Most people don’t like to spray pesticides inside their homes. I don’t blame them. I do like houseplants but so don’t certain insects and other critters. Sometimes we can use soil applied granular chemicals, but some people with pets are even hesitant to do that.
One method I sometimes use is soaping foliage in the shower with a mild dishwashing detergent. I leave the lather on the plants for a while. Then I rinse it off and send the nasty critters down the drain. This technique works great for a small number of plants. However, some people feel funny about using their shower as a plant washing station.
With recent milder weather we can treat our plants outside, either with commercial pesticides or the soap treatment. Either way, doing it outside eliminates some of the odor and mess. If we get several mild nights we can even let them stay outside for a while. If you do this make sure to keep plants out of direct sunlight. You’d be surprised how different light intensities are, even in a well-lighted south-facing room.
I have found the biggest culprits of houseplants to be mealybugs, spider mites, scale, and fungus gnats. With the exception of fungus gnats they are all difficult to control. Scale, mealy bugs and spider mites often require multiple treatments.
Fungus gnats are pretty easy. The first step usually involves reducing soil moisture levels. This is most important as these insects feed on fungi growing in the soil. Eliminating that alone is often enough to control them, but most insecticides are effective as is the soap treatment.
The other three require systemic insecticides, since those three arthropods have sucking mouthparts. This means they feed from inside the plant. Systemic means the chemicals are absorbed into the plant and aren’t simply resting on the leaf surface.
Acephate and Imidacloprid are two chemicals that are effective and labeled for most houseplants. They are also readily available and don’t require a pesticide applicators license. The most common application method is to spray the foliage completely. Usually two or three applications at ten to fourteen day intervals are recommended.
Another option is to use soil applied granules and add additional soil to cover the residue. These usually can be effective for several months. Sometimes I use an initial soap cleaning followed by systemic granules. This is also effective for aphids, which often attack tropical hibiscus, especially the flowers.
It is important not to exceed labeled rates. They are established through thorough research. Never assume if a little is good, then more is better. Many chemicals are toxic to plants at higher levels and they certainly can be unsafe to people applying or living around them.
I try to minimize my use of pesticides, but sometimes they are the best option to control pests on your houseplants. Always keep records of what, when and how much you applied. Also, it’s helpful to rotate chemicals and not use the same pesticide all the time.
Ted Manzer teaches agriculture at Northeastern High School.