If you notice a change in your coleus, such as drooping leaves, changes in leaf shape, or loss of vigor, it's time to inspect it more closely. A tip is to use a magnifying glass with at least 10x magnification and examine the undersides of the leaves. However, spider mites cannot be detected with a regular magnifying glass; a significantly higher magnification, at least 30x, is recommended.
Damage is usually much less severe and easier to manage if pests are detected as soon as possible.
The most common types of pests encountered are spider mites, thrips, aphids, fungus gnats, and powdery mildew.
Thrips:
Thrips infestation can be identified by seeing silvery spots on the leaves, and if you look at the undersides, you'll often see black dots within the silvery patches. The black dots are thrips excrement. You may also see live thrips, both in larval and adult stages. The silvery spots occur because thrips suck the sap out of the leaves, inhibiting growth and making the leaves unsightly.
Spider Mites:
Spider mites are a bit trickier and sometimes take longer to detect. Make it a habit to check the leaves; they often appear somewhat grayish and dull with small light spots on the upper side and seem dirty on the underside. What looks dirty is the webbing the mites spin. With a magnifying glass of x10 magnification, you can clearly see the eggs as small transparent pearls and probably even living mites wandering around looking to suck sap.
Top Shoot Mites:
Top shoot mites, also known as russet mites, are clearly visible on coleus plants, as the growth of the plant slows almost completely, and the new shoots are small, colorless, and very dense. Top shoot mites are extremely small and cannot be seen with a regular magnifying glass; at least 30x magnification is required, and they are still very small.
Powdery Mildew:
If your coleus has developed a white powdery/fluffy coating on the leaves or stems, it has contracted powdery mildew. Powdery mildew spreads through the air, so isolate the affected plant from others during treatment. Plants near the affected one may also have been affected, so examine them as well. Powdery mildew is a fungal disease and is easy to treat if detected early. Remove the affected leaves and treat with a fungicide like sulfur, available at garden centers. Mix 1 pinch with 1 liter of water, shake the bottle, and spray the coleus. Repeat the treatment after a week. Powdery mildew is most common at the end of spring and the beginning of autumn when the temperature varies more. It thrives in warm, slightly draughty weather and spreads through the air via spores. If you detect powdery mildew, handle the affected plant carefully and wash your hands thoroughly to prevent it from spreading to other coleus. Also, treat the coleus that have been near the affected plant.
Fungus Gnats:
If you notice small flies flying around the soil and plants, you have likely been infested with fungus gnats. The flies themselves do no harm, but their larvae do. They live in the soil and feed on the roots of plants. Young plants and seedlings are particularly affected and may die due to the larvae's voracity.
Treatment to Get Rid of Pests:
Below are treatments for unwanted insects if you've encountered any.
Treatment for Spider Mites and Thrips:
A thorough shower for your plant followed by a soapy solution with a dash of oil works on spider mites and to some extent on thrips. It's important to repeat the treatment every week for three to four weeks to eliminate the pests and cover all life stages. Raid is very effective against thrips, but consider its other effects on the environment and family members as well as pets. A substantial pruning is also preferable, as well as changing the soil in the coleus since thrips have one of their stages in the soil. A tip is to put all plants to be treated in one room and spray the Raid there. Air the room thoroughly after an hour. You can also put the plant in a plastic box or large bag and spray into it, but be careful not to spray directly on the leaves, as they cannot withstand it. Repeat the Raid treatment once a week for three to four weeks. Also, thoroughly clean the windowsill and other surfaces, and wash the curtains is a tip.
The oil in the soapy solution clogs and suffocates the spider mites, but as mentioned, repeated treatment for 3-4 weeks is required. Do not overdose on oil, as there is a risk that the coleus' pores will be clogged, and the plant may die as a result. Shake the bottle thoroughly during spraying to ensure that the oil dissolves and spreads evenly.
Treatment for Top Shoot Mites:
Top shoot mites live in the new shoots and usually at the top of the coleus leaf. So the first thing you should do is cut down the coleus leaf. Then treat with sulfur every five days, a total of three times. Mix 1 pinch of sulfur with 1 liter of water, shake the spray bottle well while spraying.
Treatment for Powdery Mildew:
Powdery mildew is effectively treated with sulfur. Mix 1 pinch with 1 liter of water, spray once a week on two occasions. If the powdery mildew returns, repeat the treatment.
To avoid reinfestation, be sure to clean all surfaces thoroughly and even wash the curtains. Surface disinfection can be used, as pests that come into contact with it will die.
Recipe for Soap/Oil Mixture against Spider Mites:
1 liter of water 1 tablespoon of paraffin oil 1 teaspoon of soap/dish detergent Mix everything in a spray bottle and shake well. The soap/dish detergent acts as an emulsifier and mixes the oil and water. Shake the bottle continuously while spraying and ensure that the entire plant gets thoroughly wet. It is the oil that suffocates the spider mites. Spray every week for 3-4 weeks. Spider mites have a lifecycle of 6 days, so it is important to continue the treatment; otherwise, you may wonder why they come back. Also, avoid spraying in sunlight or when the plant is under lighting. Moist leaves cause the leaves to burn and become unsightly.
Treatment to Get Rid of Fungus Gnats:
Let the soil dry out thoroughly between waterings, as they prefer moist soil. Place yellow sticky traps in the flower pots to attract and catch the flies. A good old-fashioned trick is to place sliced potatoes on top of the soil, which attracts the larvae. Let the potatoes sit on the soil for a day and then discard them. Something that is very effective is Nematodes, which can be purchased at a regular florist. Nematodes consist of microscopic roundworms of the species Steinernema feltiae, which are completely harmless to humans and do not attack plants or other animals. These actively seek out fungus gnat larvae in the soil and attack them. Nematodes can also be used as a preventive measure as they can remain dormant for up to a year.
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