Houseplant Pests To Watch For This Winter Back »

An orchid with aphids on the flowers.

The latest snow storm is blowing in so it is nice to be able to stay inside where it is nice and warm. While the outdoor landscape may be shades of brown to white it’s nice to be able to enjoy a little bit of green indoors, thanks to some houseplants. Some may even provide flowers from time to time to really help brighten those dark days of early winter. You may have brought in a few geraniums, took some impatiens cuttings and gotten them to grow, maybe a hibiscus, an orchid or two, or perhaps some variegated foliage like that of the spider plant, snake plant, or a weeping fig. Unfortunately all of these plants are good subjects for pest problems, particularly if you had them outside during the summer.

Many insect pests, like aphids for example, are pretty common outdoors. There are actually dozens of different kinds of plant-damaging aphids, some of which are pretty specific as to which plants they feed on while others will feed on just about any plant. If your houseplant happens to be sitting within crawling distance, aphids from nearby plants might crawl over, fall onto or fly to your plant.

Predatory insects and mites as well as insect parasites are fairly common in most landscapes too. Insects like lady beetles, lacewings, minute pirate bugs and beneficial mites can keep many pest populations low outdoors but are far less likely to do their job once you move your plants inside for the winter. Consequently, what was just a few scattered insects on an outdoor plant could turn into a major infestation after several weeks sitting in a warm home.


Aphids

An aphid colony.

Aphids or plant lice are one of the most common outdoor insect pests that might be brought in for the winter. They are small, about 1/8” long, rather rounded in shape, with six legs and two small antennae. They will often have also two small projections off the back of the abdomen called cornicles. Aphids prefer to feed on tender young shoots or flower buds. They can cause leaf malformation or interfere with flower bud opening. Aphids are sap-sucking insect pests. They insert their hypodermic-like mouth parts down into plant cells to extract the sap. They may reproduce by eggs or by live birth with dozens of generations possible in a single growing season. You usually will not just find one aphid but more likely a whole colony on the underside of a leaf or lined up along the outside of a tender shoot or flower bud.

Aphids normally just crawl on plants, but they can also develop wings.

Aphids are usually wingless but at times may develop wings and fly off in search of other plants to feed on if it gets too crowded on the “home plant” or when it gets cooler in the fall of the year. Ants have been known to move aphids from one part of a plant to another or to a different plant entirely. They “farm” the aphids, much like we do cattle. But instead of meat, the ants are looking to collect honeydew from the aphids. The ants actually stroke the aphids to induce them to excrete the honeydew, almost like milking a cow, which gives rise to another common name for aphids, “ant cows”.

Aphids can be fairly easily controlled. My preferred method is to use a forceful jet of water, like from the sprayer in your kitchen sink. Be sure to give the entire plant a good spray to dislodge as many of the aphids as possible. Even then, you will likely have missed a few that survived your treatment so you will have to treat again in a few days to a couple weeks later. Insecticidal soaps and horticultural and botanical oils are also a great treatment for aphids. Most of these are contact killers, which means that you have to get the spray on the aphids in order to kill them. Once the spray is dry, there is little residual effect. Be sure that the product you use is meant to be used on houseplants and will control aphids. These methods of treatment do not work well, however, for small plants, like pepper seedlings that you may start in the spring, one of the aphid’s favorite foods. Do not use a dormant oil spray product or you could seriously damage your plants! Some people will use “dish soap” instead of insecticidal soap. This can be effective too but be careful as some brands may damage the plants too. Other houseplant insecticides, usually containing permethryn or pyrethrum, may also be used but be sure to follow the instructions on the label.


Mealy Bugs

Mealy bugs.
Mealybugs on plant.

Mealy bugs are another very common houseplant insect pest. Like the aphids, these soft-bodied, sap-sucking insects will attack a wide variety of plants. They also produce honeydew. Mealy bugs have six legs but they are usually hidden from view, beneath the wider body of the insects. Mealy bugs also produce waxy filaments from their abdomen to protect them from the elements and predators. The females are wingless and may produce several generations of 40-60 eggs over the course of several weeks. The males do have wings and fly or crawl from one female to another, fertilizing them as they go. The young mealy bugs are the most mobile, often moving some distance from their birth place to finally settle down and begin feeding. They will often be found in leaf axils, on the undersides of leaves, particularly along the sap-rich mid-vein or amongst new shoots or just along the stems of a plant.

Mealybug adult and nymphs.
Mealybugs on False Aralea.

Mealy bugs are more difficult to control because they often hide in tiny crevices between leaves or stems where they are difficult to treat. They will even feed on the roots of plants and can be seen accumulating on the rim of a pot of a heavily infested plant! The same treatment strategies effective for aphids will also work for mealy bugs but expect to have to treat multiple times. A cotton swab, dipped in rubbing alcohol is the classic weapon against mealy bugs. Just dab the insects with the tip of the swab. The alcohol dissolves the waxy outer coating of the mealy bugs, causing them to dry out and die. Perhaps a nasty way to go but they will get no sympathy from me. I have thrown out many heavily infested plants in the hopes of stopping the spread of these little white, fuzzy beasts. I would suggest you follow the same advice – if you have a heavily infested plant, throw it out to try to avoid a plague of mealy bugs in your entire plant collection.


Scale

Scale infestation on a moth orchid.

Scale insects are another insect pest that is common outside as well as inside but there are usually different species of scale that you might find on your trees and shrubs, compared to a plant in your home. Consider scale as one of the most difficult insect pests to control. Once again, if you determine that you have a scale infestation, even a small one, it might be wiser to get rid of the plant than risk it spreading to other plants. I tell my students that one of the best treatment methods is to move the infested plant outdoors when the temperatures are at about -10° or so and leave them there for a few days.

Several immature scale insects on moth orchid.
Scale insects like to feed on succulent foliage and flowers.

Scale is another true insect with six legs, but again the legs are usually not seen except for the first stage of development known as the crawler stage. The females crawl around to find a new spot to begin feeding, then usually lose the use of their legs and remain in that spot for the rest of their life. They soon develop a thick, waxy, scale-like coating that protects them from predators. Male scale insects usually live for just a short time but use their time to crawl around in search of females to fertilize. The female scale insect may bear dozens of eggs beneath her body. When the eggs hatch the babies crawl out to start the process again.

The waxy covering of adult scale insects make them very difficult to control. A simple jet of water will usually not dislodge them but it may be effective to get rid of the crawlers. The insecticidal soaps and horticultural oils can likewise be effective on the crawlers but do little harm to the adults. Scale insects can often go undetected for weeks or months, looking like nothing more than a greenish or brownish, small bump on the side of a stem, leaf or flower. Disposal of the entire plant is my best recommendation for a heavily infested plant. In some cases you might be able to prune off the infested part of the plant then monitor the plant closely for any additional scale sightings. An old toothbrush or soft cloth might also be used to wipe or scrub the adult scale insects off the plants. But if you keep a plant that has had scale, isolate it from your other plants or you could lose them all to scale.


Whiteflies

Whiteflies are yet another insect related to aphids that might be brought in from outdoors. As the name implies, this small insect is white and has white wings. It is usually about 1/8” long and looks like a minute moth. They are most commonly found sitting on the underside of leaves or may be seen flittering around if you disturb a plant. Poinsettias are one of the favorite host plants for whiteflies along with hibiscus, primroses and geraniums. So, you may have unwittingly brought the whiteflies into your home on one of these plants. Whiteflies are quite mobile and can easily spread to other plants but they are a little pickier in the kind of plants they feed on. They are especially fond of young tomato plants so keep that in mind if you plan to start some this spring to plant out in your garden when the weather warms up.

Whiteflies are fairly difficult to control, mostly because they move around so much. The earlier treatment methods can help with a light infestation, being most effective on the egg stage of the insects. Prune off heavily infected foliage and get rid of it.


Honeydew

Honeydew from a scale infestation.

Honeydew is the sweet, sticky excrement of aphids and other related insects like the scale, mealy bugs and whiteflies. Honeydew may also become infected with a dark colored mold, called sooty mold. If you see or feel a sticky substance on the leaves of your plant, or perhaps on the windowsill beneath your plants, check them out for aphids or these other related pests.

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