There olive fly (Bactrocera oleae), or oil fly, is a parasite particularly feared by olive growers because it attacks exclusively olive trees (Olea europaea), causes extensive damage to olives and degrading the chemical and organoleptic properties of the oil. The result is a percentage reduction in yield and a downgrading of the commercial product that no longer meets the legal requirements to be classified as extra virgin olive oil. In particularly critical years, it has been found that the fly can cause economic losses over a billion dollars in the area of Mediterranean producing countries. It prefers warm, humid climate conditions, and climate change is expected to favor its expansion, allowing it to colonize higher latitudes and altitudes with prolonged autumn activity. According to an analysis by Coldiretti, the 2025/26 olive oil campaign is going quite well, with an increase of 30% compared to last year, but the collection is uneven on Italian territory, with central and northern regions particularly affected by the activity of the olive fly.
Oil fly: characteristics and biological cycle
Bactrocera oleae, dipteran native to the Mediterranean basin, accidentally introduced into California and India, it is a parasite species-specific to the olive treebecause it only attacks this plant: adults are particularly attracted to ripe fruits which have softer texture and darker color. The adult is a fly about 5 mm long, black and brown in color, with a yellowish head and iridescent blue-green eyes. In midsummer, with the temperature around 25°C and a certain humidity, the females prick the drupes (olives) and, for each sting, insert an egg into the fruit with the ovipositor. From each egg a larva develops which, feeding voraciously on the pulp, digs a tunnel in the olive; this is why it is said leafminer larva. In approximately 4 – 6 weeks it undergoes the next two moults into pupa and then into adult and thus a development cycle is complete and a certain quantity of fruits destroyed.
If the climatic conditions of an area remain hot and humid, the insect can develop even 5 generations per year. The fly overwinters in the soil at the pupal stage. In general, winter and spring temperatures influence the timing and severity of infestations. Mild winter temperatures increase the survival of pupae in the soilwhile autumn temperatures below 15 °C reduce the development of the insect, as the adults fly less and the larvae develop more slowly. However, it is negatively affected by very high summer temperatures, above 32°C. Temperature, the presence of nutrients and humidity are also crucial for development soil: for example, low soil temperature and high humidity after a rainy period increase pupal mortality.
The oil fly is such an “ancient” insect (evolutionarily speaking) and the relationship with the plant is so specific that it is considered a classic example of “coevolution between host and parasite“: both have developed adaptations, the fly to use the olives, the plant to defend itself. One of the defense mechanisms adopted by the olive tree is the production of highly volatile metabolites (therefore aromatic secondary chemical products, such as phenols and terpenes) which in some cases discourage the fly from approaching the drupes or some cultivars have developed a thicker peel which makes pricking the fruit more difficult.
Direct and indirect damage to the olive and oil
The oil fly is a real one scourge considering that the harmful effects are on the fruit, both for table olives and for pressing olivesas well as on the oil produced. The larvae, nesting and digging tunnels in the olives, cause their premature fall from the tree (technically called drop) and above all they cause chemical alterations of the pulp, thus favoring the entry of other pathogens such as the fungus that causes the so-called “olive leprosy”.
The oxidative chemical alteration obviously affects the oil at the time of production; the most important effects are a increased acidity and a modification of the organoleptic characteristics of the product. A high free acidity (i.e. with a high content of free fatty acids) prevents the contaminated batch of oil from being classified as extra virgin olive oil according to the law: the extra virgin olive oil must in fact have a component of free fatty acids below 0.8%. The sensory alterations, therefore the flavor and aroma, involve a defect called “fly” characterized by an unpleasant odor and taste that is very reminiscent of rancid chocolate.
The situation of this year’s harvest
In this period the 2025/2026 olive oil campaign is underway and is in full swing olive oil production throughout Italy, the most suitable moment to verify the impact that the olive fly has had on national production. To date, based on the results of an analysis conducted by Coldirettitogether with the Associations Unaprol (Italian Olive Consortium) and Foa (Frantoi Oleari Associati), Italian oil production in 2025 had a 30% increase compared to last yearbut the yield of the crops was patchy with losses in some regions of the Center and North caused by the olive fly. Regions particularly affected were Umbria, Tuscany, Abruzzo and Emilia Romagna.
How olive growers can defend themselves: treatments and prevention methods
Considering that approximately 95% of the world’s olive oil supply comes from European countries, in particular Spain, Italy and Greece which also have suitable climatic conditions for the spread of the oil fly, attempts are made to apply different defense systems various moments of cultivationbut the results are not always satisfactory because much is determined by the seasonal climate trend of each year. Each region emits periodic update bulletins of the conditions of the olive groves in relation to the climatic conditions and the presence of oil flies and other parasites. Agronomists and entomological experts generally recommend:
- Programs integrated fight which combine multiple methods and tend to reduce the use of pesticides, for example, monitoring the number of adult flies and at the same time checking the presence of holes on the olives and rot;
- Use of preventive strategies that involve the use of traps attractions with colors (especially yellow) or pheromones (chemical attractors that attract males);
- Choice of more resistant olive tree varieties (cultivars). to parasite attacks;
- Treatments with kaolin powdera whitish and non-dangerous mineral powder like a pesticide, which when sprayed on the plant and olives creates a physical barrier making the fruit skin more resistant;
- Use of insecticides in the doses and periods permitted by lawalthough resistance and adaptation of the fly to many products is being observed.
Recently, the use of precision agricultural techniques such as sensors of environmental parameters or electronic traps are also proving to be a great support for olive growers.
