Houseplant Pest Problems Likely To Increase

By David Graper
SDSU Extension Horticulture Specialist

Winter Household Pests

Many gardeners like to save some plants from Jack Frost by bringing some plants in from the flower garden or moved some of those container gardens indoors. Others took cuttings of their favorite plants, get them rooted and continue to enjoy them on their windowsills over the winter. Whatever you like to do, it probably means that your limited indoor gardening spaces will become more heavily populated with plants than they were during the summer months. As plants are more closely packed together, the greater the likelihood that pest problems may spread from one plant to another and also a plant that had a minor pest problem outdoors, may suffer a more significant infestation inside the home. Indoor growing conditions are usually quite different than they were for plants that were outside during the summer as well. Generally light levels are much lower, humidity levels are significantly lower, and plants are dependent on hand watering instead of the occasional rainfall that they received while outdoors.

Environment & Watering: When plants are grown in relatively high light, they generally develop thicker leaves, stouter and stronger stems and are more compact. As natural light levels decrease with the lower sun angle, more cloudy weather and shorter days, plants will often tend to have weaker growth, thinner leaves and grow taller than when they were outside. This weaker growth is often more susceptible to the feeding by pests. Indoor heating systems, coupled with lower moisture levels in outside air means that the relative humidity inside the home can be considerably lower than during the summer months. Low humidity fosters the development of certain pests like spider mites, particularly if a plant is near a heating duct where the humidity might be even lower. Hanging basket plants are particularly susceptible to spider mite infestations because they are not usually able to take advantage of the moisture released into the air by other nearby plants. Plus they tend to dry out faster. Even though humidity levels are lower for most indoor plants, meaning they tend to use more water, the number one reason houseplants die or suffer problems is over-watering. The basic rule to remember, when it comes to watering, is to wait until the potting soil feels dry, then water thoroughly so that water comes out the bottom of the pot. Then, do not water again until the soil feels dry once more. Over-watered plants often suffer root rot which can make the plant look wilted. Usually the first thought a gardener has when seeing a wilted plant is to water it. But in this case, that is the last thing that plant needs. Water stressed plants are also more susceptible to other pest problems too.

Mites

Pest Profile: Spider mites are one of the more common indoor houseplant pests and they are also common on plants that were outdoors during the summer. I got lots of questions about spider mites this past summer on all kinds of different plants, particularly because we had a fairly hot summer. If plants were growing on a deck, patio or someplace with lots of hardscape materials that would absorb heat from the sun, those were plants that were more likely to suffer from a spider mite infestation. If you took cuttings from those plants and brought them indoors, then you likely brought mites in with them. Spider mites are extremely tiny, barely visible with the naked eye. They usually start feeding on the undersides of plant leaves but can feed on all parts of the plant when infestation get heavy. We see mainly the two-spotted mite which is tan in color with a dark spot on each side of its abdomen. You may also see tiny black-brown fecal spots on the leaves as well as minute webbing at the base of leaves or in the tips of young shoots.

Management: Mites’ small size make them hard to see so they often go unnoticed until an infestation becomes severe. So, examine your plants carefully for mites, perhaps use a small hand lens to get a closer look, particularly if you see that the leaves have a speckled appearance. This speckling or stippling is caused by the actual feeding of the mites which suck sap out of individual plant cells, damaging them and often killing them as they continue to feed. If you do not notice an infestation early, and the plant is heavily infested, I would suggest getting rid of the plant, rather than risk the infestation spreading to other, nearby plants. Once you have gotten rid of the plant, check nearby plants for infestation. Mild infestations can be treated with a forceful jet of water in the Kitchen sink or shower or they may be sprayed with insecticidal soap or Neem oil. Be careful of using dish detergent because this can also cause damage to sensitive plant leaves. BE SURE to get good coverage of the undersides of the leaves because that is where most of the mites will be. Retreat weekly to keep the mites at bay.

Mealy Bugs

Pest Profile: Mealy bugs are usually an indoor plant pest but they can survive on plants that you might have moved outdoors for the summer. In fact, they might seem to nearly disappear while outside, partly because there are predatory insects outside that might eat them. But once you bring the plant back indoors, what was a small populations can explode. Mealy bugs are considerably larger than spider mites. They are often seen as tiny white fuzzy tufts on the plant, often on the undersides of leaves, on new growth and in tiny nooks and crannies between the leaves and stems. They feed on a huge diversity of plants and can spread quickly. Infested foliage will often be malformed, turn yellow and eventually die. Mealy bugs also excrete honeydew, a sugary waste product that will make leaves and other surfaces below where they are feeding feel sticky. The honeydew may also turn black as it is often infected with sooty mold.

Citrus Mealy Bug: There are several different species of mealy bugs but the most common is the citrus mealy bug. It is oval in shape with a few tiny, white filaments that are visible around the outside of its body when it is fairly young. As it becomes mature, it grows longer white filaments all over its body until it looks like a tiny cotton ball. The long-tailed mealy bug is another fairly common species of mealy bug. It is much like the citrus mealy bug but has longer white filaments, particularly at the rear end of the insect. This species can be a bit more aggressive and spread more rapidly on plants it likes to feed on than the citrus mealy bug. Mealy bugs are an insect with six legs that you may be able to see if you can flip one over. They too suck plant sap.

Management: Controlling mealy bugs is quite difficult. A commonly mentioned method is to dip a cotton swab in alcohol and touch it to each mealy bug you can see. While this will likely kill that mealy bug by dissolving its waxy cuticle so that it dries out, this can be a very tedious and time-consuming and unfortunately futile effort because there are usually so many mealy bugs to try to treat and many of them are inaccessible to the cotton swab. Some people resort to the shotgun approach by just pouring rubbing alcohol into a spray bottle and just using that to spray the whole plant. This will likely kill more mealy bugs but, like using dishwashing detergent, can cause damage to sensitive leaves. Neem oil and insecticidal soaps are probably better options. But, like a plant heavily infested with spider mites, it is probably best to get rid of a badly infested plant. Mealy bugs can be on all plant parts, including the roots so they are very likely to come back again in a few weeks. You might even see hundreds of mealy bugs on the rim, sides or bottom of a pot, right on the plastic! For a tiny insect they are persistent.

Scale

Pest Profile: The last serious and often fairly common insect pest on houseplants is scale, usually the soft brown scale. Like the mealy bugs, these too excrete honeydew. In fact, this is often what most indoor gardeners will notice before they see the actual scale on their plants. Scale is usually about the same size as mealy bugs but the adults are dark brown and rounded, without the white filaments. They look like small brownish bumps on the leaves or stems. They often go unnoticed, until an infestation becomes serious. Infested leaves will often turn yellow and infested leaves may also be malformed. When eggs hatch, the first generation called crawlers are mobile, moving about on the plant looking for a new place to begin feeding. Once they settle down to feed, they usually stay in that spot for the rest of their lives with the females laying eggs beneath their tough scale-like body covering which protects them from enemies and unfortunately most insecticides and other external treatments to remove them.

Management: Once again, the BEST TREATMENT for an infested plant is to get rid of it as quickly as possible. It doesn’t take long for scale to spread throughout your plant collection. Be sure to carefully check any new plants you bring home for signs of scale before you add them to your collection.