Jennifer Schultz Nelson
Extension Educator, Horticulture
I am an admitted indoor plant addict. Give me a window with good light, and I'll have it filled with plants before you know it. I even have lights so I can grow plants in rooms without good light. But along with all my plants, I've also come to know many unfriendly houseplant pests. The dim, dry winter months indoors seem to intensify pest problems if they are present.
A lot of factors contribute to pest problems, but consider that the plants we call houseplants do actually grow outdoors somewhere in the world. I was recently in Florida, and did not notice at first that some of the landscape and roadside plants were indoor plants back home. This was partly because of their size–what I assumed was a tree because of its size was in fact a Schefflera, or Umbrella tree, a plant I have only known to grow indoors.
I think we also have to consider what we're used to seeing in the landscape. I am just not used to seeing hibiscus used as a shrub border, or Sansevieria, also known as Mother-in Law's Tongue or Snake Plant used as a groundcover. We passed by a Lowe's and Home Depot and their garden department was full of 15 foot tall palm trees. If I lived there I would have some serious re-learning to do where gardening was concerned!
Most indoor gardeners encounter unwelcome pests at one time or another. Typically problems begin to escalate during the winter months. In general, during winter our houses tend to be warm and low in humidity and light, whereas houseplants typically prefer bright, warm and humid climates that are similar to their native lands. Having a less than ideal environment is stressful for a plant, and a stressed plant can develop weaknesses and be more susceptible to attack by pests.
Overfertilizing, particularly in winter, can produce succulent growth that is very weak and prone to infestation. For my plants, unless they are under grow lights I don't fertilize them at all during the winter. Those under lights get a dilute solution during the winter. Those in the windows do not get enough light to support healthy new growth, so I don't encourage it.
Consider too that pest problems indoors are more common since their natural enemies very rarely come inside. I think we'd like to keep it that way, since not many enjoy having natural pest predators like ladybugs or praying mantis flying around except for maybe my cat. I had a few praying mantis hitchhike indoors on my ficus one autumn, which my cat seemed to think were the greatest toys ever. They dive-bombed her, and she tried to swipe at them, while I was caught in the middle trying to get our visitors back outside. Anyone who looked in my living room would have died laughing!
Considering that entire books have been written and college courses taught on this subject, I've picked my "top five" most hated houseplant pests: mealybugs, aphids, scale insects, spider mites, and fungus gnats.
Mealybugs are soft bodied insects that resemble tufts of white cotton. Usually they go unnoticed until they are adults, because that's when they produce their white cottony covering. They are also extremely adept at hiding on a plant. They are sucking insects, piercing the stems and leaves to ingest their nutrient rich sap. Often affected plants are sticky, as mealybugs produce honeydew.
Honeydew is essentially what the mealybugs excrete after ingesting the plant sap. It is high in sugar and tends to attract a black powdery fungus called sooty mold which feeds on these sugars. Heavy infestations of mealybugs can distort new growth in plants. Small infestations can be controlled by handpicking or dabbing with a cotton swab dipped in rubbing alcohol.
Aphids are another sucking insect that produces honeydew. They are small soft bodied insects, ranging in color from nearly colorless, to green, to yellow to nearly black. Most of the time they can be controlled by washing or wiping them off affected plants. In extreme cases they will also distort new growth.
Scale insects come in many shapes and sizes, but the most common ones indoors range in color from yellow to nearly black, and resemble small droplets stuck to stems and the undersides of leaves. Many have a hard protective shell that prevents them from being easily washed off or affected by insecticides. They can be removed by hand, but this may be impossible for large infestations.
Spider mites are very common pests of houseplants, in part because they thrive in dry climates, and most of our homes are very dry in the winter. Treating them early on is best, but can be hard to diagnose. Early infestations look like faint yellow dots across leaves, with mites generally on the undersides of leaves. They are very tiny, but can be shaken off the plant onto white paper to confirm their presence. Advanced infestations reveal how they got their name, as plants are engulfed in material resembling fine spider webs. Increasing the relative humidity indoors is generally a good way to prevent spider mites. I have had limited success with using humidity to actually treat an infestation.
Fungus gnats are threatening a hostile takeover of my amaryllis right now. They are very common pests whose larvae thrive in wet soil with high organic content. The adults are just annoying and tend to fly up my nose, but larvae live in the soil and eat the roots of plants present, and in high numbers can seriously harm plants. The best control measure is to let the soil dry between waterings. Other options include spraying to kill the adults, or using specific nematodes or beneficial bacteria (Bt) in the soil to kill the larvae.
One treatment I read from University of Missouri recommended planting a pot of wheat sprouts, as the adults prefer to lay their eggs in these. After a few days you discard the pot of sprouts and all the fungus gnat eggs. Repeat every two weeks until the gnats are gone.
There are many different treatments available for houseplant pests. Before treating, remember pests can move from plant to plant. At the first sign of a problem, I isolate the affected plant away from the rest, usually in a different room.
As with our outdoor gardens, you have to learn to read labels. If you use insecticides, remember one spray will not always control all pests. Plus, some sprays, although labeled for indoor plants, can be very toxic.
This is especially an issue where children and pests are concerned. For me, I prefer to either handpick the pests off the plant, or use an insecticidal soap that is safe for use around children and pests.
Remember also that insecticidal soaps or other treatments will more often than not need to be repeated. While they will often kill adults in one treatment, new eggs will continue to hatch. Read the label of your spray to find the products suggested application interval.
I also prefer to let my houseplants vacation outdoors each summer. This exposes any pests on the plants to natural enemies, and if any of my plant stands or shelving are harboring any pest eggs, they have no plant material to feed on when they hatch, breaking the cycle.
With all of these pests, there are a few rules for controlling them that don't include sprays. For one, you need to look closely at your plants, both before you buy them, and while you own them. I have created my own trouble by failing to do both.
I'm a big fan of plants from the clearance table, but take time to give them a thorough inspection. My failure to look closely at a plant that was a great "deal" was in fact infested with mealybugs, which I brought into my house. The mealybugs spread and eventually wiped out both my African Violets, and most of my orchids.
I also failed to look closely at my plants regularly. Mealybugs, like a lot of pests, like to hide under leaves or in flowers, out of sight. I discovered too late in the game that there are also species that prefer to feast on plant roots rather than leaves. At the first sign of distress in my plants, I should have inspected the roots too.
This advice is extremely hard for me to put into practice myself, but if a plant is heavily infested with a pest it is often recommended that you discard the plant. In the long run, you may avoid having the pest spread among the rest of your plants. Had I heeded this advice, I wouldn't have lost so many of my orchids. Collectively they were worth a lot more than my bargain plant that spread the pests in the first place!