Silba adipata McAlpine

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Home > Infestation > Multiple ovipositions in the same fig (Study of the eggs - ostiolar scales / ostiolar canal).

 

Author: François DROUET.
Photographs: François DROUET.
All rights reserved.

 

 

Multiple ovipositions in the same fig

 

(Study of the eggs)

 

 

 

I have highlighted the existence of multiple Black Fig Fly ovipositions in a same fig by three means: study of the eggs deposited under the ostiolar scales and/or in the ostiolar canal (this chapter), direct observation of the egg laying females on the fig trees (see chapter), examination of the larvae contained in the figs (see chapter).

 

DETECTING MULTIPLE OVIPOSITIONS IN A FIG (EGGS)

 

Silba adipata McAlpine places its eggs under the ostiolar scales (or, more rarely, in the ostiolar canal) of immature figs. See details on eggs implantation in the chapter relative to fig infestation.

The fig infested by Silba adipata McAlpine may contain fresh eggs (unhatched) and/or empty eggs envelopes (named chorions). In the case of chorions, the larvae have left the eggs after the incubation period, to reach the central cavity of the fig via the ostiolar canal. I was able to observe several times under an ostiolar scale larvae which had just been born, and which had not yet begun to progress towards the fig central cavity (and I was able to identify the empty envelopes of the eggs from which these larvae originated).

To detect multiple ovipositions in a same fig, we can use three clues and one hypothesis.

 

FIRST CLUE

The first clue is the number of eggs and/or empty eggs envelopes present in the fig. If it is greater than 5 (maximum observed for one oviposition), I consider that the eggs are the result of more than one oviposition.

Example 1.

On June 1, 2021, I observed the abscission of an immature breba fig of the 'Grise de la Saint-Jean' variety, which had been attacked by Silba adipata McAlpine. It was 3.1 cm in diameter, almost completely reddened with a greenish vertical area, and it bore a single larva exit hole. No larvae inside, but it contained 13 empty eggs envelopes: a group of 10 under a reddish ostiolar scale of the first level (estimate: three ovipositions), and a group of 3 in the ostiolar canal (fourth oviposition).
 

Silba adipata McAlpine: 10 empty eggs envelopes under an ostiolar scale (immature fig collected on the ground).

Silba adipata McAlpine: 10 empty eggs envelopes under an ostiolar scale (immature fig collected on the ground).
(the ostiolar scale has been raised).

 

Silba adipata McAlpine: 10 empty eggs envelopes under an ostiolar scale (immature fig collected on the ground).

Silba adipata McAlpine: 10 empty eggs envelopes under an ostiolar scale (immature fig collected on the ground).
(they were grouped together, before the ostiolar scale manipulation with the dissecting forceps).

 

Example 2.

On June 21, 2021, I detected a fig infested by Silba adipata McAlpine among picked ripe breba figs (variety 'Grise de la Saint-Jean'). It had a largely greenish epidermis and bore one larva exit hole.

After opening the fig: yellowish infructescence, but no visible larvae. On stereomicroscope examination, detection of 29 empty eggs envelopes under the ostiolar scales. According to the following layout: group of 18, in a rather spherical disordered cluster, under a first level ostiolar scale (estimate: 6 ovipositions); group of 3 under a first level scale + 2 single, each under a first level scale (1 oviposition of 5 eggs distributed under 3 ostiolar scales); group of 4 under a second level ostiolar scale + 1 single under a second level second scale (1 oviposition of 5 eggs); 1 single under a third second level scale (1 oviposition).

That is a total of 9 ovipositions, distributed under 7 ostiolar scales (4 of the first level, and 3 of the second level). The ovipositions were carried out when the fig was immature, the latter having evolved to the maturity stage despite the infestation, instead of falling to the ground still immature (low occurrence, but which I see every year for this figs variety). Other interpretations of the ovipositions number are possible.

Example 3.

On June 18, 2023, I picked up a batch of 23 immature figs with no larvae exit holes from a young fig tree of the uniferous variety 'Bécane'. Almost all of these figs were infested with Silba adipata McAlpine, and several of them contained more than 5 eggs distributed under two ostiolar scales.

The photograph below shows the case of an immature fig 1.7 cm in diameter which contained 16 eggs, distributed under two level 1 ostiolar scales as follows: 3 unhatched eggs and 3 empty eggs envelopes under the first scale; 6 unhatched eggs and 4 empty eggs envelopes under the second scale.
 

Silba adipata McAlpine: the two first level ostiolar scales of the unripe fig contain a total of 16 eggs.

Silba adipata McAlpine: the two first level ostiolar scales of the unripe fig contain a total of 16 eggs.
(9 unhatched eggs and 7 empty eggs envelopes).

 

A possible interpretation of the ovipositions number is as follows: 1 oviposition of 3 eggs made before another oviposition of 3 eggs, under the first scale; 1 oviposition of 4 eggs made before 2 ovipositions of 3 eggs, under the second scale; i.e. a total of 5 ovipositions. Other interpretations (with a higher number of ovipositions) are possible.

 

SECOND CLUE

The second clue for detecting multiple ovipositions in a same fig is the simultaneous presence of fresh eggs and empty egg envelopes (differences in end of incubation dates = multiple ovipositions). It is particularly useful in the case where the fig contains 2 to 5 eggs, because multiple ovipositions then cannot be detected by the number of eggs.

In many cases, the first and second clues are cumulative for a given fig. See photograph below, taken in June 2022, for which the most likely hypothesis is two successive ovipositions: one of 3 eggs (which have hatched), the other of 4 eggs (unhatched).
 

Silba adipata McAlpine : simultaneous presence of 4 unhatched eggs and 3 chorions under an ostiolar scale.

Silba adipata McAlpine: simultaneous presence of 4 unhatched eggs and 3 chorions under an ostiolar scale.
 (unripe fig; ostiolar scale removed with a scalpel).

 

THIRD CLUE

The third clue, close to the second, is the freshness of the unhatched eggs. In some cases, eggs grouped under an ostiolar scale show two distinctly different aspects of freshness, which reflect different incubation times. I then deduce that they come from two different ovipositions.

The photograph below shows eggs grouped under an ostiolar scale, and we can see that the egg on the left is fresher (more recent) than the egg on the right. The latter is perhaps part of an oviposition of three eggs, two of which, located to his left, have already hatched and are in the state of empty eggs envelopes (chorions).
 

Silba adipata McAlpine: presence of two unhatched eggs of different freshness states under the same ostiolar scale.

Silba adipata McAlpine: presence of two unhatched eggs of different freshness states under the same ostiolar scale of an immature fig.
(we will also notice that the two central chorions are fresher than the one below  them, to their left).
 

The same observation can be made with two groups of chorions, the different states of freshness of which reflect different existence durations. In the photograph above, I observe that the two chorions between the two unhatched eggs are fresher (newer) than the chorion to their left below. This one is thinner and much more translucent.

A possible interpretation of multiple ovipositions under this ostiolar scale is therefore: an old oviposition (remainder : 3 or 4 translucent chorions), followed by a 3 eggs oviposition (2 of which have already hatched), then by a recent single egg oviposition (very fresh state of the egg). That is a total of 7 to 8 eggs successively deposited by 3 females under the same ostiolar scale.

 

USABLE HYPOTHESIS

The hypothesis concerns the simultaneous presence of eggs (unhatched or in the form of empty eggs envelopes) in the ostiolar canal and under ostiolar scales. I then consider that those present in the ostiolar canal are not from the same oviposition as those present under the ostiolar scales.

Without being able to assure it, in all rigor. But if the female opts for the ostiolar canal, it is because it is open enough (which rarely happens for an immature fig, and only at a certain stage of its evolution), and the female should stick to its choice for the entire oviposition duration. For me, the presence of eggs in the ostiolar canal indicates an oviposition later than the ovipositions under the ostiolar scales.

 

OVIPOSITIONS DATING BY THE EGGS STUDY

Concerning the ovipositions dating by the eggs study, the only parameter at our disposal is the eggs incubation period, which varies from 8 days in April to 3 days in summer (see chapter). And this provides weak information.

Take the case of an eggs observation in summer: the presence of unhatched eggs indicates that the oviposition occurred no later than 3 days before, empty eggs envelopes indicate that the oviposition took place at the earliest 3 days ago. The simultaneous presence of fresh eggs and empty eggs envelopes shows that the ovipositions are separated by 1 to 3 days, or more.

It should be noted that the larvae examination is richer in information for dating ovipositions. In any event, it cannot be determined whether the ovipositions carried out in the same fig come from different females, or from the same female returning to visit the fig tree on the same day or on different days (whatever the number of days separating the successive ovipositions in the same fig).

 

PERCENTAGE OF MULTIPLE OVIPOSITIONS IN THE SAME FIG

 

There is no interest in trying to determine from eggs study a ceiling value for the percentage of multiple ovipositions in the same fig, because it would be of high uncertainty. Indeed, for figs containing 2 to 5 eggs, we do not know how to attribute eggs distributed under two or three ostiolar scales to one or more females; and in the case where all the eggs are grouped under a single ostiolar scale, we also do not know wether they were deposited by one or more females).

The egg laying females observation led us to a ceiling value of 15% for the percentage of Black Fig Fly multiple ovipositions in a same fig (see chapter), and the larvae examination led us to a ceiling value of 10 % for this percentage. Of these two, as these are maximum values that the multiple ovipositions in the same fig rate cannot exceed, we retain the lowest, i.e. that obtained by examining the larvae: 10%.

This ceiling value does not allow us to determine in how many already infested figs a female lays eggs on average during a sequence of 20 successive ovipositions on the same fig tree, but it allows us to establish that this average does not exceed 2 figs.

It is important to point out that this value is not an order of magnitude, nor the maximum of a range resulting from counts during observations. It is simply a ceiling value that the percentage of multiple ovipositions in a fig will not exceed when we will have quantified observation elements to apprehend it more precisely.

 

 

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