Spider mites, thrips, mealybugs, and whiteflies are all common pests of Calathea when grown indoors. In my experience, these insects are easily handled with mild horticultural oil or soap treatments for pest control. As with many houseplants, good care is the key to preventing disease and pest pressure.
Named for their habit of making spider-like webs across the underside of plant leaves, these mites are white and tiny, like specks of dust.
They’re easy to overlook until the Calathea’s leaves are turning yellow and thinning in spots due to damage. Spider mites can attack any healthy plant and quickly build a colony.
As their name suggests, Whiteflies are white and only about 1/10th of an inch long. If you disturb the plant, these little flies tend to take off like fluttering snowflakes.
Their larvae nibble on Calathea leaves and suck out the juices, drying out the leaves and causing yellowing. Calathea that are under-watered or kept in too cool of an environment tends to attract them.
They don’t always look like bugs, but scale insects are tiny shelled pests that attach to a stem or the underside of a leaf. Sticking close to the surface, the brown or green bug may just look like a raised section of plant material. But it’s actually sucking the fluids out of the plant and leaving behind honeydew that attracts other pests and diseases. Scale insects are relatively rare indoors compared to other pests, but they can go overlooked for a long time.
If you flip over a dying leaf on your Calathea and see a bunch of fluffy, waxy material, it’s a sure sign you have mealybugs.
These white, oval-shaped bugs aren’t just fluffy themselves, but they leave behind waxy coatings that can be rubbed off with a finger. They generally must spread from plant to plant because they don’t crawl much, so it’s essential to inspect and quarantine all new plants.
Up close, a thrip resembles a tiny grasshopper. But to your naked eye, you’ll notice black spots on leaves that are drying up and turning yellow. Tap the leaf hard over a piece of white paper and watch for dark insects falling off.
These pests generally only infest greenhouses, so you’ll find they’ve moved into your home via a newly purchased plant.
Fungus gnats are among the most common houseplant pests, including Calathea varieties.
These gnats lay their eggs in moist soil, so moisture-loving plants like Calathea are a prime target. The larvae can damage roots if an infestation is severe, but mostly the adult gnats are just a nuisance.
Letting the soil dry out and practicing better watering habits can keep these gnats from ever appearing.
Bacterial Leaf Spot:
Pseudomonas bacteria are found in the air and can settle onto the leaf of a weakened or stressed plant. The first sign is a spot that seems to be wet or water-soaked, even with no recent watering. Then the spot turns dark green and finally black.
It may turn tan or brown when it dries up and have a paper-like texture. Keeping the leaves dry is the key to preventing this disease.
Over-watering can suffocate and kill off the lowest parts of the Calathea’s root system. Plants with root rot will turn yellow from the oldest leaves first and may also display signs of drooping or wilting. Only careful attention to watering and a fast-draining planting mix will prevent this from occurring.
Pesticide and Insecticide Solutions
Wiping down your Calathea plant’s leaves weekly is the best way to spot pests and early signs of disease. However, even thoroughly wiping away spider mites or thrips won’t completely solve pest problems.
Pesticides and insecticides help protect your houseplant when nothing else will work. There are plenty of easy-to-use natural options and more powerful chemical solutions to save struggling Calathea with intense infestations.
Not many pesticides are recommended or registered for use on Calathea, especially in the home. Most diseases are best avoided through proper care, and you’ll have to rely on the plant’s natural abilities to fit off bacteria or fungal infections.
It’s recommended for treating spider mites in particular by many sources, but it can be too photosensitizing for Calathea. These plants already prefer low light levels. Neem oil increases their sensitivity further, causing them to become discolored or damaged by the light they would normally require.
A better choice for suffocating stubborn pests like thrips, spider mites, and scale is the use of horticultural oil without neem as an ingredient. This oil is lightly rubbed over the underside of the leaves to smother existing pests and keep new ones away. If you can’t find horticultural oil from a local supplier, plain Canola oil works well as a substitute.
Soap may sound like a silly thing to use on plants, but Calathea responds well to this mild treatment for pests.
The soap is added to warm water and wiped over the leaves to pick up adult pests like mites or thrips along with larvae and any residue left behind that could cause disease.
An inexpensive DIY option that won’t harm your Calathea is to mix a few drops of bleach-free dish soap in water and use it.
Yellow sticky traps (via Amazon) are effective at catching adult fungus gnats and annoying whiteflies, these strips or cards must be placed all around the plant to work. Change them regularly to stay on top of an infestation before it can spread.
How to Use Pesticides & Insecticides for Calathea Plants
Oils and soaps work best when applied weekly during the entire warm season. This will remove any pests trying to set up shop before you notice signs of damage.
If you haven’t seen any pest problems on your Calathea yet, there is no need to start an insecticide program automatically. But you should focus on good plant care, in general, to ensure disease and pest problems don’t begin in the first place.
Managing Pests & Diseases
Calathea often struggles indoors because we tend to keep our homes around 40% ambient humidity. They prefer 50% humidity and up, with higher numbers better for them due to their adaptation to native jungle habitat. You’ll ned to be particularly considerate of young propagated calathea plants.
Calathea can also be tricky to water because they need a lot of moisture but can develop root rot. Ensure the soil you choose drains quickly.
Occasional fertilizing and pruning is also recommended for optimal Calathea plant health and aim to locate your plant in a spot with plenty of bright, indirect light.
You’ll also want to consider repotting your calathea plant if there’s any evidence of root rot or decay in the soil base.
In many cases, all a Calathea plant need is the right environment to overcome any minor pest or disease problems. Use some mild insecticides as needed to keep hungry pests to a minimum.
For more, see our in-depth guide to the uses and benefits of calathea plants.