Houseplant Pest Profile: Scale

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There are 8000 species of scale insects worldwide, and they vary quite a bit in terms of appearance.

I don’t currently have any plants with armoured scale (the worst one imo), but I lost a big palm to it once, and I never want to encounter it again.

Although, it was kind of fun picking the adults off.

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What do scale insects look like?

There are two kinds, armoured and soft-shelled. The soft-shelled ones are usually covered with a waxy, shell-like cover. Doesn’t reading this make your skin crawl? It’s making my head itch.

Armoured scale looks like, er, your plant has little scales. You can flick them off with your thumbnail. They hardly look like bugs at all, which makes them a thousand times worse. They’re sneaky.

Some scale insects produce a substance rather like cottonwool. Did you know that mealybugs are a type of scale insect? Well, now you do. The white stuff they leave is left by other scale insects too. It’s gross.

Before picking the scale off willy-nilly, make sure it is scale, and not just corking, which is a plant’s way of giving itself more stability and strengthening its stem. I have a whole article with pictures here.

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What plants do scale insects prefer?

As I said, I’ve only had the armoured ones on palms (as has my dad, so they definitely like palms). I had my palm next to a yucca and the yucca is fine (well actually there were two, but one killed the other, but that was fratricide, not death by scale insect).

You’d never think that having a nice hobby like growing house plants could end up so much like a soap opera, would you?

However, they’re a fucking nightmare economically and can be found on everything from crops to trees to grasses. And since there are 8000 species they’re going to turn up freaking everywhere.

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Where to look for scale insects

The adults tend to congregate on the leaves, the juveniles on the stem, but I’ve found adults hunkering down in the crevices between the stem and leaves.

I also read somewhere that the little black spots on the stem re nymphs, but I can’t find any information that backs that up.

Maybe I dreamt that.

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How your plant contracts scale

The scale insect nymphs are very fast-moving and can crawl from one plant to another. Once they’re adults they stay where they are and barely moving, hence why you can just flick off the scales.

Some of the species boast winged males, but they’re not very strong fliers. A common thread that unites many species of plant pest. Gosh, it must be awful to have wings but still be crap at flying.

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How scale damages your plant

Like aphids, they excrete honeydew, which can lead to sooty mould and fungus.

Scale insects weaken plants. They’re ‘true bugs’ (that’s the real, scientific name of their order), and they use their sucking mouth[arts to extract sap from plants.

They can cause a plant’s growth to become stunted and can cause yellowing of the leaves and eventually leaf drop.

My palm didn’t drop any fronds, it just went yellow and died very slowly. It was awful. Scale is the worst.

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Conditions scale insects thrive in

Since the species vary so much, it depends. So I’m guessing that no matter what condition you have, a type of scale will thrive there. Yay!

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Control & elimination of scale insects

There is a species of small, black ladybird that likes to eat scale insects, as well as lacewings.

Insecticides can be more or less effective depending on the type of scale and how armoured their shell is.

Neem oil (or any horticultural oil) can be rubbed onto any kind of scale insect – it can both suffocate them and prevent them from breeding.

It’s worth knowing that not all species of scale are harmful to plants, but I couldn’t tell you how to distinguish between the killers and the ones that are just looking to casually chill on your plant.

Rubbing a rubbing alcohol-soaked cotton ball up and down the stems of scale infested plants can help kill some of the nymphs and adults.

Spraying your plant with a dish soap and neem oil mixture can help, but some stubborn armoured species may remain – douse them in neem oil.

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Preventative measures to ward off scale insects

You really need to catch scale insects early – especially the armoured ones. Other kinds, such as mealybugs can cause long-term damage but can also live on plants in smaller numbers and not hinder your plant too much.

It’s still important to keep populations under control though – you don’t want a couple of bugs to turn into an infestation.

If you’re worried about scale insects, you can add a layer of diatomaceous earth to the top of your plant pot. This kills scale insects (and various other bugs) by absorbing some of the lipids in their exoskeleton. Nice.

This layer helps the bug retain the correct levels of water in its body, so when the diatomaceous earth absorbs the layer, the water evaporates and the bug perishes.

And to think I thought those little yellow traps were cruel.

Using diatomaceous earth to top-dress your plants is a great way to keep pests under control. It’s something I’ve been considering, but need to do a bit more research into. I’ll be sure to share my information when I find it.

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  • Asexual reproduction. Again.

  • Adults are almost totally immobile. Sad.

  • You know those red cochineal bugs that vegans can’t eat (not that we want to)? They’re a kind of scale insect. As is shellac.

  • Actually, a number of scale insects are used for making dyes and waxes.

  • You can get giant mealybugs. Terrifying. I don’t know how giant, because Wikipedia didn’t have a photo, but giant suggests they’re, you know, sizeable.
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I fear scale, because they’re the only bugs that have killed a perfectly healthy plant. I know mealybugs killed my Dieffenbachia, but that was more of a mercy killing. The poor plant was absolutely on its way out.

My palm had little black flecks around the base of the stem which I think were the baby scales. My palm was too infested for it to make a difference, but I bet if you attacked them with neem oil you might stand a chance.

Now let’s all pray that none of the 8000 (!) species of scale insects darkens our door again.

Tips in the comments are appreciated!

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Caroline Cocker

Caroline is the founder and writer (and plant keeper) of Planet Houseplant

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