Orchids, just like any other plants, indoors or in the garden, are prone to pests and diseases. Orchid pests and diseases are nothing out of the ordinary and many can be controlled or treated. Here are the most common orchid pests and diseases, including scale insects and thrips, and sooty black disease.

Before I talk about pests and diseases, sometimes people confuse pests and disease issues with environmental issues. If your orchid has yellow leaves, read this guide. If your orchid has brown leaves, then this guide is worth reading. If this isn’t an issue then read on to learn more.

Orchid Pests

Diapsid hard scale insects that attack orchids

What are Diapsid hard scale insects?

I’ve written quite a few articles about scale insects in relation to other plants, but the Diapsid scale insects are different. Diapsid scale insects are hard shell scale insects, whereas the others I’ve given advice on all have a soft shell. The hard shell makes these ones somewhat resistant to insecticide sprays.

diapsid scale insects don’t leave honeydew behind, while soft shell scale insects do. Of the many (about 100) types of these scale insects, about 25 varieties are in gardens or houseplants.

diapsid scale insects suck the sap out of the leaves and stems of orchids. While most plants can handle a few of these bugs without too much trouble, a heavy infestation can seriously harm your orchid. Infestations of diapsid scale insects spread in the crawler stage. They may travel quite far before stopping to feed. They’re also light enough to be blown about by the wind.

How to recognise these pests?

diapsid scale insects are about 2mm in diameter so they are extremely small. The adult females have a hard outer shell that’s brownish white in color. The adult male has a more elongated shape. These scale insects are often covered in a white wax that covers the egg masses.

Look around the underside of leaves and the stems of your orchids for these pests. You generally find them in a cluster at the base of the leaves. They’re often immobile, just lying there. They look like scales as they hide their legs.

As I mentioned above, hard shell scale insects don’t leave behind the tell-tale clear sticky residue of honeydew. That’s good news, as ants aren’t attracted to the plant, and black sooty mould won’t find a welcome environment there.

How can you control diapsid scale insects?

Prevention and control

The best form of prevention for any pest and disease is to create a strong and healthy orchid plant by placing it in the best environment for it. (See How to grow and care for your orchid.) If you do spot a few diapsid scale insects, don’t worry too much. Healthy plants can stand a light infection. Just pick off the insects or lightly wash them off.

Treatment

For heavier infestations, start with a non-pesticide remedy, especially for indoor orchids. Keep on removing these scale insects when you see them, but that may not be enough in the long run. You can purchase Chilocorus nigritus, a black ladybird/ladybug as a biological control agent, which is great if you have orchids in a greenhouse or even outdoors.

If you need to move to a pesticide, start with one with natural pyrethrum oil such as Bug Clear Ultra. Or use another plant oil.  Bug Clear Fruit and Veg is another alternative worth trying. Remember to reapply these pesticides as they have a short working life. Plant oil products such as these do less harm to larger insects than synthetic chemical solutions.

If this doesn’t work, you’re at the point of deciding between throwing away the plant or moving to harsh, synthetic insecticides. Be careful how you use these products as they don’t discriminate between good and bad bugs plus, synthetic insecticides are not ideal for orchids grown inside the home.

Mealy bugs

What are mealy bugs?

Mealy bugs are another of the many sap-sucking insects to prey on plants. Your orchid plant weakens when these insects take away some of the sap and it becomes unhealthy. The plant doesn’t have the strength to maintain the healthy foliage its developed and can’t create any new buds and shoots.

How to recognise mealy bugs?

Mealy bugs aren’t that small. But they’re difficult to find as they hide away in the parts of the orchid plant that are out of sight. As they live together in groups, they’re somewhat easier to spot than individual insects. Look in the leaf sheafs and under the outside layer of the stem.

But there’s good and bad news here. Mealy bugs leave behind a clear sticky substance called honeydew. They excrete this after eating the plant’s sap. You can look for signs of this substance on the leaves and then look for the insects that are probably close by.

The bad news is that honeydew attracts ants and is the right growing environment for a fungal disease called black sooty mould (see below). But other insects as well as mealy bugs leave honeydew, so it’s best to actually catch sight of these orchid pests to make sure of what insects are bothering your orchid plant.

How can you control mealy bugs?

A few mealy bugs is a big deal. A small population of these pests can quickly grow into a dense colony that swarms over the entire plant. Remove the parts of the orchid plant that have the mealy bugs them. Place the plant away from other plants (especially orchids) as these bugs really do spread quickly.

However, mealy bugs don’t travel far, so inspect any nearby plants for similar infestations. Remove any infested bits.

And that’s really the only way to get rid of these pests. If you don’t catch their infiltration to your orchid early enough, and they do take hold of your plant, the only thing left to do is to destroy the plant.

Soft scale insects

What are soft scale insects?

I’ve talked about diapsid hard scale insects above but orchid plants are also prone to soft scale insect encroachment. The soft scale insects are around 6mm long and have a waxy shell. This acts as a kind of armour from spray insecticide treatments.

Soft scale insects also suck the sap from your plant. They also feed on the stems, so nothing escapes them. This weakens the plant and takes away the strength needed for healthy growth. A really heavy infestation can cause the orchid to die back, though this rarely happens,. It just continues to look poorly and sad.

How to find soft scale insects?

These orchid pests are pretty small and hang out on the underside of leaves. Taking a magnifying lens to your plant may help you spot them, especially on the stems.

Conveniently (or not), soft scale insects leave behind honeydew. This is a clear, sticky, sweet substance that you can easily see on your orchid plant. Look first on the leaves that are yellow or brown or are wilting. Honeydew a substance that doesn’t directly harm your plant. But it is a breeding ground for black sooty mould (see below) that does cause damage. And it also attracts ants.

How to control soft scale insects?

A few soft scale insects won’t harm your plant. Just remove them by picking them up when you see them. But keep an eye on your orchid in case their numbers increase.

Try spraying them with natural plant oils such as neem or pyrethrum oil. A synthetic pesticide is your last resort. But be aware that you are indoors so look for house plant indoors spray.

Thrips

What are thrips?

Thrips are one of those orchid pests that you can have but never see. They’re only about 0.1cm long so you have to search hard for them even with a magnifying lens. The adults have feathery wings and black, greenish brown or tan bodies.

These are yet another bug that survives by sucking the sap from your orchid plant. They especially like flowers, especially buds and new growth. Thrips also spread disease so they harm your plant in different ways.

How to recognise thrips?

As you probably won’t see thrips themselves, look for silvery coloured discolouration on the orchid’s flowers. If your blooms shrivel and drop before opening, thrips have probably got to them. Mature flowers with spots or streaks, or that are shrivelled or discoloured, are the result of thrip attack.

Open a flower that’s been infected and carefully look inside for these tiny insects. For prevention of an infection, do the same thing in flowers that look to be untouched. If you see thrips, deal with them before they cause harm.

How to control thrips?

The first thing to do is to isolate the plant. Thrips can travel quickly between plants carrying disease. Then try an insecticidal soap that’s safe for orchids, following the manufacturer’s instructions. Leave lots of space around the plant for good air circulation. Apply the treatment weekly until all signs of the thrips are gone.

If this doesn’t work, then bite the bullet and get rid of the plant.

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Orchid diseases

Sooty black mould

What is sooty black mould?

Sooty black mould is a fungal disease that’s attracted to plants that have honeydew on them. Honeydew is a clear, sweet and sticky substance that’s excreted by most sap-sucking insects (see section above). While this mould doesn’t attack the leaves, it does cover them and so reduces the leaves’ ability to photosynthesise energy.

How to recognise sooty black mould?

If you find a what looks like a black powder on your orchid’s leaves, that’s sooty black mould. To start with, the mould comes in splotches but over time it can cover the whole leaf. Without sunlight reaching the green leaves, the plant weakens considerably, being unable to remain healthy and to produce new growth.

How to control sooty black mould?

This one’s easy. Just use a mild soap solution to wash away the black mould and the honeydew. The green leaf reappears and can function well again.

That’s controlled the sooty black mould. But you still have the underlying problem of the sap-sucking insect that produced the honeydew that the sooty mould grows on. Read the section on Pests in this article to try to track down which insect that is and find out what to do about it.

Root rot

What is root rot?

Root rot is a common fungal disease that any plant, either indoors or in the garden, can get. It’s usually caused by overwatering or letting the plant stand in a pool of waterlogged soil. The roots become waterlogged which stops oxygen getting to the plant.

How to recognise root rot?

You should be able to recognise root rot quickly in your orchid plant as the roots aren’t buried out of sight in soil. (See Do orchids need special soil for information about what to plant your orchid in.)  If the roots of your orchid are shrivelled or spongy, or black or brown, they have root rot. You’ve been overwatering the plant and not draining away the excess water.

How to control root rot

Get control of your watering regime for the orchid. My article When and how to water my orchid? gives a detailed description of the different ways of watering the plant.

But the general rule of thumb is to wait until the potting mix is dry and then to water the plant, making sure that all excess water drains way. This may be once a week in the winter and twice a week in the summer. But the frequency depends on your own individual orchid plants.

For more information about root rot, please read Preventing and treating orchid root rot.

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