Detect, Prevent and Treat Houseplant Pest Infestations

Last fall I wrote several columns on the basics of buying and caring for houseplants. Now though, it’s time for some critical maintenance work. This includes brushing up or gaining new skills in knowing what and who to look for on these plants.

Someday you’ll buy a houseplant that will come with visitors of the insect type. A friend may give you a cutting of one of their plants that has an unnoticed scale or mealybug on it. Be wary of friends bearing gift plants. Always isolate these new arrivals until you’re certain they don’t have insects on them. And new plants, right from the store, should always be isolated until you’re certain that no one has hitched a ride and is just waiting to meet your other plants.

New pesticide regulations have changed what nurseries can spray on them. There are fewer pesticides used on houseplants, and nearly all the growers have switched to far less chemical use and more use of organics and biocontrols. This is good for us and good for the plants, but it does call for more vigilance on our part.

The biologicals being used by growers include a number of agents that will fight diseases and insects, including predatory mites that kill the bad bugs and other insects that feed on others like aphids, mealybugs and spider mites. These new control devices can work very well in greenhouses but aren’t practical in our homes.

One of the most popular insects that growers use to feed on the bad insects are a group referred to as the assassin bugs. They’re very effective in controlling caterpillars, aphids, whiteflies and others. Last week a friend told me that he’d just purchased two cartons of raspberries in the market that he noticed were from Mexico. In each pint container he found not only the raspberries but an Assassin bug. A much better thing to find on an edible berry than spray residue.

The problem with not knowing what or who to look for on your plants means you will usually find the bugs when the populations are out of control and are putting your other houseplants in jeopardy. The solution is easy and even if you find an intruder, the control can most likely be organic and natural.

For the most part we only deal with five specific houseplant pests. These are the two-spotted spider mite, scale, mealybugs (a type of scale), aphids and fungus gnats. Each has its favorite hangout and feeding spots, and when caught early, each can be managed and eliminated — but only when found early. None of them chew leaves, but all will suck the life out of them.

There is one common clue that all these insects leave behind. You might notice it as a stickiness on the top surface of the leaves or as a black fungus (sooty black mold) that grows on the sticky residue that we refer to as honeydew. The honeydew is essentially the sugars that the insects excrete while the black mold is an ever-present mold that grows on the honeydew. If you touch a leaf and if feels sticky, then it’s time to become a detective. Remember, the honeydew comes from above the leaf where the insects are feeding and not necessarily from that leaf.

When most houseplant owners find they have 2SS (two-spotted spider mites) it’s too late. These very tiny eight-legged pests are invisible to most of us unless you know what their damage looks like and what plants they prefer. They adore Gardenias, roses, citrus and Hibiscus but can be found on other plants. Two clues to their presence are their thin and wispy webs and the fine stippling they can leave on leaves as they insert their needle-like proboscis into the leaves to feed.

Spider mites need to be found early and controlled immediately. They can move from plant to plant on the slightest wisp of air that carries the mite attached to a strand of webbing. Washing your plants regularly will reduce the chances of a mite infection, and sprays of cold-pressed neem oil will kill not just the adults but the eggs left behind as well. The neem oil is not toxic to people, plants and pets, and it will also shine the foliage of large-leaved Ficus (rubber) plants.

Mealybugs and scale are closely related. Neither fly and neither will walk into your home and attack your houseplants. Both need to be on your plant when you buy it or can move from another plant if your plants are touching. The mealybugs are cottony and white when mature and will almost always be on the bark or at leaf-stem joints. They can be rolled off with a Q-tip dipped in alcohol, but this is just a stop gap.

Other types of scale will usually be hard and small bumps you notice and can occur singly or in groups. If you use a pin or fingernail to get under them you can make sure it’s a scale and not just a scab. Spraying them is generally useless as their hard shells are impervious to most insecticides, but there is one thing that works.

Both mealybugs and other scales are also susceptible to cold-pressed neem oil. In this case the oil covers the bug and smothers it. It will work on both hard scale and mealybugs, but you need to treat the entire plant and it will take about three applications at two-week intervals to get control. And if you ever have either of these scales never assume that they’re gone until you haven’t seen them reappear for a year or longer. Just one missed scale can result in a reinfection that won’t be noticed for months.

Aphids on the other hand do fly and can move from plant to plant. They prefer to feed on buds and flowers and always occur in social groups so if you’ve got one you’ve got dozens. Look for them at growth tips, new buds and flowers of plants like Gardenias, roses, citrus, hibiscus and others. They are easily controlled with sprays of oils like neem and light hort oils. Pyrethrin sprays will also knock them down very quickly and may only need to be repeated once.

Whitefly is a lesser problem on houseplants, especially in the cooler months. There are several varieties but all are treated the same. Cold-pressed neem oil is again the best choice and will have some residual effects, killing both the adults and eggs. Again, at least two applications will be necessary. Yellow sticky traps are great to have and use for monitoring for whitefly. You can buy the cards on Amazon, and if you need help using them drop me a line.

You may notice fungus gnats on the soil of your plants and maybe on the lowest leaves near the soil. They don’t feed on your plant but their larvae may damage new roots. If you notice them flitting about your soil, simply get some Mosquito Bits (8-ounce jar for about $17) and follow the directions for mixing with water. Apply the mixture to the soil of all your plants and repeat in three weeks. The “bits’” contain a bacteria species (Bt) that is very effective on fungus gnats and not harmful to humans or pets.

That’s it, and those will be your major problems and what to look for regularly. I’d also suggest trying to get your plants outdoors once in mid-spring and again in mid-fall. Give them a shower or a spray of neem oil, cold-pressed only. Always be careful to spray and clean both sides of the leaves when you can.

Diseases on houseplants are very, very rare, and chances are that if you had a disease there wouldn’t be a way of managing it with the tools available to us. Some things may show up thought that masquerade as diseases.

If the tips of your plants — say Dracaena marginata or Ficus — turn brown, you probably have a watering problem and not a disease. When roots get damaged the damage is often expressed in the plant’s foliage, and this can happen from over- and underwatering. Overfeeding can also result in leaf-tip burn, and you can pretty much ensure this won’t happen by not feeding most houseplants in the winter, and when you do feed them only use organics and follow the directions. Chemical fertilizers like Miracle-Gro (15-30-15) can and will burn when not used correctly, and highly concentrated fertilizers like this should only be used on indoor flower plants and fruits by experienced gardeners.

Some houseplants will drop their leaves. Don’t panic. Ficus benjimina, the ficus with smaller leaves, naturally drops older leaves and replaces them with new ones. Many houseplants will do this as part of their natural life cycle. However, plants like Aralias and Crotons will drop leaves if it gets too cold, and leaf drop on other plants can also be a sign of the plant being pot-bound and in need of transplanting.

However, if you are growing herbs indoors like basil the best thing to do is grow varieties from seed that are noted as disease resistant. Rosemary can also get mildew, but the pesticides used to control it are not recommended for use indoors and not on plants that may be edible.

It’s actually much easier than you thought if you just treat your plants well and observe them carefully and regularly. In several weeks it’ll be repotting time. With most plants it’s easy, but it’s still weeks too early. Keep growing.