Il visual inspection of the drupes to evaluate any infestations of olive fly completes the one implemented with the capture of adults by traps.
This monitoring practice It should be performed weekly and is based on several steps, which allow to evaluate the extent of the infestation and to decide on the actions to combat the diptera.
Let's look at these steps in detail, trying to accompany the olive grower punctually in all the various control phases to be able to correctly evaluate the extent of the infestation and determine the subsequent necessary actions.
Sampling
a) Selection of plants: olive trees are randomly selected throughout the field or area of interest, trying to uniformly cover the entire surface. The number of sampled plants must be representative of the size and variability of the field.
b) Olive harvest: From each selected plant, a certain number of olives are collected (usually between 10 and 50 per plant).
Olives should be harvested from different parts of the plant (low, high, sunny side, shaded side) to best represent the entire plant.
The visual examination
The olives harvested are visually examine to see if there are signs of oviposition punctures, which appear as small depressions or scars on the surface of the olive. The female fly uses an ovipositor: it is similar to a thin, sharp needle, which pierces the skin of the olive and lays eggs inside, creating a round and precise hole in the skin of the olive without causing excessive damage (usually about 1-2 millimeters in diameter). Such a small hole causes less stress and damage to the olive, allowing it to continue to develop normally for a certain period, which is advantageous for the larva that will develop inside. In addition, very small holes reduce the likelihood of the eggs being disturbed or removed by predators or external agents, and this increases the chances of survival of the eggs inside the fruit.
During the oviposition process, the fly inserts the ovipositor directly and perpendicularly to the surface of the fruit.
The olive skin tends to close around the hole soon after the puncture, maintaining the round and well-defined shape of the hole, which are a distinctive sign of olive fly activity and can be used to quickly identify infested olives.
Oviposition punctures do not only cause aesthetic damage. The larvae that develop inside the olives can seriously compromise the quality of the fruit, reducing the oil yield and altering the organoleptic characteristics of the final product.
External inspection tools
For external inspection, you do not need particularly sophisticated tools, but the use of a magnifying glass that facilitates the identification of the punctures, especially if they are small or not very evident. Good lighting is also important to clearly see the differences in color and surface.
The lens – It should be able to achieve 5x to 10x magnification, enough to see egg-laying punctures.
Higher magnification may be useful for seeing finer details, but it may also reduce the field of view and ease of use. The lens diameter should be 50-75mm, it offers a good balance between field of view and ease of manipulation.
A larger diameter allows you to examine a larger portion of the olive surface without having to constantly move the lens. The most common and easy to use form is the classic hand lens, which is easy to use and comes in many variations.
If you have a lot of olives to examine, a stand or table magnifier allows you to have both hands free, as well as a headband magnifier, which is useful for long work sessions. Prices for these magnifiers range from 20 to 60 Euros.
The cut – To check for the presence of eggs, larvae or tunnels dug by the olive fly larva inside the fruit, you must proceed by cutting with a sharp blade, cutter or scalpel, so as to expose the inside of the olive, making visible any signs of infestation that are not detectable externally. Between a scalpel and a cutter, it is advisable to use the former, because it has a very sharp and thin blade, ideal for delicately opening the olives and checking for the presence of larvae or damage without damaging them and obtaining a more accurate view of the infestations. The cutter may be too large or less precise for this purpose, it could damage the olives or compromise the assessment of the infestations.
The cut should be made lengthwise to expose the inside of the fruit and a crosswise cut to have a different view and ensure you do not miss any signs of infestation.
Characteristics of puncture wounds
When the olive fly lays eggs, the penetration holes can cause changes to the skin of the olive, around the oviposition holes the skin of the olive appears slightly darker or discolored than the rest of the fruit.
• Dark halo: Around the holes, the skin of the olive may appear slightly discolored or have a darker halo than the rest of the fruit.
• Fruit reaction: When the female olive fly lays her eggs, the fruit reacts to the damage caused by the sting and causes a slight discoloration or a dark halo around the hole.
• Oxidation: Exposure to air through the hole can cause oxidation of the olive pulp, which manifests itself as a dark halo
• Staines: In some cases, there may be small brown or black spots around the holes.
Dark halos and spots around the oviposition holes are visible signs of damage and reaction of the fruit to the attack of the olive fly, they take on the dark color, which can help identify the affected olives, because they become entry points for fungi or bacteria, which can be the cause of the brown or black spots around the holes. In addition, the bite and the subsequent activity of the larvae can cause the death of the surrounding tissues, which also manifest as dark spots.
Exposed sides of stings
It is important to pay attention to the exposed sides of the stings.
• Upper part: On the surface of the olive, punctures can be found anywhere, as the upper part, which is more exposed to sunlight and atmospheric agents, is a common spot for punctures.
• Part covered by leaves: even the sides of the olive that are not covered by leaves or other fruit are easily accessible to flies, therefore, more likely to be found with puncture marks.
• Petiole area: The area around the stem, where the olive is attached to the branch, may also show signs of puncture, as it is an easy area for flies to reach.
• Lower part: Although less common, the underside of the olive may show puncture marks, especially if the fruit is hanging and not protected by leaves.
Fruit consistency
The consistency of the olives is an aspect to consider during the inspection and can reveal signs of infestation, including the softness or mushyness of the pulp, due to internal decomposition, as the larvae of the Olive Fly develop inside it, feeding on it and damaging its tissues. Infested olives can thus be softer to the touch than healthy olives: it is more evident around the oviposition holes and the difference can be noticed by gently pressing the olive with the fingers.
In addition to softness, you may notice that the skin of the olive appears more wrinkled or shriveled in the infested areas, due to the loss of structural integrity of the fruit.
Opening the olives
Using the scalpel, the suspect olives are cut lengthwise and inside, the eggs and larvae are looked for.
A) Egg Examination
Appearance: The eggs are very small, about 0,7-1 mm long, white in colour, and have an elongated, cylindrical shape, usually deposited just under the skin of the olive.
How to find them: After cutting, carefully examine the area just below the surface of the olive; it can be difficult to see with the naked eye, so use a magnifying glass.
What they indicate: the presence of eggs is a sign of initial infestation, meaning the fly has just laid eggs.
B) Detection of Larvae
Appearance: the larvae are whitish or yellowish, without legs, elongated in shape and can vary in larvae of:
Age: small, elongated, whitish or yellowish, without legs, about 2 mm long.
Age II: larger and longer than the first ones, whitish or yellowish.
Age III: larger and more developed, whitish or yellowish, they can reach 8 mm.
Dolls: After the larval stage, the larvae transform into pupae, they are inactive, immobile and protected by an external structure.
How to find them: After cutting the olive, the pulp should be observed to identify any larvae; they may be visible to the naked eye and, if alive, could move.
What they indicate: The presence of live larvae indicates an active infestation, their size can also help determine how long the infestation has been ongoing.
C) Identification of the tunnels
Appearance: The galleries appear as larval feeding cavities within the pulp.
How to find them: After cutting, look for galleries in the pulp, often there are small dark granules, which are excrement of the larvae.
What they indicate:
The galleries show the feeding activity of the larvae and, together with the droppings, confirm an active infestation and potential damage to the fruit.
D) Assessment of pulp damage
Appearance: An infested olive may have blackened, soft, or disintegrated flesh in areas where the larvae have fed.
If the infestation is advanced, the entire pulp may be damaged.
How to find it: After cutting, observe the pulp for any signs of decay or significant deterioration.
What it indicates: Extensive damage to the pulp indicates that the infestation is advanced, with potential repercussions on the quality of the oil or the pulp itself.
Even at a presence of 8 – 10% it is possible to detect hints of vermiculture in the oil.
More indicators of infestation are the smell (long-infested olives may emit an unpleasant odor, a sign of decay) and the external appearance (In some cases, a heavily infested olive may appear softer on the outside, discolored, or show signs of rot)
Damage classification
• Light infestation: occurs when eggs or small tunnels without live larvae are present.
• Limited infestation: in this case, the presence of live larvae or more developed galleries is found, which represent approximately 2-3% of the damage.
• Severe infestation: occurs when large larvae or extensive damage to the pulp is present, exceeding 5%.
At this point, the percentage of infested olives on the total number of olives examined is calculated by proceeding as follows: if we have sampled 50 olives and among these we have found 1 infested olive, then we will carry out this operation:
1 (infested olives) : 50 (sampled olives) x100 = 2% (Percentage of infestation)
Data recording and interpretation
All data collected during the visual analysis should be recorded in order to evaluate the progress of the infestation over time and the effectiveness of the control strategies applied. After visually examining the sampled olives, it is possible to evaluate the extent of the infestation and determine the actions necessary for control.
If signs of oviposition punctures are detected, it is important to consider timely interventions to prevent damage to olive production.
In summary, visual control of infestations, together with monitoring with traps, is an essential practice to effectively manage olive fruit fly infestations.
