orchid problems MEaly Bugs

Orchid problems Mealybugs on Orchids

Paul J. Johnson, Ph.D.

Insect Research Collection

Box 2207A, South Dakota State University

Brookings, SD 57007

Updated: 25 February 2009

Mealybugs are serious pests of orchids and next to scale insects are probably the most

difficult to control pests of orchids in homes and greenhouses. Most definitely, they need

to be dealt with immediately upon discovery. The damage done to plants by mealybugs is

considerable, causing a loss of vigor and a weakening and loss of leaves, buds, and

flowers through their feeding. In addition, mealybugs create copious amounts of

honeydew which make plant parts sticky, attracts ants, and provides a substrate for sooty

mold. Though some mealybugs vector plant viruses apparently no orchid viruses are

known to be transmitted by these insects. Mealybugs are not particular about their host

and probably all species of orchids are susceptible to mealybugs, especially when

cultivated.

Identification

Nearly 300 species of mealybugs are known from

Canada and the United States. Fortunately, only a few

species are common or serious pests of orchids.

Mealybugs are classified in the family

Pseudococcidae, and are closely related to the scale

insects. In fact, mealybugs can be thought of as a kind

of soft scale that does not form the protective cover

that most scales produce for protection. The pest

species are in the genera Pseudococcus, Planococcus, Phenacoccus, and Dysmicoccus.

Immature to adult mealybugs may measure 0.5-8.0 mm in body length. All of the known

orchid feeding species are coated with a waxy secretion that hides the body of these

insects. The more common species of these odd insects that infest orchids are

immediately recognized in the adult stage by the white, yellowish-white, whitish-grey, or

pale pink to pale blue in color coating. The body is oval and the sides of the body have

short waxy filaments and there may be 2-4 short to long filaments on the posterior end of

the body. These filaments sometimes give the impression of numerous legs.

Mealybugs can be found on all plant parts, but especially roots, rhizomes, pseudobulbs,

and the underside of leaves. They are adept at hiding on roots and rhizomes deep in the

potting media, in crevices and under sheaths. Unlike scales, mealybugs wander in search

of feeding places and will leave plants, be sure to check for them in cracks and in joints

on benches, under lips of pots and trays, and other hiding places. The immatures are

small, and white to yellowish or pale pink. Hatchling nymphs, or crawlers, are not easily

seen without a magnifier and hide under cover, but older nymphs appear like diminutive

adults.

Orchids become infested with mealybugs in some combination of three methods:

purchase of an infested plant, movement from infested to un-infested plants that are in

contact with each other, and windblown colonization. Mealybugs are active and will

crawl from one plant to another, pot to pot, and across benches. Mealybugs will leave

plants and hide under rims of pots and trays, in bench crevices, and even drop from

overhead plants. Spread of crawlers can occur both indoors and outdoors by floating on

breezes or air currents produced by circulating and heater fans. The occurrence of

infestation hotspots may be due to crawlers settling on plants where the air currents are

the weakest. Similar effects are found with aphids, scales, and spider mites.

The identification of mealybugs is difficult and often requires the services of a taxonomic

entomologist specializing on these insects. Because of this difficulty, accurate

information on the identification and biology of species that may infest orchids is much

poorer than one would hope. Undoubtedly, all the orchid infesting species were tropical

or subtropical in origin, but the most problematic species have adapted to indoor life and

may feed on hundreds of species of ornamental plants other than orchids.

According to identification records kept by the

Systematic Entomology Laboratory, U.S. Dept. of

Agriculture, 39 species of mealybug are reported from

orchids. Fortunately, only a few species are problematic

in Canada and the United States. However, it is very

easy for any of these species to be transported unseen.

Consequently, extreme caution and due care is urged to

anyone transporting orchids between states or countries.

In most of Canada and the United States, the longtailed

mealybug (Pseudococcus longispinus) is probably the

most common and problematic species on orchids,

particularly in homes and greenhouses. This is also the

most easily recognized species because of a pair of very

long filaments on the posterior of the body.

In California the longtailed is very common. However, five additional orchid feeding

species are known: orchid mealybug (Pseudococcus microcirculus), imported mealybug

(Pseudococcus importatus), obscure mealybug (Pseudococcus obscurus), pineapple

mealybug (Dysmicoccus brevipes), and the solanum mealybug (Phenacoccus solani).

Apparently, the orchid mealybug is the most problematic species in California,

particularly in greenhouses.

In Hawaii the longtailed and pineapple mealybugs

are common on orchids. In addition there is the

dendrobium mealybug (Pseudococcus

dendrobiorum), Jack Beardsley’s mealybug

(Pseudococcus jackbeardsleyi), and the grape

mealybug (Pseudococcus maritimus).

Life Cycle

Mealybugs have a three-stage life history: egg, larva (nymph or crawler), and adult. Eggs

are laid within a waxy coated egg sac produced by the female. The eggs hatch after about

a 10 days into the mobile nymphs, the crawlers, that appear as diminutive adults. The

crawlers are the most active stage that can move between plants and will develop through

several growth periods before becoming adults. Adults of most species are also active.

Thus, unlike scales where the crawler finds a suitable site for feeding and remains fixed,

mealybugs will move about to find feeding sites. However, the most common pest

species is the longtailed mealybug and it is parthenogenetic; no males are known of this

species.

Male mealybugs do little feeding and only in their youngest crawler stages. Mature males

are small (1.5-2.5 mm) winged creatures whose primary function is to mate, and then

die. Females and immatures do not fly, but they will crawl off of the plant and migrate

thoughout a growing area.

In temperate regions, mealybugs usually have only one or two generations per season. In

a warm greenhouse or indoors there may be upwards of 8 overlapping generations per

year. Out-of-doors in cold climates, cold-tolerant species of mealybugs hide in protected

places, such as under tree bark, among roots, and in compost.

Management

Outdoor mealybugs are vulnerable to a variety of parasitic and predatory insects,

including wasps, brown and green lacewings, and lady beetles. Weather, especially

heavy rains, also help to keep mealybug populations low. Indoors, mealybug

management is difficult because of their propensity to move into the potting medium and

feed on roots, or for the crawlers to work their way into tight places. Repeated

application of any treatment is required to kill the immatures, and treatments are at their

greatest effectiveness against the small crawlers. Hand removal is effective only for the

obvious adults and larger nymphs. All control efforts must begin immediately following

discovery. Even light infestations

restricted to one or a few plants can

explode rapidly and necessitate chemical

methods. When possible, immediately

isolate infested plants from others to

prevent the mealybugs from moving

amongst them. Also, check the lips and

cracks of pots, trays, and benches because

females will wander and leave the plant to find

hiding places. If plants other than orchids are

grown, check those also as they may be a source of infestation.

Because the life cycle of mealybugs can be so short combined with the overlapping of

generations, you will need to do a treatment every 10-14 days in order to bring a serious

problem under control. Because mealybugs are such a problem there are few effective

"home remedies" available. To deal with an established infestation, the use of an

insecticide will likely be necessary. Be aware that non-insecticidal treatments are often

not very effective for elimination of mealybugs without diligent application and followup

treatments.

Rubbing Alcohol

Probably the most popular home remedy against mealybugs is to swab and daub plants

with a cotton-tipped swab or ball of cotton dipped in isopropyl (rubbing) alcohol. Do not

use other alcohols, such as ethanol or methanol, that can penetrate the plant tissues and

cause considerable damage! The common 70% isopropyl available in stores is

satisfactory. On hard-leaved plants, gentle rubbing with the fingers, a cotton ball, cottontipped

swab, or a soft infants toothbrush is effective. Remove all mealybugs, large and

small. Afterwards, you will still need to repeat the alcohol treatment to remove the tiny

yellowish spots which are the recently hatched crawlers. Pay particular attention to the

folds, crotches, branch bases, midrib areas, and roots. Spraying the alcohol with a

misting bottle or small pump sprayer is effective, but dribbling alcohol into tight areas is

necessary. To avoid get a spray solution on window-sills, table tops, furniture, non-target

plants, etc., move the plant(s) to a large sink, bathtub, or shower stall, then move them

back to the growing area when they dry.

Many home growers will mix with alcohol a small amount of mild liquid dish detergent,

and sometimes mineral oil, neem oil, or horticultural oil. Vegetable oils will work, too,

but in sunlight they can turn rancid quickly, and become smelly and lose effectiveness.

One recipe for a 1.5 liter spray bottle is to mix a 50:50 solution of isopropyl and water,

with a few drops to about a teaspoon of liquid soap to act as a spreader, and a teaspoon of

one of the oils. But, it seems that every grower has their own proportions of these

ingredients, none of which seem to work significantly better than another. Caution is

urged, however, as excessive amounts or too strong of a detergent, or use of an ammoniabased

chemical cleaner may damage your plants, particularly buds and flowers. This is

true of dish-soaps and household detergents that could remove natural protective waxes

from plant tissues. Also, spraying of alcohol is not always effective against eggs which

are often well hidden, hence the need for thoroughness and repetition.

Repotting

Even a light to moderate infestation of mealybugs should be of concern. These

insects like to move into the potting media and feed on roots, or move off of the

plant to find hiding places to lay eggs. Unless the roots are checked and the

media changed, removal of mealybugs from only the upper plant portions is not

a guarantee of success. The potting medium can harbor eggs and crawlers, so

dispose of it in a compost pile or in the garbage. When repotting, a close

inspection, and if necessary a very gentle cleaning and spraying of the roots

before repotting is essential.

Oils and Soaps

Horticultural oil, neem oil, mineral oil, and insecticidal soaps are effective for

mealybug suppression. The oils and soaps are often regarded as "organic" or

non-chemical methods, but this is a misconception or an extremely broad and

nearly meaningless concept of "organic." Indeed, neem oil is extracted from

the neem tree, but horticultural oils and mineral oil are petroleum distillates.

Likewise, insecticidal soaps are a solution of synthetic pyrethroids mixed with

a mild detergent that is made from petroleum products. However, all of these

solutions are generally considered safer for humans, pets, and plants than usual

insecticides. None provide absolute control over mealybugs, but frequent use

during the presence of crawlers can serve to reduce their populations

dramatically.

Horticultural, mineral, or neem oil solutions smother the insects, so complete

coverage of all sprayed plants is essential. These oils are mixed with water and

usually a plant-safe detergent for enhancing the spreading and sticking of the

oil. The main caution with these oil solutions is that they should never be

applied to plants on hot days (>85° F) or in direct sunlight, as to prevent

burning of tissues. Leave the plant in shade until the application has dried.

Unpublished anecdotes suggest that the flowers of some orchids are sensitive to

neem oil, such as species of Miltonia and Masdevallia.

Insecticidal soaps are usually solutions of a synthetic pyrethrin and a plant-safe

detergent. As with oils the detergent acts as a surfactant and spreader for dispersing the

pyrethrin evenly, and as a mild caustic against the insects. Also, to prevent sunburning

apply the chemical and allow it to dry in shade. Pyrethrins are synthetic analogs of

pyrethrum, the natural extract from certain Asteraceae. Caution should be urged with

so-called "safe" insecticidal soaps as some plants are sensitive, particularly

tender new tissues. Some non-orchid ornamentals will drop leaves and abort

flowers when sprayed with insecticidal soaps, so caution is urged with prized

orchids.

Insecticides

Persistent populations of

mealybugs or infestation in many

plants may demand the need for

use of synthetic insecticides.

There are several common,

inexpensive, home-and-garden use

pesticides labeled for ornamental

plants. Insecticide 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 insecticide itself, often produce phytotoxicity and

may seriously damage or kill plants. Thus, never use any insecticide that is not

specifically labeled for ornamental plants. Some of the more available and effective

insecticides that come in various brand names are acephate (e.g., orthene), malathion,

carbaryl, and diazinon. Pyrethrins and rotenone have limited effectiveness. 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 tough plants, but 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 testing is justifiable.

Some insecticides are occasionally discontinued for use because of some

discovered hazard. For example, Cygon used to be available, but it no longer

recommended and labeled for orchids because it will damage many plants,

especially the buds and flowers, and is extremely hazardous to use. Although

most insecticides with discontinued labels are legally allowed to be "used up",

it may be best to dispose of such chemicals rather than continue their use and

risk damage or loss of plants, or increase your own health hazard.

Most home orchid keepers and growers in northern states that need to apply insecticides

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. Again, you may have to consider removing the potting medium, spraying

the plant, and repotting it with new media in a clean pot when the spray has dried.

Growth Regulators and Chitin Inhibitors

These classes of insecticides have great potential for use in orchid pest

management. Growth regulators are relatively expensive, but the cost per

application is less than botanical oils.

Kinoprene (tradename = Enstar II) is a synthetic form of juvenile hormone which is

highly important in insects at critical stages of their metamorphosis. The use of

kinoprene interrupts the normal development of the insects, including mealybugs, scales,

aphids, and whiteflies. This insect hormone appears safe for humans and pets under

usual use precautions. Experience on its use in greenhouses and home collections

suggest that this may be the best new generation pesticide for controlling many orchid

pests, including mealybugs.

Bifenthrin and other growth regulators are also available for use on ornamentals, but little

information is available for orchids. Some of these new chemicals are very effective but

are also highly regulated and may not be available in some states for non-commercial

uses.

Azadirachtin (tradenames = Azatin and Neemazad) is a plant derived chemical that is a

chitin inhibitor. Chitin is a primary compound used by insects when developing their

integument, or exoskeleton. Azadirachtin reduces the insects’ ability to properly develop

its integument and causes mortality through incomplete development. There is little

information available on this chemical for use on orchids, but it is available on a wide

variety of ornamentals, is labeled for greenhouse applications, but may be too expensive

for most home greenhouse uses.

Biological Control

There are many parasitic wasps and various predatory insects that feed on

mealybugs outdoors, but these

species are rarely of value in a

small greenhouse or in the home.

Usually for the small collection

orchid keeper the use of biological

control agents in general is very

limited or not effective. However,

the keeper of many plants in a large

greenhouse or a commercial

grower may wish to consider the

use of one or more parasitic or

predatory insects to help keep mealybugs under control. As in all biological

control efforts eradication is not possible. Also, anyone wishing to use

biological control agents needs to balance their use with proper timing or avoid

the use of insecticides so as not to kill the beneficial insects.

Biological control agents that are available commercially include a variety of

tiny parasitic wasps, brown lacewings, green lacewings, and lady beetles.

Montrouzier’s lady beetle, or mealybug destroyer, Cryptolaemus montrouzieri,

is highly effective for control of mealybugs in greenhouses.

Final Considerations

Heavy infestations of mealybugs, especially on many plants, may require

severe control methods using insecticides. On the extreme side if you have a

plant showing signs of decline from infestation you may have to seriously

consider destroying that plant, as the low 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 mealybugs for long periods of time (e.g., >9 months) and

have been using the same insecticidal control method then you probably

developed a resistant population. The best resolution to this is to change

methods and chemicals occasionally; 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.

For example, if you used insecticide then switch to an oil, soap, or different

insecticide. Resistance is not generally a problem with growth regulators, such

as kinoprene.

Generally, never use an insecticide not labeled for ornamental plants.

Whenever using oils, soaps, and insecticides, be thorough, change formulations

frequently, and do not use less than the minimum concentration of mixture, or

more than normally recommended. Too little of a chemical enhances

resistance, while too high of a concentration may damage the plant. Unless you

are a commercial grower rotating mixtures of chemicals do not use chemicals

prophylactically, that is do not routinely use chemicals as a preventative as it is

a waste of chemical (and money!) and such use allows resistant mealybugs to

develop. Finally, keep up the manual removal of all mealybugs, if possible.

Mealybugs are an excellent example of pests that are easily transported and

create tremendous problems. Although most orchid keepers in North America

obtain their plants from conscientious growers in either Canada or the U.S.,

many persons do purchase plants while traveling, in exchange from friends, or

from questionable sources. Everyone needs to be aware of the great potential

of inadvertently dispersing species to new areas, particularly from international

originations. There cannot be enough stress placed on the recommendation that

all plants come from a reputable and quality grower, and are clean of pests.