Call me a meanie, but the first time my houseplants got scale, I didn't even notice! I missed the little brown bumps on the stem. I didn't notice some leaves were suspiciously sticky.
To be fair, the common species of scale that we get in the houseplant hobby were made to win hide and seek, looking like nothing more than tiny raised brown or black bumps on leaves and stems, easily mistaken for a part of the plant, especially because they don't move.
This guide covers everything you need to know so you don't make the same mistake I did. Including how to spot scale on your indoor plants and how to treat scale successfully, once and for all.
The good news is scale normally won't kill their host plant, as least not in any great rush. Although it's a pretty miserable existence for our plants being constantly fed on to the point that don't have enough left for themselves to be able to grow anymore.
Your plant may not die outright, but stunted or no growth is common, growth tips or entire stems may die off, and leaves may die and drop off. Never say never though. A bad infestation can be too much for some houseplants, especially if there's anything else already making them weaker, or if they are smaller plants.
What to look out for if you think you have scale
How big are scale?
They vary in appearance depending on the species, but the most common houseplant scale species are small but visible, around 2mms to 6mms long and 2mms to 5mms wide, getting larger as they mature.
What colour are scale?
They normally start out lighter-coloured, often greenish- or yellowish-brown and change to darker brown as they mature, often with darker-brown mottled speckles and spots, and darker in the middle. They are usually slightly oval shaped but can have a different appearance and shape depending on the species.
Where do scale hang out on houseplants?
You'll typically first spot them on stems, but if you take a closer look underneath leaves you'll often see them there too, usually feeding along the veins. The sap that flows under leaves is the good stuff they're after as it's more nutrient-rich than the sap that flows along the top of leaves.
PS: Watch out for honeydew
Not all species of scale produce it, but if you feel or see something sticky on the leaves, you've found honeydew. It's a sticky, sugary residue which most pests that feed on plant sap excrete.
Honeydew can also grow a black fungus called sooty mould that can end up covering the leaves and eventually prevent a plant from photosynthesizing. The honeydew that they excrete can also attract ants. That's a lot of unpleasantness all from one pest!
Know your scale lifestages
There are differences between different species of scale, but they tend to share the same life cycle, which goes something like this...
Depending on the species, scale usually produce one to five generations a year and multiple generations can overlap which is why you normally need to use two treatments at the same time to break the life cycle to stop them coming back endlessly. Scale typically over-winter in the egg stage, so it may not be until spring or summer that you realise something's wrong.
Eggs hatch in spring
Eggs tend to stay hidden under the female's protective covering over winter, waiting for the right conditions. As the weather warms up, eggs hatch into mobile nymphs called crawlers, become active and start to feed.
The crawler stage (on the move)
At the crawler stage, nymphs are on the move to find good feeding sites, often underneath leaves. Once they settle, they pierce the plant with their mouthparts, and start feeding on the sap.
It's this mobile, juvenile stage of the life cycle that's the best chance at treatment. This often coincides with early spring but they typically don't spend long in this stage (sometimes a matter of days).
If caught early and treated at the right time, and there's only one generation doing the rounds, you may get away with just using a mineral oil spray (my go-to is Enspray 99 by Grosafe). More about what to use and when coming up.
It's this mobile stage where scale can spread easily without you realising since you can't see the tiny nymphs as they're about half a mm in size and coloured to blend in with your plant. The wingless crawlers can be moved by wind, other animals, ants and us when we touch or move our plants or don't clean our tools between plants.
The adult stage (bye bye legs)
This may be the first stage you discover you have scale. Since it's more common to miss scale until they reach the adult stage - and you may have more than one generation using your plant as a buffet at the same time - you'll normally need two different treatments, used in tandem, to target two life stages at once, in order to break the life cycle and fully eradicate them. Treatment options coming up next.
After the mobile crawlers have settled in, they moult and lose their legs, forming their permanent, armour-like protective layer that gives them their common name of scale.
Many species of scale don't need males to reproduce. The visible, armoured scale you see on your plants are the females. Once they reach the adult stage, females stay put, often feeding for months, eventually maturing enough that they produce the next generation of eggs. One female can lay from hundreds to several thousand eggs depending on the species.
Watch out for ants!
If you've got ants as well (honeydew is like lollies for ants), controlling the ants is an important but often missed step to help you win the battle against scale and stop them spreading. Ants will keep scale clean and protected as well as pick up scale in the mobile crawler stage and shift them to more of your plants! I use Exterm-An-Ant to target the ants. Weird to rave about ant bait but that stuff is superb.
How do scale feed on plants?
One of the many 'vampire' pests of the plant world, scale are sap-suckers. They insert their stylet, a needle-like mouthpart, and suck out nutrient-rich sap, usually from underneath leaves along the veins where sap is more sugary and nutritious.
Why conditions do scale like?
Unfortunately scale can be hanging out on your plant in one form or another (often in more than one life stage at the same time), throughout the year. However, they do prefer the warmer months, with females hiding and protecting eggs over winter, nymphs hatching in spring, maturing and feeding over summer and autumn, until it starts all over again next spring.
Which species of scale do houseplants get?
Treatment is much the same and the life stages don't differ too much between species; but to help with identification, the species you'll see most often on houseplants are soft brown scale (Coccus hesperidum), the most widespread species in New Zealand.
Scale come from the superfamily Coccoidea, an insect group with about 7,500 species worldwide, but we have about 400 species in New Zealand.
Other species of scale you might see on ornamental houseplants include the white cottony cushion scale (Icerya purchasi), juniper scale (Carulaspis juniperi, which prefers woodier plants like Ficus and Bonsai), soft wax scale (Ceroplastes destructor, which tends to favour citrus but can find its way indoors), and brown scale (Parthenolecanium corni).
How to treat scale
Don't stop here! Make sure you keep going to the end (below the short version), to get lots of tips for successfully treating scale once and for all, instead of having to repeat this all over again every few months.
Here's the shortest version:
- Isolate the plant
Optional: Remove visible adults, treat ants, clean off honeydew - Treat with insecticide every 7 to 10 days for 1 to 2 months
- Keep plant isolated for another month
The products to get
(or Enspray 99 Ready to Use which comes with a Super-Fine Spray Mister bottle included)
Optional > Rubbing Alcohol, Exterm-An-Ant, BioNeem, Super-Fine Spray Mister
Also handy to have cotton buds or cotton face wipes (to apply the rubbing alcohol)
Treatment plan for scale
DAY 1
Always isolate the plant until after treatment. Scale can spread.
Optional: Instantly kill visible adult scale with rubbing alcohol using cotton wipes or cotton buds (check the directions and tips below before use).
Optional: If ants are present, use ant bait (like Exterm-An-Ant), as ants will protect scale and spread them to other plants (directions and tips below).
Optional: If leaves are sticky, use rubbing alcohol to wipe honeydew off leaves (this reduces growth of black mould and reduces food supply for ants). Check the directions below for how much to use, mixed with water, and how to apply.
WEEK 1
Spray with a mixture of Enspray 99 and Groventive together.
(how much to use of each and how to apply them is coming up below)
WEEKS 2, 3 and 4
Spray again every 7 to 10 days with Enspray 99 by itself.
(or with BioNeem, or for best results, alternate Enspray one week, BioNeem the next)
WEEK 5
Spray with Enspray 99 mixed with Groventive.
Do you have to combine Groventive with Enspray?
No you don't. You can spray with Groventive by itself if you don't have Enspray. Enspray is a contact insecticide. Groventive is a systemic insecticide. Enspray also works as a spreader, helping Groventive work better when you use them together.
How long do you need to keep treating for scale?
The typical life cycle of soft brown scale in good conditions is around 60 days however multiple generations may be present at the same time. One treatment with Groventive turns your plant's sap toxic to pests for up to one month but can take one to two weeks to work its way around the plant. Do not apply Groventive more than 4 times in a 6 month period to avoid pests becoming resistant.
Spraying with Groventive at week 1 and again around week 5 or 6 should provide over 2 months protection, but Groventive is best used along with Enspray 99 or BioNeem to break the lifecycle. Using only a contact insecticide, or only a systemic, is less effective on scale compared to other pests.
If you still see signs of scale, repeat the treatment schedule above one more time. Keep the plant isolated for one month after the last treatment. Check frequently on stems and under leaves to make sure there are no signs of scale or honeydew before returning your plant to your collection.
Step 1. Get the alcohol
Even if you skip this step, always start by isolating the plant. At the mobile crawler stage, scale can jump ship and set up home on other plants. Make sure to treat for ants too as they can transport scale to your other plants. More about treating for ants at the end.
How to use rubbing alcohol to instantly kill visible scale
This first step is optional but I highly recommend not skipping this step if you have visible adults right now. One female can lay hundreds to thousands of eggs so each adult you get rid of makes a huge difference (all visible scale are females, adult males fly and are short lived).
Grab a bottle of rubbing alcohol and get to work. At this stage your goal is to quickly eradicate visible females (and any eggs they are protecting along with them). Wet a cotton wipe or cotton bud with the rubbing alcohol. Target the visible adults, wiping them away or touching the cotton bud to them individually. They will die immediately, turning an orangey colour, and you can wipe them off.
How to use rubbing alcohol to remove honeydew
You can also use rubbing alcohol to remove honeydew (that sticky residue), from leaves. Cleaning the leaves will reduce the spread of the black fungus called sooty mould, will remove the food source for ants, and help the plant recover by allowing it to photosynthesize better and breathe (transpire) properly again.
Mix 1 part rubbing alcohol with 3 parts water and wipe leaves (making sure to do both sides of the leaves). Letting leaves air dry is fine if the plant isn't sensitive, but if it is or you're unsure, wipe again with plain water or wash / spray leaves with plain water after using rubbing alcohol.
You can also treat a couple of test leaves first if you're worried and wait 2 to 3 days before using it on the rest of the plant to check there's no reaction.
How to check you have the right strength of alcohol
Make sure it's rubbing alcohol, not pure alcohol. Always check the strength before using it on plants. I use a brand called Isocol. Rubbing alcohol is normally between 60% and 70% isopropyl alcohol but always check the strength on the label, even if they call it rubbing alcohol.
Which plants are more sensitive?
As a general rule, plants with thinner, velvety or furry leaves are usually more sensitive. Orchids and ferns are also known as being more sensitive to oil-based treatments in particular.
What are the best times of day to apply pest treatment?
As with all insect treatments applied to foliage, it's best to use rubbing alcohol early morning or early evening. Apply on a day not expected to reach over 30 degrees Celsius. Avoid direct sun on leaves during treatment.
Even at the lower strength, rubbing alcohol is strong stuff. There is always some risk when treating plants with any insecticide. You can do test applications first and wait two to three days to check for a reaction first, especially for sensitive foliage.
Some leaf browning, wilting, leaf damage or leaf drop can happen when using insecticides, which should come right quickly after treatment when your plant can put out healthy new growth again.
Step 2. Apply a contact insecticide
Continue to keep the plant isolated during treatment. As mature scale have a protective, waxy 'shell' they are harder to target with most insecticides, but when the nymphs are in the mobile crawler stage, they lack that protective shell.
There are a couple of options for contact insecticides to use for this step. I use Enspray 99 mineral oil, but BioNeem is another option. In fact, because they work differently on pests, you'll often see it recommended to alternate between them for best results (usually using one or the other, applied every 7 to 10 days).
How does Enspray kill scale?
Enspray 99 is a very fine mineral oil. It's not a poison. It can be used frequently without pests building up a resistance to it. It kills by smothering pests, preventing them from breathing.
Although we're talking about treating scale on houseplants, Enspray is also certified for organic use, can be used on fruit and vegetables, and is a safer option for people, pets and beneficial insects.
Enspray is most effective against crawlers, but can kill adult scale also as well as preventing eggs from hatching BUT it does need direct contact to work and should be repeated every 7 to 10 days (or if used with BioNeem, every 14 to 20 days, using BioNeem in-between).
Enspray has the added benefit of preventing fungal growth at the same time. The honeydew that scale excrete can also cause a black fungi called sooty mould to grow on leaves, which suffocates the plant and stops it photosynthesizing. Enspray also makes Groventive work better (Groventive is the next step, coming up).
How does BioNeem kill scale?
Enspray 99 works faster, suffocating pests on contact and stopping them breathing. BioNeem works slower but helps break the pest's life cycle in multiple ways. Using them either mixed together or alternating between them, is recommended for harder to kill pests or bad infestations.
Like Enspray, BioNeem won't harm beneficial insects. The active ingredient (azadirachtin), is an antifeedant, meaning it stops pests feeding. BioNeem also gets in the way of pests molting so they can't progress to the next stage in their life cycle. It also interrupts their ability to reproduce.
The active ingredient is of low toxicity. It does not kill insects on contact. It starves insects to death, disrupts their development so they don't become adults, and reduces the population.
Why neem 'doesn't work'
Just using any old neem product will NOT work. All neem products are not the same. Some are not even insecticides. BioNeem is enhanced with 10,000 ppm of pure azadiractin (the active ingredient), as well as neem oil extract. It's a registered, tested, proven insecticide.
A lot of neem oil products do not work as insecticides (even though companies still claim they do!), as neem oil by itself is often too low in azadiractin. They have not been proven to work as insecticides and are not registered.
Some neem products DO still offer mild pest repellent properties when used as a leaf shine, but won't work on an active infestation (like BioNeem does). That's why you'll see people say 'neem doesn't work'. It does! They just didn't get one that's been proven / tested / registered.
How to apply Enspray or BioNeem
Both Enspray and BioNeem are contact insecticides. so good coverage is important. Spray all the foliage and stems, especially underneath leaves. Best applied using a fine mist sprayer like the Mister 360 or Egmont 360 or the free refillable spray bottle included with the ready-to-use Enspray 99.
For an active infestation, spray Enspray 99 every 7 to 10 days, or BioNeem every 7 to 14 days. A clever approach is to use Enspray the first week, BioNeem the next, and repeat.
Enspray directions for scale
For indoor plants, use 10mls Enspray per 1 litre water and apply with a fine spray mister, covering leaves (top and bottom) and stems. Use fresh.
BioNeem directions for scale
For indoor plants, use 1ml BioNeem per 1 litre water and apply with a fine spray mister for good coverage of leaves (top and bottom) and stems. Use fresh.
When using a spray mister for any pest products, discard leftover mixture and clean the spray mister thoroughly with warm soapy water, spraying it through the nozzle also to clean it out. Pest control left in spray bottles can clog the nozzle and other small parts.
Step 3. Apply a systemic insecticide
Scale are one of a few pests where a contact insecticide alone often won't fully eradicate them. Most insecticides are contact insecticides. That's why you'll often find scale keep coming back again and again. That's where a systemic insecticide - like Groventive - is needed...
How does Groventive work?
Once absorbed through the leaves, Groventive travels throughout the system of the plant. It travels through the sap highways, both up the plant in the xylem, and down the plant in the phloem. It normally takes one to two weeks to make its way around all the plant (longer in winter as sap travels slower when it's cold).
When sap-sucking pests like scale feed on your plant, they end up getting a dose of Groventive. The active ingredient is spirotetramat which stops pests growing, developing and functioning properly.
Groventive stops pests maturing from nymphs to the adult reproductive stage. It also targets adult scale, reducing their ability to produce and lay eggs.
Groventive is particularly effective on immature stages of scale's life cycle, so while Enspray or BioNeem targets the visible adults, Groventive gets to work against the almost invisible nymphs when they hatch.
Groventive does NOT have to come in direct contact with the pest like Enspray, BioNeem or other contact insecticides do. It also stays active for longer than contact insecticides.
How often to use Groventive for scale
Spray with Groventive at the start of treatment, and again around week 5 to 6. Use it by itself or ideally use mixed with Enspray for better results.
Groventive gets absorbed through the leaves and travels around the plant in the sap, turning plants toxic to pests for 4 to 6 weeks.
Do not apply Groventive more than 4 times in a 6 month period to avoid pests becoming resistant.
How to apply Groventive
Groventive does NOT work when applied via the roots. That's a myth that does the rounds every so often (or wishful thinking?), but Grosafe (who make Groventive), have confirmed with me directly that it will not be as effective and may not even be absorbed into the plant at all.
Apply Groventive by spraying foliage well, including underneath leaves, using a fine mist sprayer. Make up and use fresh. For best results, apply Groventive mixed with Enspray. Mix Groventive (or Groventive and Enspray) outside of the spray mister, then add to the bottle.
Groventive is not as fine as Enspray (one reason Enspray helps Groventive work better), so it can block the nozzle and small parts of a super fine spray mister if left sitting in the bottle. Discard what you don't use.
Always give the bottle a good spray through and wash out with warm soapy water straight after use, and store dry (the same as you should after using any pest product in a spray mister).
Once mixed, you can't store Groventive to use later. I make up small batches at a time so nothing goes to waste. When pest products are prepared most start to break down and lose efficiency in a short period of time. Best made up and used fresh each time.
Groventive directions for scale
Use 10mls Groventive per 1 litre of water.
Optional: Treat the ants too
Ants are like scale's bodyguards. They will pick up nymphs and shift them to more plants. They also keep scale clean and protected. All so the scale can produce more of that delicious, sticky, sugary honeydew that ants love to feed on.
There are a lot of ant species in New Zealand. Not all ant products work on all species. But Exterm-An-Ant is one formulated in NZ that works on far more ant species than most. So if you've tried other ant baits and the ants keep coming back, that might be why.
Rather than go into all the detail here, since this guide is focused on scale not ants, you'll find all the directions for use on the Exterm-An-Ant product page including tips for best results.