite Tenuipalpus pacificus and the oncidium mite Brevipalpus oncidii. Three other species are recorded from orchids, Brevipalpus phoenicis (red and black mite), Brevipalpus californicus (omnivorous mite), and Brevipalpus russulus, but these reports are not verified and may represent misidentifications. Flat mites are native to tropical and subtropical habitats and hosts, and are moved globally by the plant trade.
Flat mites are smaller than two-spotted spider mites, difficult to see without magnification, and move very slowly.
Other mites frequently found associated with orchid culture include predatory mites that feed upon pest mites. There are many innocuous mite species that feed on fungi, bacteria, and decaying organic materials. There are also a number of beneficial mites that are predators on plant-feeding mites, insect pests, and other critters. Oribatid mites that look like tiny round, dark-colored beetles feed on fungi on plant parts and decaying organic materials. A large diversity of yellowish to light brown mites are frequent in potting media and may occasionally be found on plants. These are usually large, >1.0 mm in length and easily seen.
The two-spotted spider mite is probably the most important mite pest of cultivated orchids in all areas, but flat mites are very common and are often not diagnosed properly. Both two-spotted and flat mites can become problems in greenhouses and homes. Because of the small size of these mites, and great similarity among related species, their accurate identification is difficult and often requires the help of an experienced entomologist with a high quality microscope.
In general, two-spotted and flat mites are small sized, with two-spotted mites reaching a grand 0.5 mm in length and flat mites reaching a mere 0.3 mm in length. All of these mites are pale yellowish-green to orange-red color and often with two or more black areas visible through their integument. All bear conspicuous pale hairs. Two-spotted spider mites spin networks of silk webbing that protects their colonies from predators and helps maintain high humidity near the leaf surface. This webbing is also protective against pesticide sprays. Flat mites do not spin this webbing.
Typically, mites are always present in low numbers. This makes managing cultural conditions important for mite control. Mites will readily move between plants, float on air currents, be introduced on new plants or those brought indoors from the garden, and the eggs or resting stages may be in potting media. Colonisation of your plants by mites can be done at any time, but severe problems may not show themselves until favourable environmental conditions are present. In the home and hobby greenhouse spider mites will readily move to orchids from other plants.
All of these mites may be found on a wide variety of orchids. In addition, the two-spotted spider mite is known to feed on hundreds of different plant species. The larvae, nymphs, and the adults all feed by puncturing cell walls and sucking cell contents, particularly chloroplasts. The killing of individual cells or groups of cells produces the transparent, yellow, or tan patchwork of damage that indicates mite infestation. Feeding may be done on many plant tissues, but mostly on leaves and buds and can cause these to drop prematurely. Heavy feeding produces a patchy chlorotic appearance to leaves, and portions of or the entire leaf may turn dry and brown. This damage generally reduces the vigour of plants and may kill plants. Mites may also transmit certain viruses.
Flat mites often feed on the upper surfaces of leaves and this will create a pock-marked appearance from empty and collapsed leaf cells. This type of damage is particularly easy to see on infested Phalaenopsis leaves. Flat mite feeding on thin leaves, especially the underside, is similar to the stippling caused by spider mites, but there is no webbing. Mite damage is permanent, so it is best to manage mites at low populations than to experience heavy infestations. Thin or soft-leaved orchids are more susceptible to mite damage than those with thicker leaves, but no species or variety is immune.
Both two-spotted spider mites and flat mites have five life stages: egg, larva, protonymph and deutonymph (or nymphs), and adult. The larva has only six legs, but the nymphs and adults have eight legs. Eggs are laid by females on the surface of plant structures and are often hidden in crevices. Eggs and larvae are very tiny and are nearly impossible to discern without magnification. A good hand-lens is useful for seeing even the adults.
Developmental rates of mites are dependent upon temperature. In general, the higher the temperature the shorter the life cycle. The egg may take upwards of three weeks to hatch for flat mites, but only 1-2 days for two-spotted spider mites, at standard indoor temperatures. While larval and nymph stages usually take 5-6 weeks to reach adulthood for flat mites, it may take only 1-3 weeks for two-spotted spider mites. Optimum temperatures for development are 30-32°C
Management and Control
Two-spotted spider mites and flat mites are small and relatively delicate creatures. The easiest method for keeping mites under control is to regularly spray, or syringe, the plants with water. In the home placing your plants in a shower or using a sink sprayer is very effective. Mites are readily washed from the plants or are damaged by a heavy spray. In a greenhouse regular spraying and misting is effective.
Biological control of mites is feasible even in small hobby greenhouses. Numerous predatory insects attack mites, including lacewings, ladybugs, and wasps. The use of predatory mites is particularly successful in greenhouses. Most of the predator mites that are sold by suppliers are from several genera. See https://www.bioforce.co.nz/ for what you can get in New Zealand.
Light infestations restricted to one or a few plants can usually be treated with household products.
When possible, immediately isolate infested plants from others to prevent the mites from moving through out them.
Probably the most popular home remedy is to spray plants with a mixture of isopropyl (rubbing) alcohol and liquid mild dish detergent, such as Earthwise.
Do not use other alcohols, such as ethanol or methanol, as these will penetrate the plant tissues and cause considerable damage! The concentration of the isopropyl seems to make little difference, the common 70% concentration available in stores is satisfactory. Alcohol treatment is effective against all the life stages of mites, except eggs.
Horticultural oil, neem oil, mineral oil, and insecticidal soaps are readily available, inexpensive, and effective against mites. Oil solutions smother the mites so a complete coverage of all sprayed plants is essential. These oils are mixed with water and usually a plant-safe detergent or commercial spreader-sticker should be used for enhancing the effectiveness of the oil. The main caution with these oil solutions is that they should never be applied to plants on hot days (>85ºF/29ºC) or in direct sunlight, as to prevent burning of tissues. Leave the plant in shade until the application has dried. Some plants or parts, such as buds and blooms, are sensitive to oils so due care and consideration is urged.
Because the life cycle of mites is so short and there are overlapping of generations, to bring a serious problem under control you may need to do treatments every 1-3 weeks. The time period between control efforts will depend upon the growing conditions, especially temperature: greater frequency in a warm greenhouse, less inside a house. As with any pest, persistence is a key to success and correlating the control method to the mite species is important for effective management. Cultural conditions are a key to managing mite populations.
Heavy infestations of mites, especially on many plants may require extensive control methods. Since the damage done by mites is permanent, constant management of the population is more effective than control of a major infestation. On the extreme side if you have a plant showing signs of severe change or general decline from mites you may have to seriously consider destroying that plant, as the likelihood of rejuvenating that plant may not justify the expense and effort of continued treatments. Destruction of a sick plant can be used to justify the purchase of a new and healthier plant! YAY!
If you are battling mites for long periods of time (e.g., >2 months) and have been using the same miticide then you likely developed a resistant population of mites. Remember the short generation times of mites. The best resolution to this is to change methods and chemicals frequently; that is do not use the same chemical mix more than 3-4 times sequentially. After isolating infested plants give them a thorough application of something different from what you have been using. Resistance is not a problem with alcohol, oils, and soaps as these suffocate or desiccate the mites.