Sam’s Spade: Houseplant pest care

9 mins read
Christmas cactus next to a window.
Sam’s healthy Christmas cactus next to a window. (Sam Hill photo)

It has been relentlessly cold this past week, cold enough that the over-wintering pests may have been substantially killed off. Perhaps the Japanese beetle in particular, which plagues our gardens each year, will not be around to give us any trouble. Instead of dwelling on insects that are frozen, however, let’s consider the insects that are very much alive, and a threat to your existing houseplants.

There is a lot to know about using insecticides, and even more to know about foliar-spraying in general. I’ll try to touch on the important stuff, with maybe just a few interesting tidbits here and there. Once you know the science behind what you are doing, it’s a lot easier to understand why something may or may not work for you at home. It’s important for me to state here that purchasing an insecticide for your houseplants, such as an insecticidal soap, is a safe alternative to mixing your own, simply because the ingredients are usually chosen specifically for insect control on plants and the ratio of ingredients used in the spray recipe is generally more precise than what the DIY horticulturist typically blends together. With that being said, if you have the know-how and you enjoy doing this kind of stuff, why not give it a try?

If you have a problem with insects inside the home then you’ll want to deal with it immediately. If you notice only a few plants are infested with bugs, remove them to a separate room until you can eradicate all harmful pests. Also, if you bring new plants into the house, it would be a wise to quarantine them for a few days as well, just to make sure there are no insects or fungal infections that could (and most likely would) spread to the rest of your houseplants.

There are different types of sprays you can mix at home to treat your houseplants from the nasty little bugs that have hijacked your indoor plants. Oils, soaps, and plant extracts all have their place in the realm of pest management, but they don’t work the same as each other and some are more effective at controlling certain pests.

Horticulture oil (any petroleum-based, paraffinic oil refined for use on plants) has been used to control pests on fruit trees for years, and a lighter version of this oil makes a great insecticide on many of our other plants. Something with a high UR rating and a low viscosity will work, but if this isn’t something you have laying around, vegetable oil is a great alternative. Cottonseed, soybean, canola, olive oil, as well as the aforementioned horticultural oils, can be used to control aphids, mites, scales, white-flies, mealybugs and thrips.

An oil solution works simply by smothering the insect or egg and shutting down normal body functions. The oil not only suffocates the bugs, but it keeps them from eating, which is helpful in some cases in order to prevent the spread of disease. Oil does not readily mix with water, so a wetting-agent (surfactant) is required to bond the two together, which allows the spray-solution to thoroughly wet the leaf, rather than beading-up and sliding off the leaf uselessly. Soap is often used in preference to detergents (liquid dish detergent, not laundry detergent!), because of a gentler interaction with the cuticles on the leaf surface. Oil sprays are generally mixed at a 1-2 percent solution, and should be sprayed on all plant surfaces (especially the bottom of the leaves where insects and eggs are generally found).

Fungus gnat. (Sam Hill photo)
Fungus gnat. (Sam Hill photo)

Using soap alone with water has also proven itself as a valid method of dispatching insects, including aphids, thrips, mealybugs, spider mites, white-flies, some scale insects and even Japanese beetles! Once emulsified in water, the soap works by dissolving the waxy surface of the bug’s exoskeleton. Without this water-resistant layer, the insect quickly dries out and dies. Soap sprays are used at .5 – 3 percent solution. Like oil sprays, soap solutions are only effective by direct contact with the insect, not from residuals left on the leaf after evaporation takes place. Also, hard-water reacts adversely with soap (creating soap scum) so finding a different water source, or using detergent instead of soap, would be beneficial in this situation.

A popular organic insecticide to mix at home is one that incorporates the insecticidal properties of garlic, onion, and cayenne pepper. To utilize these properties, dice up a small onion, a half to 1 bulb of garlic, and two cayenne peppers (or 1 tsp flakes). Add one quart of really warm water and let it sit 12-24 hours. After you strain the mixture and add a few drops of soap it is ready to be sprayed. Be careful not to get the mixture in your eyes or lungs! Unlike oil and soap sprays, there are plenty of residual effects from this form of insecticide, which generally allows you to go longer between each spraying. By adding a form of vegetable oil you may be able to prolong these residual effects.

If you have read some of my other articles then you probably know I like to use neem oil on my plants. It can be used to disrupt the normal cycle of insects, particularly the pesky fungus gnat, by affecting their feeding habits and preventing female insects from laying eggs and starting a new generation. This type of insecticide is effective for a wide range of bugs that you might find on a plant, and is surprisingly harmless to beneficial insects. Neem oil has other qualities that are beneficial to plants, namely it’s ability to subdue powdery mildew. Mix neem oil at 1 tsp (5 mL) per quart of water, with a few drops of soap to create an emulsion. Spray plant thoroughly.

These sprays should be used in the mild morning sunlight unless you notice the insects are showing up at a different time, in that case, the soap and oil sprays should wait until the insects are present. Avoid evening spraying to protect against molds that might form, as well as spraying in strong sunlight or high heat.

I like to designate a place to spray my plants, rather than spraying them where they stand. Excess moisture is never a good thing in and around a plant’s environment, so finding a sink to use as a backdrop for your spraying can be helpful.

Do not spray plants when they are very thirsty or were watered very recently; wilting leaves can be damaged by foliar spray and spraying freshly-watered plants puts a lot of moisture in one place, encouraging microbial life.

Some plants don’t agree with certain types of spray. Research your house plants and find out if there is anything you should stay away from.

Store-bought sprays generally incorporate one or more of the types of spray solutions described above. For example, soap is typically referred to as “potassium salts of fatty acids” on the ingredient list for many of the soap sprays on the market.

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  1. you can also use dish soap and vegetable oil and it wont hurt your plants the oil will coat the leaves and the soap will hurt the bugs and not anything else

  2. The soap/detergent is the surfactant. You’ll notice that the oil you use will float to the top of the water and refuse to bond with the water when you shake it around. If you add some liquid soap/detergent and shake, the oil will start to bond with water. Add soap in increments until the oil is no longer visible floating to he surface and/or suds start to form.

  3. Hi Sam,

    Thank you so much, I will try this on my roses. After babying them and seeing the leaves healthy and bright green, to only notice these tiny little pest, was very upsetting.

    I remembered using this method a long time ago, but could not remember the other ingredient in the soap/water mixture.

    I will also spray the basil and oregano, that grows next to them, so the bugs do not infect them. I think the oil I will use, willl be olive oil. lol.

    Have a great day and thanks again.

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