The olive fruit fly Bactrocera oleae (Rossi) (Diptera; Tephritidae)

This monophagous tephritid poses the greatest arthropod threat to olive cultivation, with Australia being the only Bactrocera oleae-free (B.oleae) zone within the global olive-growing area. The brown-yellowish thorax of the adult flies, with 2 to 4 parallel grayish bands, and the hyaline wings with a brownish spot at the apex, are the most important B. oleae morphological features. The eggs are creamy white, and larvae are white-yellowish, growing from 1 mm at L1 to 7-8 mm at L3 with the pupa confined inside the puparium.

The olive fruit fly is a polyvoltine species with 2 to 5 generations per year, depending on local and regional relative humidity and temperature conditions. Temperatures over 30ºC and low relative humidity conditions lead to egg resorption and reproductive diapause, resulting in a higher number of generations in littoral areas, which is progressively reduced inland and at higher altitudes. Sexual maturation is highly synchronised with the availability of suitable fruits. The gravid olive fly female pierces the epicarp and lays 10 to 12 eggs daily and between 200 and 250 eggs per lifetime. One egg per fruit is frequently found when B. oleae population density is low and fruit availability is high, but several eggs per fruit appear when fly population density is high and fruit availability is scarce. After hatching, the emerging larva bores a gallery, grows, and reaches the third instar. Then, these larvae pupate either inside the fruit, when it is still green with a hard mesocarp, or at a shallow depth in the soil surface beneath the tree canopy, leaving the fruit when its mesocarp is soft and rich in oil. Most of the olive fly population overwinters as pupa several centimeters below the soil, whereas there may also be overwintering adult populations, either taking refuge in the olive groves or outside.

This key tephritid pest causes both quantitative and qualitative crop damage. The larvae (maggots) feed inside the fruit, destroying the pulp and allowing the entry of fungi and bacteria that rot the fruit. It can degrade the commercial (physical-chemical and organoleptic parameters) and nutritional quality of the oil extracted, leading to the product not meeting the legal specifications for extra virgin or virgin oil (highest commercial categories). On table olives, oviposition punctures cause a serious reduction in crop value, made even worse by indirect damage associated with the settling of pathogenic microorganisms in these punctures, as the fungus Camarosporium dalmaticum (Thüm.) Zachos & Tzav.-Klon.

When the adult or larval population reaches intervention levels, chemical pesticides are administered as cover or bait sprays to reduce olive fly populations in most growing regions. To this end, population density is assessed by adult capture in traps and fruit sampling. The progressive reduction of chemical insecticides necessitates the development of new effective alternative non-chemical methods to control B. oleae in accordance with current environmental policies or strategies. The role of microorganisms in indirect biological pest control, such as the use of entomopathogenic fungus Metarhizium brunneum (Petch.), has proven effective, along with other agronomic practices such as early harvesting and semiochemical-based strategies.

New lines of research for the control of this pest are based on the study of defence of the olive tree against B. oleae. The interaction and correlation of physical (size, weight, volume, break force, elasticity, firmness and colour are among the most studied to date), chemical (compounds secreted and/or emitted by the olive fruit, including skin waxes, phenolic and volatile compounds) and molecular factors (up-regulated gene expression in response to wounding) appear to be determinant in the complex relationship between plant and insect.

These mechanims could explain the ovipositional preferences of B. oleae adult females for certain varieties, indicating cultivar susceptibility. More recently, the identification and characterization of sources of genetic resistance to (B. oleae) is being included in olive breeding programs in order to develop resistant cultivars and control methods against this dipteran.

Table 1. Susceptibility from varieties of the True Healthy Olive Cultivars (THOC) to attack by the olive fruit fly (B. oleae). The susceptibility index scale corresponds to the percentage of fruit infestation: 1 (0%), 2 (1-20 %), 3 (21-40%), 4 (41-60%), 5 (61-80%) and 6 (81-100%). Different letters (A-E) indicate the corresponding source of information. 

*.- Scale and info source mainly from: 

A) Quesada-Moraga, E., Yousef, M., Garrido-Jurado, I., & Santiago-Álvarez, C. (2015). Evaluation of the susceptibility of varieties of the World Olive Germplasm Bank of Córdoba to the attack of the olive fly Bactrocera oleae. Phytoma 271: 53-58., as well as from current study proyects. 

B) Medjkouh, L., Costa, A., Tamendjari, A., Bekdouche, F., Bouarroudj, K., & Oliveira, M. B. P. P. (2018). Susceptibility of eight Algerian olive cultivars to Bactrocera oleae infestation – a pomological and nutritional quality perspective. Phytoparasitica, 46(5):595–605. 

C) Garantonakis, N., Varikou, K., Markakis, E., Birouraki, A., Sergentani, C., Psarras, G., & Koubouris, G. C. (2016). Interaction between Bactrocera oleae (Diptera: Tephritidae) infestation and fruit mineral element content in Olea europaea (Lamiales: Oleaceae) cultivars of global interest. Applied Entomology and Zoology, 51(2), 257–265. 

D) Rizzo R, & Caleca V. (2006). Resistance to the attack of Bactrocera oleae (Gmelin) of some sicilian olive cultivars. Olivebioteq 2006, II, 35–42. 

E) Malheiro, R., Casal, S., Pinheiro, L., Baptista, P., & Pereira, J. A. (2019). Olive cultivar and maturation process on the oviposition preference of Bactrocera oleae (Rossi) (Diptera: Tephritidae). Bulletin of Entomological Research, 109(1), 43–53.