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Plantwise Technical Factsheet

olive kernel borer (Prays oleae)

Host plants / species affected
Anemone (windflower)
Jasminum (jasmine)
Ligustrum (privet)
Olea europaea subsp. europaea (European olive)
List of symptoms/signs
Fruit  -  frass visible
Fruit  -  internal feeding
Fruit  -  premature drop
Inflorescence  -  external feeding
Inflorescence  -  frass visible
Inflorescence  -  webbing
Leaves  -  frass visible
Leaves  -  internal feeding
In the leaf-feeding generation, P. oleae presence is most easily detected by examining the leaves for mines. Frass is ejected from the underside of the leaf. In the flower-feeding generation, florets are spun together with silk containing grains of frass. In the fruit-feeding generation, early fruit drop is likely to be caused by this pest.
Prevention and control


A review of control methods in Portugal is given by Sobreiro (1993); in the Near East by Katsoyannos (1992); in Spain by Lopez Villata and Dominguez de la Concha (1989); and in France by Loussert and Brousse (1978).

Host-Plant Resistance

Cultivar Aglandau is resistant to P. oleae (Anon., 2000), whereas cv. Bouteillon is not (Anon., 1999). Delrio et al. (1996) tested 20 varieties in southern Sardinia, Italy, and found that following June drop, the percentage of fruits with larvae penetrating the flesh ranged from 4 to 48%, depending on cultivar. Cultivars Palma, Corsicana da Olio, Sivigliana da Olio, Olieddu and Bosana had fewer than 10% of fruits with larvae. Further fruit drop in the autumn caused by the pest varied among the cultivars from 0.2 (Palma) to 37% (Pizz'e Carroga). The reduction index (the ratio between the percentage of infested fruits at the end of June and that of infested fallen fruits in autumn) was negatively correlated with the average weight of fruits at harvest, with the highest values (>5.5) being observed mainly in oil cultivars. The high resistance found in some cultivars with small fruits, such as Semidana, Palma and Bosana, was due to high levels of early fruit drop of infested fruits and to factors as yet unidentified causing larval mortality.

Biological Control

Chrysopid predators are the most commonly used agents for biological control. In Portugal, Bento et al. (1999) found that the rate of predation by chrysopids on P. oleae eggs varied among different generations of the pest and in different years, reaching 34% for the carpophagous generation in 1996. Releasing 360 larvae of Chrysoperla carnea per tree halved the potential damage by P. oleae. Morris et al. (1996a) surveyed a large number of predators in Spain and Morris et al. (1999b) found that ants had a detrimental effect on the number of other predators. Afellah et al. (1998) in Morocco found that the rate of parasitism was low (0.12-0.36%).

Often P. oleae has to be controlled with other pests of olive. In Turkey, Yalya (1983) recorded 25 species of pest and 24 species of predators and parasites and concluded that a natural balance had been established due to the lack of chemical treatment.

Pheromonal Control

Mazomenos et al. (1999) described the use of a pheromone. Pheromone trap catches were reduced by up to 96-100% in the mating-disruption plots. During the first year of mating disruption, a treatment with Bacillus thuringiensis kurstaki (Bt) was applied to reduce the first generation of larvae. Fruit damage in the mating disruption plots was lower than that in Bt, insecticide and untreated plots. In high-fruiting years, the proportion of fruit damage was lower than in low-fruiting years. Mating disruption applied in the same olive grove over several years progressively reduced the P. oleae population from year to year.

Chemical Control

Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:

The species has been known as a pest of olive since the third century BC (Pelekassis, 1962). Throughout the Mediterranean area where olives are grown, P. oleae is one of the major pests, although incidence varies from year to year. Iannotta et al. (1998) found that losses in Calabria, Italy, amounted to 5.5-10%. In a 28-year study in Granada, Spain, Ramos et al. (1998) concluded that when fruit drop exceeded 40%, control methods should be considered. This happened one year in three. Also in Spain, Arias et al. (1990) found that it was viable to spray when 16% of fruit contained eggs. In Turkey, Kaya et al. (1987) found that when the damage level reached 8.3-19.3% in the generation produced at flowering time, fruit loss amounted to 37.0-41.1%, and this level was taken as the economic threshold.
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