Its attacks tend to increase in wetter and colder areas, with great variability according to the variety of olives cultivated. It is less noticeable in areas with dry and hot summers.
Morphology and biology
The adult insect is a reddish-grey or orange colour which is highlighted by black spots.
It has transparent, iridescent wings and the lower part of its thorax is yellow. Its eggs are long and white. The larva is maggot-like and white. The pupa is long and has a yellowish colour. When it grows, its colour darkens.
On average, two to three generations can be observed every year, depending on the climatic conditions. They usually overwinter in pupal stage which is located underground at little depth.
The adults of the first generation appear in March or April. They spend some time feeding on sugary substances, such as the honeydew from some Homoptera, until they reach sexual maturity and begin to lay eggs. The female lays
the eggs underneath the skin of the chosen olive. The olive should have an appropriate level of maturity and must not be occupied by another egg. It is common for this insect to lay only one egg per olive.
After hatching, the larva feeds on the olive by excavating a small and torturous gallery. Once the larva has finished growing, it pupates inside the fruit near the skin.
Flight activity of the olive fruit fly reaches its highest peaks in July and October. However, there is a greater increase in flight activity during October.
Damages
There are two types of damage caused by the olive fruit fly: quantitative and qualitative.
Regarding quantitative damages, they are caused by second stage larvae and especially third stage larvae. They consume an important amount of the olive’s pulp which, as a consequence, reduces the oil quality. Part of the production
is lost because the olives that have been attacked fall prematurely from the tree. In olives used to produce oil, damage caused by the egg laying and the first stage larvae do not have significant economic impact. However,
for table olives, the egg laying depreciates the value of the product.
Third stage larva
© Giancarlo Dess
Olive with a fresh
of Dacus (Bactrocera) oleae
Exit holes created by the fly
© Giancarlo Dess
Regarding qualitative damage, the olives with a high percentage of this pest produce oil of the worst quality, particularly if they are third stage larvae. The oil extracted from these olives can have a level of acidity (expressed
in oleic acid) from 2 to 10% depending on the percentage of infestation. It is also less possible to preserve these oils because they contain a higher percentage of Peroxide.
Due to the attacks from this fly, secondary qualitative damages also occur. The remains that the larva leaves inside the fruit creates fungi that deteriorate the quality of the oil extracted. This secondary damage in quality
is more obvious in oils extracted from olives that have been attacked, then picked from the ground or that they may also have been stored for many days before being ground in the olive press.