Olive trees are grown all over the world, with high yields in regions with a Mediterranean climate. Olive oil is currently produced in more than 40 countries, with more than 750 million olive trees cultivated worldwide, 95% of which are in the Mediterranean region1. It remains the most widely cultivated perennial crop in this region and is well suited to its climate, which is characterised by high temperatures and hydric stress2.
Globally, on average, between 10% and 28% of crop production is lost to pests34. In the case of olive crops, alarming statistics show that up to 40% of global olive production is lost annually due to their impact2. Several insects, pathogens, and nematodes affect olive trees and threaten olive production although olive cultivation is considered a very stable crop. In this cultivation, some exclusive insect pests have been associated with a different incidence in the olive fruit, affecting the yield and/or quality of oil and olive table. In order of economic importance, the pests studied are as follows:
1. The “olive fruit fly” Bactrocera oleae (Gmelin, 1790) (Diptera: Tephritidae). It is considered to be the most important olive pest in the Mediterranean area and has been recorded in 34 countries in the Mediterranean and the Middle East, as well as in southern and eastern Africa, India and Pakistan, and since the end of the 20th century, in California and Mexico2. This insect has several generations per year (usually 2 to 4, but up to 6 depending on the prevailing temperature and availability of fruit). The female can lay up to 20 eggs/day on the fruit, producing several hundred eggs during her lifetime. The fruit is damaged by oviposition. The larvae feed inside the fruit mesocarp, damaging it and reducing its quality. It can also cause premature ripening or dropping of the fruit, loss of oil quality, or complete loss of damaged fruit in table olives. Infestation can be severe enough to cause 100% fruit drop in years of low yield5. Some authors have noted that some fungi share at least part of their life cycle with the olive fruit fly because they mainly affect fruits from the beginning of olive ripening and could be favoured by insects that can act as vectors6.
2. The “olive moth” Prays oleae (Bernard, 1788) (Lepidoptera: Plutellidae: Prayninae)7. It is widespread throughout the Mediterranean and is considered the second most important olive pest in the EU. P. oleae completes three generations on the olive tree each year, damaging flowers, leaves, and fruit in each generation. In the carpophagous generation, which is the most dangerous for yield, the eggs are laid on the calyx of a young olive fruit; the larvae enter the young fruit endocarp near the pedicel during the summer and feed on the olive seeds, dehydrating the fruit and causing it to drop, especially when the larvae leave the olive fruit to pupate in the soil, reducing the productivity of the tree2). The 28-year study by Ramos et al.8 in southern Spain (Andalusia) showed that this pest can reduce olive production by 50–60%, with heavy infestations occurring approximately every three years, causing 40% premature fruit drop and consequently significant economic losses.
3. The "olive fruit rot" appears because of a fungal infection, which can be caused by different species of fungi. The most common is Botryosphaeria dothidea (Moug.) Ces. & De Not9,10Haga clic o pulse aquí para escribir texto.. A revision of the different names given to the fungus involved in olive fruit rot and taxonomic changes can be found in Lazzizera et al.10. This pathogen is the causal agent of the Dalmatian disease of olive, which is found in most of the Mediterranean basin11,12. B. dothidea produces a sunken, necrotic, and circular lesion (approximately 8 mm in diameter, never larger than 1 cm) with a sharp edge that separates the infected tissue from the healthy fruit tissue; this appearance is called "escudete" (little shield) in Portuguese and Spanish. As the disease progresses, the necrotic depression expands to cover the entire fruit. As the fruit ripens, it falls to the ground and becomes mummified12. Overall, the incidence of olive fruit showing Dalmatian symptoms is relatively low, but it often exceeds the tolerance level for the "Extra" class standard for olive fruit, which is usually at a level of 2 % or 4%, according to the Spanish law13 or the FAO’s odex Alimentarius14. Its presence in table olives is particularly important15. Thus, oviposition wounds may facilitate the infection process of fungi. Iannotta et al.11 observed a correlation between the presence of Botryosphaeria olive rots, formerly associated with the fungus Camarosporium dalmaticum (Dalmatian disease), and olive fly infestation, but did not provided evidence of the role of the insect in favouring fungal infections6. The midge Lasioptera berlesiana Paoli, 1907 (Diptera: Cecidomyiidae)16 is an active predator of eggs and larvae and has been used as a biological control agent of the olive fruit fly17. There is evidence on its role in transmitting fungi, but its interaction with the olive fruit fly and both with the transmission of fungi remains controversial18–20.
To study the effects of these pests and diseases on olive fruits, ocular inspections followed by dissection of the fruits, accompanied or not by light microscopy or electron microscopy studies, have been carried out (i.e.21,22). Recently, X-ray images have been used to cheque the degree of infection of the fruit23. In this work, the technique of computerised microtomography is used for the first time to study the fruit as a whole and, without altering it, to obtain high-resolution 3D rendered images of the effects of the three pests. Studying each of the components in their original situation, allowing observations from different perspectives, obtaining high quality 3D rendered images, and additional supplementary videos useful tools for future research and as teaching aids.