Olive black scale and sooty mould: the H on its back

If the leaves and branches of your olive tree are coated in a black, sooty film, the blackness is not the problem. It is only the symptom. The cause is the small, hemispherical scales sitting on the branches, each carrying an "H" on its back: the olive black scale (Saissetia oleae).

At heavy infestations crop losses reach 60-70%; in later years the tree bears nothing at all and turns shrubby.

Where does the sooty mould come from?

As larva and adult the scale feeds by sucking the tree's sap. As it does so it excretes sugary substances that coat the whole tree. Saprophytic fungi grow on that sticky layer and produce sooty mould (fumagine) — the black film over leaves and branches.

The tree is harmed on two fronts at once:

1. Sap is drained — direct loss of vigour

2. Sooty mould blocks photosynthesis — the leaf cannot work

As numbers build, leaves and fruit drop and branches die back.

How to recognise it

The "H" on the back is this pest's signature — no other scale insect carries it.

The year's cycle

• Winter: usually passed as 2nd and 3rd instar larvae on the leaves.
• Spring: as the weather warms the larvae migrate to the shoots, develop there and become adults.
• First crawlers: May in the Mediterranean, June elsewhere. The emergence period lasts 1-3 months.
• Summer: crawlers wander over leaves and shoots for 1-7 days, then settle and feed.
• Late summer: they become 2nd and 3rd instars and enter winter in that state.

Some individuals complete development quickly and produce crawlers again in autumn — a partial second generation.

Reproductive capacity is very high: 500-3,000 eggs may be counted beneath a single female's shell.

Nature pushes back, though: if winter temperatures fall below 0°C for five or six days, or if summers bring dry heat, there is substantial natural mortality.

⚠ Spraying manufactures this pest

The instruction is explicit:

t; The olive black scale becomes a problem essentially in groves sprayed heavily against various pests and where cultural management is not carried out regularly.

The reason is the wealth of natural enemies. Studies in Türkiye have identified 8 parasitoids and 9 predators — Chilocorus, Exochomus and Scymnus ladybirds, Metaphycus parasitoids, Chrysoperla carnea.

The instruction sets a specific rule around this:

t; When natural enemies hold the pest above 50% under control, no chemical treatment should be applied.

In other words, before spraying you measure the parasitism rate. Above 50%, no spray.

The cultural measure: pruning

For this pest the most effective tool is not a chemical but pruning. The logic is simple:

t; The olive black scale finds less chance of survival on vigorous trees.

What to do:

• Cut out dead branches
• Prune so air and light reach the inside of the tree
• Fertilise to good practice
• Prune after the last frost and hoar frost — the date varies by region and year
• Use certified nursery stock when establishing a grove

Pruning lowers the population directly and strengthens the tree at the same time.

If spraying is needed: shoot counts

If early-season checks show natural enemies falling short (parasitism below 50%), treatment may be applied. Timing follows crawler emergence:

1. Take a 20-25 cm shoot from each of four sides of a representative number of trees

2. If 80-100 cm of shoot carries 4-6 adult female scales on average, the threshold is crossed

3. From late May, the first spray when 50% of eggs have hatched

4. The second spray when 90% have hatched

⚠ If the grove is being treated against another pest (olive scale, for example) during the 50-90% hatch window, that application also suppresses black scale — one treatment may be enough.

Preference goes to summer white oils, the least harmful to beneficials.

⚠ No product names or doses are given in this article. Plant protection products and doses appear in Annex 4 of the Ministry's instruction and registrations change. Consult the current licensed product list and your Provincial or District Directorate of Agriculture and Forestry before treating.

Hosts

Its main host is the olive. It also damages citrus, plane, quince, pomegranate and bay — such trees around the grove can be a source of infestation.

Summary

• Signature: a hemispherical 4-6 mm scale with an "H" on its back.
• The black film is sooty mould (fumagine), fungus growing on the scale's sugary excretion.
• Damage is twofold: sap loss plus blocked photosynthesis. At heavy infestation, 60-70% crop loss.
• 500-3,000 eggs beneath one female.
• Five or six days below 0°C in winter, or dry summer heat, cause natural mortality.
• ⚠ Heavy spraying manufactures this pest — it has 8 parasitoids and 9 predators.
• ⚠ If parasitism exceeds 50%, do not spray.
• The most effective tool is pruning: black scale cannot hold on a vigorous tree.
• Threshold: 4-6 adult females per 80-100 cm of shoot. Spray at 50% and 90% egg hatch.
• Prefer summer white oils.

Source

t; Republic of Türkiye, Ministry of Agriculture and Forestry, General Directorate of Agricultural Research and Policies, Department of Plant Health Research. Olive Integrated Pest Management Technical Instruction. Ankara, 2022.

Related reading: The olive moth · Olive fruit fly control · Olive knot · Verticillium wilt