Green scale
You will often spot the black sooty coating and the ants tending it long before you notice the flat, still insect actually causing the damage.
- Pest type
- Soft scale insect (sap-sucker)
- Attacks
- Leaf undersides, tender shoots
- Worst in
- Shaded, humid conditions; heavy ant activity
- Main risk
- Honeydew + sooty mould cutting plant vigour
Overview
Green scale is a small, flat, oval, greenish sap-sucking insect that settles on the underside of coffee leaves and on tender shoots and stays still while it feeds, which makes it easy to mistake for part of the leaf at first glance.
As it feeds, it excretes sticky honeydew onto the leaves below, and a black sooty mould soon grows on that honeydew, coating the leaf surface. Ants are strongly attracted to the honeydew and will often be seen moving up and down the stem in numbers — they protect the scale insects from their natural predators in exchange for this food source, which is often the first clue a farmer notices, well before spotting the scale insects themselves.
How to identify it
- Small, flat, oval, greenish or yellow-green insects sitting motionless on the underside of leaves and on tender shoots.
- A sticky honeydew coating on leaves below infested growth, followed by a black sooty mould growing over it.
- Heavy ant traffic moving up and down the stem is a strong early clue, since ants tend and protect the scale insects for their honeydew.
The damage it does
- Sap-sucking weakens the plant over time, reducing vigour and, in a heavy infestation, cutting yield.
- The black sooty mould that grows on the honeydew blocks light from the leaf surface, cutting photosynthesis on top of the direct sap loss.
- Ants that tend the scales actively chase away the scale insect’s natural predators and parasites, which lets numbers build up further than they otherwise would.
When to act
- Check the underside of leaves and young shoots regularly, especially in shaded, humid parts of the plantation where the pest is worst.
- Treat ant trails on the stem as an early warning sign worth investigating, even before you spot the scale insects themselves.
- Act before sooty mould has spread widely across the plant, since preventing it is easier than cleaning it off afterward.
Organic & cultural control
- Control the ants tending the scales — for example with a sticky barrier around the stem — since removing their protection lets natural predators and parasites bring the scale population back down on their own.
- Prune to open up shaded, crowded growth, since dense shade favours the scale; better light and airflow make conditions less comfortable for it.
- A soap solution or neem-oil spray smothers scales on reachable growth and is gentler on natural enemies than a broad-spectrum insecticide.
Chemical control
- A recommended systemic insecticide can help where infestation is heavy and ant-protected, since contact sprays struggle to reach scales sheltering under ant-guarded honeydew.
- Combining chemical control with ant management gives better results than spraying alone, since ants will keep protecting new scales otherwise.
- Confirm the current recommended product and dose for coffee with your local KVK or coffee board advisory.
Crops most at risk
Where this problem is called out in our crop guides — open a crop for the full package of practices.
Frequently asked questions
My coffee leaves have a black, sooty coating, but I cannot see any obvious insect. What is causing this?
This is very likely green scale. The insects themselves are small, flat and greenish, and sit still on the underside of leaves and on tender shoots, so they are easy to miss. The black coating you are seeing is sooty mould growing on the sticky honeydew the scales excrete — check the underside of nearby leaves closely for the flat insects themselves.
Why are there suddenly so many ants on my coffee plant?
Ants are strongly attracted to the sticky honeydew that scale insects produce, and they protect the scales from natural predators in exchange for this food. Heavy ant traffic up and down the stem is often the first sign of a scale infestation, even before you notice the insects themselves.
What is the best way to bring a scale infestation under control?
Controlling the ants that protect the scales — for example with a sticky barrier on the stem — is one of the most effective steps, since it lets natural predators do much of the work. Combine this with pruning to reduce shade and, for heavy infestations, a recommended systemic insecticide confirmed with your local KVK or coffee board advisory.
Symptoms and control measures follow ICAR, KVK and state agricultural-university package-of-practices; any named chemical is a registered use. Always read the product label and confirm the dose for your crop and zone. editorial policy.