Mites on Cultivated Orchids
Paul J. Johnson, Ph.D.
Insect Research Collection
Box 2207A,
Last update: 22 November 2008
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 amongst the most serious pests of cultivated orchids. Common orchid cultural conditions in homes and hobby greenhouses can favor mites, and the use of pesticides removes natural predators and allows development of resistant populations.
Sources and Identification
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. There
are other pest species of mites, but they are generally of less importance.
The most common spider mite
recognized as a persistent pest of orchids is the common two-spotted spider
mite (Tetranychus urticae),
but the carmine spider mite (Tetranychus cinnabarinus) may be an unrecognized pest species in
North America. The spider mites are a
yellowish-green and usually with two large dark areas on either side of the
body at about midlength. They are active species that is easily 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. However, it is possible that in some cases
the red form of the two-spotted may actually be the carmine spider mite. Both species are global, feed on many kinds of
plants (polyphagous), and are easily transported on
many kinds of plants.
Flat mites recognized as
pests on orchids are the orchid mite (Tenuipalpus
orchidarum), the phalaenopsis
mite (Tenuipalpus pacificus)
and the oncidium mite (Brevipalpus
oncidii). Tenuipalpus orchidofilo
was described recently and was reported as a pest of Arundina
graminifolia in 
elsewhere.
Three other species are recorded from orchids, Brevipalpus
phoenicis (red and black mite), B. californicus (omnivorous mite), and B. 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. There are
probably more species on orchids, but the taxonomy of tenuipalpid
mites is poor as is accurate information about their occurrence on
orchids. 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’s
and flat’s are small sized, with two-spotted’s reaching a grand 0.5 mm in length and flat’s
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. Colonization of your plants by mites can be
done at any time, but severe problems may not show themselves until favorable
environmental conditions are present. In
the home and hobby greenhouse spider mites will readily move to orchids from
other plants.
Damage
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 vigor 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 handlens 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 nymphal 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 (86-90°F). Both
kinds of mites will have many generations per year under favorable
conditions. While flat mites may take
6-9 weeks to complete a generation, the two-spotted spider mite can complete a
generation in as little as 5 days in optimum conditions. Like other orchid pests the overlapping of
generations creates a significant mite management problem.
Pesty mites tend to increase in numbers during “rain-less” periods due to the lack of rain, fog-drip or other sources of free water on plants. It is the physical presence and force of impact of water that help keep plant-feeding mite populations low. Spider and flat mites require high relative humidity and occupy a thin static air layer next to plant integument. Under `normal' conditions the mites are widely dispersed on and among plants. In dry conditions, the mites concentrate in the most protected areas in the static air layer next to the plant and between hairs. Further, the silk spun by spider mites acts as a tent to deflect air flow and hold humidity close to the plant. Low humidity and lack of free moisture is also bad for predatory mites and insects, important natural control factors for spider and flate mites. In-doors and in greenhouses, spider and flat mites become serious problems during the winter under the combination of reduced overhead watering and absence of predators, and use of insecticides. In the home, typical gentle misting methods simply are not effective on mites when dry air is circulating from furnaces and heaters. Overall, it is a rather delicate balance between high relative humidity, free moisture, a dry air source, breeziness, temperature, and the presence of predators that keeps mites well managed.
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, ladybeetles, 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. Phytoseiulus persimilis is a commonly used and readily available species. Of course, the use of insecticides and miticides when biological control agents are active is self-defeating, and mite problems can be exacerbated by use of general insecticides.
Rubbing Alcohol
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 amongst 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 Ivory. 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.
Repotting is not very
effective against mites. However, with
an extreme infestation it may be worth repotting a plant as eggs and resting
adults may be in the growing media.
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.
Insecticidal soaps are
usually solutions of a synthetic pyrethrin and
potassium salts of fatty acids, otherwise known as soaps. Pyrethrins are
synthetic analogs of pyrethrum, the natural extract from certain Asteraceae, particularly certain species of Chrysanthemum. Caution is urged with so-called “safe”
insecticidal soaps as some plants are sensitive, particularly tender new
tissues. Piperonyl
butoxide is a common enhancer of pyrethrins
but can cause allergies in some people and may affect plants, too. Some non-orchid ornamentals will drop leaves
and abort flowers when sprayed with insecticidal soaps, so again caution is
urged with prized orchids.
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.
Insecticides and Acaricides
Persistent populations of mite or infestation in many plants usually demand the
need for synthetic pesticides. Mites are
unrelated to insects and most common insecticides are not effective against
mites. Pesticides designed for mite
control are called miticides or acaricides. There are few miticides
specifically registered for use on orchids, but there are many miticides for ornamental plants in general and several are
available as inexpensive home-and-garden solutions. Miticide
formulations not labeled for ornamental plants are often mixed with solvents
that aide in the application of the active ingredient for specific
purposes. These solvents, not
necessarily the miticide itself, often produce phytotoxicity and may seriously damage or kill plants. Thus, never use any chemical that is not
specifically labeled for ornamental plants.
Common insecticides are not
effective against mites, though some do have some weak suppressive action, but will kill mite predators. Acephate (Orthene), malathion,
and disulfoton (Di-syston)
are labeled for mites but are not very effective. Resistance by mites to pesticides is a serious
problem and is in part due to the excessive use and weak action from common
insecticides. Dimethoate,
diazinon, and chlorpyrifos were
recently removed from the market in the
There are many miticides available for ornamental
plants, but some are not tested on orchids, and others are generally too
expensive or otherwise not readily available for the small-collection
grower. Effective miticides
for ornamental use include avermectin (Avid), bifenthrin (Talstar), dienochlor (Pentac), fenbutatin-oxide (Vendex), and fluvalinate (Mavrik). Fenbutatin-oxide is
mixed with acephate and sold in home-and-garden
formulations. Avermectin
is probably the least toxic of these chemicals to people and pets.
Of course, always follow
label directions and never exceed the minimum recommended concentration given
in mixing directions! Recommended
solutions are based on extensive testing for selected pests and plants. Orchids are sensitive to many chemicals,
particularly under direct sunlight or high heat, and while certain species may
not react to a given formulation others may, so your own testing on plants
before general application is recommended.
Home orchid keepers that need
to apply miticides during inclement weather need
special care for applications. If you
cannot spray out of doors, place your plant(s) inside a large plastic bag
(remove the bag after the spray has settled!) and let the plant ventilate where
the fumes will not be wafted around the house or work area.
Final Considerations
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 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. Too, destruction of a sick
plant can be used to justify the purchase of a new and healthier plant!
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 dessicate the
mites.
Generally, never use a miticide not labeled for
ornamental plants. Be thorough. Prophylactic use of miticides
is tempting but does little good as it is a waste of chemical and money, and
allows resistant mites to develop.
Orchid growers with an
entomological penchant and desiring detailed information are referred to the
excellent book Mites of Greenhouses:
identification, biology and control, by Zhi-Qiang Zhang (2003), CABI
Publishing, Oxon (UK) and Cambridge (USA), ISBN 0 85199 590 X.
Image
Credits: False spider mite and spider
mite images are from the North Carolina Cooperative Extension Service and
USDA-ARS, respectively. Other images are
those of the author.