Brochures
Indoor Plant Pests
 Whether they are houseplants on a windowsill or ornamentals in a hobby greenhouse, indoor plants
can suffer from infestations of insects and mites. Because indoor plants occupy
the same rooms as people and pets, it is important to use the safest pest
controls. This brochure describes effective, least-toxic ways to control indoor
plant pests. Most of the controls described are suited for use in home
greenhouses, while some are more applicable to houseplants.
Prevention
Getting a new plant is the most common way of bringing pests into a house. Many serious
pests of indoor plants are not found outdoors in B.C. They can only be spread by
moving infested plants from greenhouses. The best way to prevent pest problems
is to make sure new plants are not infested. This means carefully inspecting
plants (with a magnifying glass, if possible), before you buy. Unfortunately,
you can't do this with plants given as gifts, such as poinsettias or azaleas.
You can, however, keep any new plant well away from the rest of your plants for
at least a month until you are sure it isn't infested.
Controls
Discard It: Unless you are strongly attached to an infested plant, the best remedy is to discard it. This may be painful, but it is a quick way to get rid of difficult problems, such as mealybugs or scales, before they spread to other plants.
Water Washes: For plants with sturdy foliage, washing the entire plant every week or two with a
strong spray of water is an effective way to control aphids and spider mites.
Use room-temperature water and thoroughly wash the leaves, paying particular
attention to the undersides. This is best done outdoors or in a large sink or
bathtub.
Pesticides: The following pesticides are the safest registered for use on indoor plants. Look
for these ingredients on the label as they are sold under a variety of product
names. Always follow directions on the label for mixing and using any pesticide.
Wear rubber gloves to apply sprays. Store pesticides out of reach of children
and pets.
- Insecticidal soap: Soap sprays for houseplants are widely available at garden centres. Soap sprays must
contact the insect or mite to kill it. Plants such as ferns, nasturtiums, sweet
peas and some varieties of impatiens and schizanthus, are damaged by soap
sprays. Before spraying a tender plant, it is a good idea to test spray a few
leaves, wait for a day or two, then check for signs leaf damage.
- Diatomaceous earth (silicon dioxide): This is a non-toxic, white powder made from ground up shells of fossil diatoms mined
from ancient lake beds. It kills insects by causing them to dry up. Because it
also kills beneficial insects, silicon dioxide is not a good choice for outdoor
gardens, but it can be used around indoor plants where there are no beneficial
insects.
- Pyrethrins: These are the active ingredients extracted from flowers of pyrethrum daisies.
Pyrethrins control sucking and chewing insects and must contact the insect to
kill it. Do not use pyrethrins in hot, sunny conditions (wait until evening) or
on sensitive plants, such as cyclamen, delicate ferns or seedlings. Take
infested plants outdoors to spray them or at least move them out of rooms where
there are young children, pets, birds or aquariums.
Biological Controls: Biological controls are natural enemies that prey on pests. The many beneficial insects and
mites that keep pests in check outdoors are absent indoors, but several species
are reared commercially for use in the greenhouse industry. Most can be used
with success in hobby greenhouses; a few species can be used in some situations
on house plants. The advantage of using natural enemies is that they are
non-toxic. They can also completely eliminate pests under good conditions. The
main disadvantage is that they are relatively expensive. The minimum order from
a commercial source is usually enough to treat a small greenhouse, but it is an
expensive way to treat a few plants in a home. Also, biological controls would
not be popular with people who do not want any insects, good or bad, indoors.
Tips on using biological controls:
- Do not use pesticides on the plants. Even low-toxicity sprays and dusts harm beneficial insects. If it is necessary to knock down a high pest population before releasing biological controls, insecticidal soap can be used before natural enemies are released.
- It is essential to identify the pest problem correctly. Most natural enemies are very particular
in their tastes. For example, the aphid midge eats only aphids, while the
whitefly parasite, Encarsia formosa, only attacks whiteflies. An exception is
the convergent lady beetle sold in some garden centres. If kept from flying away
(see next paragraph) they will eat aphids as well as other small, soft-bodied
insects.
- Confine the natural enemies to the plants. In a greenhouse, this means screening
openings so that insects cannot get out of the greenhouse (this also keeps out
some pests). To treat a few infested plants, especially houseplants, this means
keeping the beneficial insects on the plants using a simple cage. Drape a very light-weight, sheer curtain material, or other very fine screening, over the infested plant. Put 20-50 lady beetles or other predators on the plant, then tie the bottom edge of the fabric around the pot to make a simple cage. Let the predators feed for 4-5 weeks before removing the cage. Once they eat all of the pests on the plants, the natural enemies die from lack of food. If a new infestation occurs, you will need to buy more biological controls.
- Be patient. Biological controls take time to control pests. This may take from a few weeks to several months. The best time to start a biocontrol program is in the spring, when conditions favour the beneficial insects.
- Get information from the suppliers. Before buying biological controls, check with the supplier to make sure you are
ordering the right ones for the job. Follow all handling instructions included
with the package. Biological controls are sold at some garden centres and
through gardener supply catalogues; commercial distributors also sell directly
to home gardeners.
Common Pests on Indoor Plants
Spider mites: The first symptom of spider mite attack is yellowish speckles on the leaves. Look on
the undersides of the leaves for fine webs. This is easier to see if you lightly
dust the underside of the leaf with flour. If you see webbing, look closely,
preferably with a 10X hand lens, for tiny (under 0.5 mm) mites moving on the
webs. Plants in hot, dry conditions suffer most from spider mite attack. Mite
outbreaks are common in greenhouses in mid- summer. Rinse plants with water
frequently or mist them daily to keep up the humidity. Spray insecticidal soap
or pyrethrins to control mites on plants. If you want to use biological control,
release the predatory mite, Phytoseiulus persimilis, on plants. These are tiny, reddish-orange, mites that move quickly. They can be kept on individual potted plants by a simple water barrier. Set the potted plant, with its saucer, in a shallow pan. Fill the pan with water up to the lip of the pot saucer so that the plant is surrounded by water, but is not actually sitting in water. If predatory mites are trapped on plants they usually eliminate spider mites completely.
Aphids: If leaves have a sticky coating, look for very small (2-6 mm long) insects on the
undersides of leaves and on the tips of branches. Aphids are pear- shaped and
may be green, black, yellowish or even pinkish, depending on the species. They
are usually found in clusters, often within curled or distorted leaves. There
are many natural enemies of aphids outdoors and it often works to put the
infested plants outside for the summer to let the wild predators control them.
Aphids can be controlled by spraying plants with strong spray of water,
repeating weekly until they are eliminated. For plants that are hard to wash,
spray insecticidal soap or pyrethrins. If you want to try biological controls in
a greenhouse, release a small package of the aphid midge, Aphidoletes aphidimyza, whose larvae feed on aphids. If the greenhouse vents are screened, lady beetles can also be released. Lady beetles can also be caged onto houseplants to control aphids.
Whitefly: Greenhouse whiteflies are tiny, (1 mm long), snow-white, flying insects. They rest on the
undersides of top leaves and fly up when disturbed. Immature whiteflies, or
nymphs, look like minute, whitish scales on the undersides of lower leaves.
These suck the plant sap and excrete sticky honeydew onto the leaves. They are
not hardy outdoors, but are moved around on plants from greenhouses. Control
them by picking off older leaves, where most of the immature whiteflies are
developing. Spray plants with insecticidal soap or pyrethrins to control the
immature stages. To trap adult whiteflies, hang yellow sticky traps among tall
plants or place them on stakes in pots of short plants. Using a hand held
vacuum, you can also gently vacuum the adults from the undersides of leaves
where they congregate. The parasitic wasp, Encarsia formosa, can be used to eliminate whiteflies in small greenhouses over the course of the summer. These mini-wasps are only 1 mm long and they don't sting. Release them two or three times at 2-week intervals in the spring. As the wasps parasitize the whitefly scales, the scales turn black and look like specks of black pepper on the undersides of the leaves.
Soft brown scale: Usually the first noticeable symptom is sticky spots of honeydew on leaves. Scales are
very hard to see, but look closely for small brown, oval bumps on the stems and
leaves. These don't look anything like insects, although they are actually
females scales that have come to rest in a one place. They suck plant sap and
produce thousands of tiny scale offspring, which crawl to other parts of the
plant. These are very difficult to control without using toxic insecticides. The
best approach is usually to discard an infested plant before the infestation
spreads. Pruning out infested branches and regularly wiping scales from leaves
and stems using soapy water will keep infestations down. Sprays of insecticidal
soap provide some control. The predatory beetles, Lindorus spp. and Chilocorus nigritus, can control soft brown scale if they are caged onto plants. These species are available from suppliers, but may not be as easy to get as the other biological controls described.
Mealybugs: Plants infested with mealybugs have cottony white masses on the stems and leaves. Under
the white fluff are clusters of tiny, pinkish, oval, segmented insects, 2-3 mm
long. They suck plant sap, especially on new growth, and drip sticky honeydew on
leaves. Mealybugs are very difficult to control with sprays because they are
protected by the waxy fluff. If only one plant is infested, it is best to
dispose of it before the infestation spreads. Wash mealybugs from plants with
water, or spray insecticidal soap or pyrethrins. A predatory beetle, called the
mealybug destroyer or Australian lady beetle (Cryptolaemus montrouzieri) is an effective control if caged on the infested plants.
Fungus gnats: Tiny black or dark grey insects (2 mm long) that seem to be hovering around the soil
and resting on pots or window glass are likely to be fungus gnats. The maggots
of this tiny fly are in the soil, feeding on fungi. When they are too numerous
they can damage the fine hairs of plant roots, but in most cases they are just a
nuisance. Where fungus gnats are a problem if often means that plants are being
overwatered. This should be corrected as soon as possible. The adult gnats are
attracted to bright yellow and can be trapped on yellow sticky traps mounted on
stakes in pots or hung from overhead. A light dusting of diatomaceous earth on
the surface of the soil of the potted plants controls fungus gnats as they
emerge.
1995
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