It is possible that the prolonged use of mass traps has exerted a selective pressure towards individuals less sensitive to sexual pheromones, as suggested by field observations and preliminary studies in other insect species (Tephritidae). High summer temperatures reduce fly flight activity during the central hours, altering capture times and complicating data interpretation.
The surface microbiota of olives.
Recent studies (e.g. Malheiro et al., 2015; Konstantopoulou et al., 2020) indicate that the microorganisms present on the peel they emit olfactory cues that guide the fly in choosing the fruitThe use of rock dust and antimicrobial treatments can alter these signals, making the olives less recognizable and influencing the behavior of the pest.
Summer heat: a partial ally
Climate change has brought milder winters, favoring the survival of overwintering forms of the fly. Summer, however, brings with it a thermal compensation that can be beneficial for the olive grower. Above 30–32°C, females tend to suspend egg-laying, taking refuge within the canopy until more favorable temperatures. Eggs laid in extremely hot conditions show a mortality rate of over 50%, especially if the olives are exposed to the sun and poorly hydrated.
In irrigated or shaded olive groves, the internal temperature of the fruit can remain lower, favoring larval development. In dry and windy conditions, heat can inhibit the life cycle, offering the olive grower a window of respite.
August refreshes the fruit, but rekindles the risk
In the second half of August, a decreasing trend in heat-related mortality is observed, linked to agronomic and climatic factors.
The olives have now reached their final size, with greater hydration that makes them more resistant to thermal stress.
Direct sunlight hours decrease in August, reducing prolonged exposure of eggs and larvae. In the absence of significant water stress, olives offer more effective thermal protection, but this very condition can reactivate the fly's infestation cycle in favorable microclimates.
Traps may not detect apparent activity, while infestations may still develop in shaded or irrigated areas.
Late summer: the fruit becomes welcoming, the risk increases
As August progresses, the olives present ideal conditions for oviposition, they are hydrated, mature and more attractive.
This often coincides with an increase in the adult population, favored by milder temperatures than the July peaks.
The summer generations of the fly tend to intensify precisely between August and September, when the fruit is most vulnerable.
The increased availability of susceptible drupes and the drop in maximum temperatures can reactivate the parasite's reproductive activity, making this phase particularly delicate. In irrigated or shaded olive groves, where the fruit retains moisture and freshness, the risk of infestation can increase significantly.
Nocturnal egg laying: what happens?
A frequently asked question concerns the nocturnal activity of flies. Available evidence indicates that the Bactrocera oleae It is a diurnal insect, with peaks of activity concentrated in the cool hours of the morning and late afternoon.
Females avoid the hottest midday hours, but are not active at night. Egg-laying occurs in bright light, as the fly uses visual and olfactory cues to locate suitable fruit.
However, the longer, cooler nights of late summer can indirectly influence the insect's behavior, encouraging greater activity in the early hours of the day. This makes timely monitoring and direct observation of the drupes even more important, especially in the morning.
Towards an adaptive defense
In a context of climate change and behavioural variability of the phytophagous, defense must evolve towards an adaptive model, in line with the principles of Integrated Pest Management (IPM, Directive 2009/128/EC). In addition to monitoring with traps, it is essential to perform direct inspections of the drupes by sectioning, evaluating:
– number of oviposition punctures;
– presence and stage of development of eggs and larvae;
– percentage of infested fruits.
Combining these data with weather forecasts and the phenological stage of the olive tree allows for targeted interventions: for example, the use of ovilarvicides when early-instar larvae predominate, avoiding unnecessary adulticides; or postponing treatment if temperatures above 32°C are forecast for several days, taking advantage of the natural suspension of oviposition.
Treatments: when to act to really hit the mark
Given the fly's diurnal behavior, the timing of pesticide application is crucial. Treatments carried out in the early morning or late afternoon/evening are more effective, as they coincide with the insect's peak activity. During these times, adults are more exposed, and the active ingredients are more persistent, preventing evaporation and photodegradation due to exposure to UV rays.
Acetamiprid: best results if distributed during times of parasite activity.
Azadirachtin: photosensitive, should be applied in shaded conditions.
Deltamethrin: Synthetic pyrethroid with knockdown action; most effective when adults are in flight or moving, preferably during the cooler hours to increase their direct exposure.
Flupyradifurone (Sivanto Prime): systemic and translaminar, less sensitive to the time of application; its effectiveness is linked more to the timing of the infestation phase than to the precise time of day.
Spinosad and Cyantraniliprole (food baits): effective when the fly is feeding, typically in the cool hours.
Natural pyrethrum: with rapid knockdown action, it should be applied during the cool hours to maximize direct contact with moving adults; sensitive to light, therefore, it should be avoided during the central hours.
Beauveria bassiana (strain ATCC 74040): Entomopathogenic fungus with contact action and egg-laying deterrent; useful in small olive groves or those with high pest pressure, effective in the final stages of the cycle (September/October); apply during the cool hours to maximize the effect.
Not all labels provide explicit instructions on the time of application, but agronomic experience and regional guidelines clearly suggest avoiding the hottest central hours and favoring the cooler hours of the day. The effectiveness of the treatment depends not only on the active ingredient, but also on the timing of its administration.
In this scenario, the olive grower is no longer just an implementer of protocols, but an active interpreter of biological and climatic signals, capable of adapting the defense day by day.
