The olive fruit fly (Bactrocera oleae) is a species of fruit fly which belongs to the subfamily Dacinae. It is a phytophagous species whose larvae feed on the fruit of olive trees, hence the common name. It is considered a serious pest in the cultivation of olives.
Until 1998, the fly had not been detected in the United States, and its range coincided with the range of the olive tree in the Eastern Hemisphere: northern, eastern, and southern Africa, Southern Europe, the Canary Islands, India, and western Asia. In the Western Hemisphere, it is currently restricted to California, Baja California, and Sonora. The olive fruit fly was first detected in North America infesting olive fruits on landscape trees in Los Angeles County in November 1998. It can now be found throughout the state of California.
History In the final years of the 18th century, Italian scientist Giuseppe Maria Giovene (1753–1837), in his work Avviso per la distruzione dei vermi che attaccano la polpa delle olive (1792), provided some suggestions for peasants to effectively destroy the fly musca oleae, which infested the pulp of olive trees.necrologio-giovene, pag. 44, note 3avviso-distruzione
Distribution and importance This species is associated with plants of the genus Olea. It is found throughout the Mediterranean basin and in South Africa. Since the late 1990s, it has also been present in California and may have spread throughout the area of olive cultivation in the Nearctic region. It is considered the most serious pest of olives in regions where it lives, significantly affecting both the amount and quality of production in most olive-growing areas.
The impact of its attacks tends to worsen in the more humid and cooler growing areas, with significant variations depending on the variety grown, where it affects olive cultivars and areas that have hot summers and less drought.
Morphology
The egg is around 0.7 to 1.2 mm long, elongated, and slightly flattened in its stomach, with a small, white microfleece nodule, which is important for the respiration of the embryo.
The larva is vermiform and has a conical-cylindrical, narrow front. It develops through three stages (larva, first, second and third stage). The mature larva is 6–7 mm long, white-yellowish in colour, elongated, and subconical. The front sensors are bipolar and the second conic feature, the rear sensor, has eight sensilla. The cephalopharyngeal skeleton has very short dorsal and ventral apodemes, the hypostomal scleritis is triangular. It lacks a subhypostomal and the jaws are hooked. The oral lobes have 10–12 indents, preceded on each side by a sensory plate similar to the larva of the Ceratitis capitata. The frontal stigmas have 9–10 lobes. The three larval stages can be distinguished in different ways by their cephalopharyngeal structures. The different shapes of the frontal stigmas allow determination of the larvae of the second and third stages, while the larva at its first stage is metapneustic, equipped with one pair of posterior stigmas.
The pupal stage takes place inside the puparium, an elliptical shell formed by the last exuvial transformation of the larva. The puparium is 3.5 to 4.5 mm long, varying in colour from creamy white to yellow-brown, when it is dry. The change in colour of the puparium can determine the age of the pupa.
Adults are 4–5 mm long. In Italy, they are easily recognized in conjunction with other Tephritidae for the small dark spot at the apex of the wing and the length of the narrow, elongated anal cell.
The adult male has a hardened wing at the top of the anal cell, which is longer than the female's. The third urite shows the pectorals.
The adult female has a yellowish head with two strong circular spots under the antennae close to the compound eyes, whilst the eyes are bluish-green. The chest can show various specks instead of the typical bands and lines. The mesonotum is bluish-gray with three blackish longitudinal lines. The humeral callus and areas mesopleurali, metapleurali, and mesoscutello are ivory. The wings are hyaline, with part of the pterostigma with brown specks at the apex. The abdomen is light brown with variable colourings: typically there are pairs of blackish spots on the first to fourth urotergit, which often come together in bands. The ovipositor is clearly visible, partly invaginated in the seventh urite, which is always black. The length is 4–5 mm.
In the Asian variety, the whole body of the mesonotum is yellowish with strong, visible dark lines.
Life cycle
The females lay their eggs in the summer when the olive is at least 7–8 mm in diameter. Egg-laying is done by making a puncture with the ovipositor into the skin of the olive, leaving only one egg in the hollow below. The bite has a characteristic triangular shape due to an optical effect. A puncture has a dark green colour, whilst older bites have a yellowish-brown colour as a result of wound healing.
Hatching occurs over a variable period depending on weather conditions: from 2–3 days in summer, to about 10 days in autumn. The newly hatched larva initially digs a tunnel on the surface, but later moves deeper into the flesh to the core, which is not affected in any way. During larval development, two changes occur, which in turn increase the size of the larva.
Around the third change, larvae at their third stage move toward the surface and prepare the exit hole for the adult, gnawing at the flesh to leave a thin surface layer. During this phase, the olive clearly shows signs of the attack because its appears darker in conjunction with the tunneling. On the surface, a circular hole due to the remaining residual skin becomes apparent. The pupae remain dormant in the hollow below, protected within the exuviae produced by the mature larvae.
At maturity, the adult breaks the exuvia and emerges from the pupa. It breaks the skin surface left by the larva by force and leaves the exit hole. In late autumn and winter, its behaviour changes; the mature larva emerges from the olive and drops onto the ground, where pupation takes place.
Adults are glycogenic and feed primarily on honeydew. Since their basic diet is low in protein, they are particularly attracted to materials that emit volatile nitrogenous substances, such as bird droppings, for purposes of supplementing their protein requirements. This behavior is important because it can be used in programs for the fly's control and monitoring by using attractants such as hydrolysate proteins and ammonium salts.
Environmental needs
The development cycle is closely linked to environmental conditions, in particular the climate and the state of the olives. Knowing these parameters, together with the monitoring of the population, is needed to implement effective pest management programs.
The climate influences the cycle, especially with the temperature and less humidity. The duration of each stage is summarized below: State Summer Autumn-winter Egg 2–3 days 10 days (autumn) Larva 10–13 days 20 days or more Pupa 10 days up to 4 months (hibernating pupae) Adults several months
The duration of young larva therefore varies from a minimum of 20 days to a maximum of 5 months in the overwintering generation.
Temperature has an important role on the viability and rhythms of reproduction. Temperatures above 30 °C cause resorption of ovarian follicles by reducing the fecundity of females; a female lays two to four eggs on average per day in summer and 10-20 eggs in autumn. Persistent temperatures above 32 °C for several hours a day also cause mortality of over 80% of the eggs and larvae of that age.
Low temperatures, therefore, have very limited effects because its activity is undermined by temperatures below 0 °C. Given ordinary climatic conditions, low temperatures and harsh winters clearly interfere with population dynamics only in the northernmost areas of olive vegetation.
In general, the optimum temperatures for oviposition and larval development are between 20 and 30 °C, together with a need for humid weather.
The second controlling environmental factor is the obvious characteristics of the olives and the phenological stage of the plant. Females receive sensory stimuli to denote the degree of receptivity of the olive, a phenomenon that allows them to choose the olive; before oviposition, the female first "analyses" the size, colour and odour and, it seems, the presence of certain bacterial species. They are especially frequent in summer, caused by the females with a sterile puncture to test the receptivity of the olive. The ethology of the fly has been paid particularly regard in recent years when analysing study control methods based on the use of prior insect repellents (copper, kaolin, etc.).
Larval development is instead influenced by the consistency of the pulp and especially the size of drupe. In table olives, in fact, mortality of the larva is lower in summer because they can escape the lethal effects of high temperatures by migrating deeper. The consistency of the pulp is instead an intrinsic characteristic. Not even susceptibility of olive fruit fly attacks make much difference according to cultivar.
Population dynamics
Unlike other species, the succession of generations of the Bactrocera oleae is not markedly different for the scaling of ovipositioning and longevity of adults. Within a year, generally three to five generations occur, but in many years, a sixth generation can grow in the spring on the olive tree, but does not remain on the tree.
The population size varies throughout the year, but with two peaks: the first in the middle of spring, at the development of the winter generation of adults, and the second, more intense, in early autumn when the olives are at the highest degree of receptivity, the temperatures drop slightly and the climate becomes wetter. In Sardinia, the larval population peaks typically occur during the months of April–May and September–October.
Predisposing factors The predisposition to the flies' attacks is tied to several factors, both intrinsic and extrinsic. The main ones are climatic (temperature and rainfall), so marked differences can occur from year to year. However, other genetic or agronomic factors should not be overlooked.
Ultimately, the environmental conditions favorable to fly attacks are:
Conditions of moderate heat, with temperatures not exceeding 32-34 °C A humid climate Premature cultivation Mass or dual purpose cultivation Cultivation under irrigation
Because of these factors, the incidence of olive fruit fly attacks increase, passing from the southern to northern areas, and coastal to inland regions. As for the season in general, the summer infestations are usually contained with the exception of cooler environment and more susceptible cultivars. On the other hand, infestation peaks happen from the months of September until cold weather arrives, especially with a rainy climate.
Another unique factor is the relationship between the alternation of production, a phenomenon in which the olive tree is particularly susceptible, and the intensity of the attacks. The attacks are usually most intense in years of low production and high production. The cause of this behaviour is partly biological, partly agricultural.
In low seasons following a high one, usually, a significant amount of olive residue is left on the plants from the previous year, so it has a higher population at its production peak in spring, and a higher reproductive potential occurring with more intense and earlier attacks. The attacked olives fall early in autumn and this causes a higher incidence of mortality during the winter.
In the subsequent good year for the population of first generation olives, production is quite low, with modest reproductive potential. The attacks will therefore be later and help a mass-production.
Damages
The damages caused by the olive fruit fly are of two types: quantitative and qualitative.