Help! My Frangipani Has Scale Insects!

‘Hi Doc, Rico here! 

Can you help with my frangipani? I’d like some advice regarding black leaves and white fluffy spots under the leaves.  Before I go and administer a systemic pesticide to my lovely trees, I’d like a second opinion – are you willing to help, or even visit and consult?

Thanks – Rico’


There are a few things going on here, hold off on the pesticide for now and read on…..

This description is characteristic of a severe mealybug infestation, a type of scale insect from the family Pseudococcidae. They are oval shaped and covered in a mealy or floury looking wax, hence their common name. They are usually around 5mm in length (the size of a small fly) with a white powdery or fluffy appearance. They feed by sucking the sap from leaves, buds and stems, causing wilting, and distortion of new leaves. In heavy infestations they will conglomerate, and resemble cotton wool like masses and for this reason a colony is often mistaken for fungal growth.

The blackened leaves indicate a fungus known as sooty mould, which is secondary to the infestation of the bugs, and resembles a layer of soot on the leaves as the name suggests. The sooty mould thrives on the sugary honeydew excreted by piercing sucking insects, such as mealybugs, aphids and other types of scale. The result is foliage and stems smothered in a sticky black goo, blocking light and impeding photosynthesis to the further detriment of the plant.

Now this is where it gets even more interesting.

Ants are also attracted to the honeydew, as a food source and once on the scene will farm mealybugs around the plant and then from plant to plant…what a bunch of cheeky little buggers!

But still it gets worse from here, because the ants protect them from predators and parasites, this means that mealybug infestations tend to be much more serious in the presence of ants. it gives a whole new meaning to the term ‘ant-farm’. And to think that humans would like to take credit as the only species to conduct agriculture on a large scale, pardon the pun.

It is also known that by stroking the back of some aphids (another type of scale) with their antennae, that ants can induce a honeydew droplet. A plant covered in ants is the canary in the coal mine, which will alert you to the fact that there is a scale insect problem, and may allow you to nip it in the bud early on before a more serious infestation takes hold.

Left unchecked a plant can be severely weakened, even destroyed in a surprisingly short space of time. In short, the bugs suck the sap, the ants harvest the by-product, farming the bugs around to produce even more, which then leads to sooty mould and the cycle repeats until you either kill the bugs or they kill the plant, whichever comes first. What started with the bugs has snowballed into a threefold problem, bugs, ants and sooty mould. It is a relentless coordinated attack.

The solution is simple. For a badly infested plant much of the affected foliage should be cut away before attempting any chemical control, organic or otherwise. Prune off all severely affected foliage, to stimulate new growth. Foliage smothered in sooty mould is about as useful as a gangrenous limb. Cut it, bag it, and throw it out.

Concoct a home remedy, by mixing vegetable oil with dish washing soap. This is preferable to a highly toxic chemical spray and just as effective. Mix a cup of vegetable oil with a cup of dish soap. Dilute this mixture at a rate of 1 part to 20 parts of water, then apply to scale infestations. The dish soap is toxic to the scale and will break down their waxy protective shell and suffocate the critters in the process. The vegetable oil ensures the soapy water adheres to and smothers the scale, whereas soapy water alone will simply run-off and not be as effective. The soap must stick!

What we have created here is essentially a home-made pest oil with much lower toxicity to the environment and humans but with just as much efficacy. It is also more economical than buying chemicals, and you probably have the ingredients ready to use, right now sitting under your kitchen sink. Just mix it up and spray on with a garden applicator, alternatively using a wet sponge will also suffice. Make sure to wet the undersides of foliage and the stems, as scale tends to colonise most heavily on the underside of leaves and close to the trunk. Leave the solution overnight and then attack the weakened bugs with a strong jet of water or wipe them off with a cloth. Alternatively let them drop as they die. Repeat applications will be necessary.

Mealy bugs are found in the open garden but also like the sheltered conditions of a greenhouse and can even find their way indoors. If you spot them early getting rid of them can be as simple as wiping them off with an old cloth. Remember, even mild pesticides such as our homemade pest oil should only be used as a method of last resort as it can also harm the beneficial insects responsible for pollination, and others that help keep pests under control such as spiders.

Even so called ‘organic’ pesticides should be used sparingly, formulations that contain neem and pyrethrum are very poisonous until they biodegrade. You can use confidor or horticultural oil, the residue will last much longer and you’ll also be poisoning the bees, pollinators and yourself. What’s the point of gardening if you must spray potent poison all around the place. It turns a natural pastime into a dangerous one. Don’t forget your gas mask, and full protective body suit while you’re at it. You wouldn’t want to get it on your skin let alone inhale the vapours also. Even organic pesticides such as neem are extremely toxic to humans!

An alternative method that works well in the earlier stages of infestation, is to blast the bugs off with a hose at high pressure, although this may not be an option depending on how advanced the infestation or how delicate the foliage is. To be effective this treatment should be repeated every few days until the mealy bugs are gone.

Try to avoid all commercial chemical sprays even for other pest problems, as they will also kill beneficial insects such as ladybirds and lacewings, which are known predators of scale. The mealybugs however often escape unscathed if not directly targeted because they reside underneath the leaves and are covered in a protective wax. Remember, even natural sprays should only be used sparingly and as a last resort, because they can also harm beneficial insects.

Regular inspection of your plants is essential. If you ever see ants crawling up and down stems and across leaves you should realise that you have a problem. Ants are typically found on the ground, and will only inhabit plants if there is a honeydew source, which means either mealy bugs, aphids or other types of scale. If you spot a colony of ants wandering around on your plants alarm bells should be ringing! A quick check of the growing tips and the undersides of foliage will reveal the bugs.

Now that you know all about mealy bugs, how to spot them and how to stop them, you’ll be able to nail them as soon as you come across them!


Dr. Kris

Garden Doctor


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