Mealybugs (MB) (Pseudococcidae) are sap‑sucking insects that promote the growth of sooty mould (SM) through honeydew secretion, posing an emerging threat to oil palm (Elaeis guineensis Jacq.) productivity in Sabah, Malaysia. This study quantified the field‑level relationship between MB infestation and SM colonisation in oil palm plantations and developed a standardised Mealybug-Sooty Mould Severity Index (MSI) for field surveillance. Study was conducted across four plantation blocks in the FGV Sahabat Region from February to September 2025, representing a gradient of palm age: Young (1–3 years), intermediate (4–6 years) and mature (13 years). A total of 120 palms were assessed, with fronds examined at upper, middle and lower canopy levels using binoculars to detect MB colonies and associated SM. Mature palms exhibited the most severe infestations, with a strong positive correlation between MB density and SM colonisation. MSI values increased with palm age and were consistently highest in the lower canopy. These findings highlight the close linkage between pest infestation and fungal development and emphasise the importance of field‑based surveillance to support timely intervention within integrated pest management programmes.