Silba adipata McAlpine

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Author: François DROUET.
Photographs / videos: F. DROUET.
(unless indicated).
All rights reserved.

 

 

Female behavior on a fig

 

 

 

Below, personal observations (no information reported from the literature).

 

When an egg-laying female has targeted a fig, it always lands in the ostiolar region. Then, having located the ostiole (in my opinion, before landing), it generally immediately assumes a first egg-laying position by crossing the ostiole to place its body beyond it. Then, the position being taken, it exteriorizes its retractile ovipositor and inserts it in the ostiole. But, quite often, the female walks a few seconds around the ostiole, before taking up its first egg-laying position.

It is important to point out  that the female has crossed the ostiole to take the egg-laying position. It is a constant of the Black Fig Fly egg-laying behavior on the fig. The female could simply turn around to present its ovipositor facing the ostiole, or move backwards to approach the ovipositor to the ostiole. But it does not. It knows only one way to present the ovipositor facing the ostiole: it is the crossing of the ostiole by the whole body, head forward. It is this curious and immutable behavior that transforms oviposition into a characteristic ballet composed of successive comings and goings above the ostiole.

 

EGG-LAYING IN A FIG

 

The Black Fig Fly female never lays the egg through the fig skin (its ovipositor is not strong enough to puncture the hard green unripe fig). But it does not release the egg into the fig central cavity by crossing the ostiole with its ovipositor, as the formulation "oviposition in the fig" might suggest. In fact, it introduces the ovipositor under an ostiolar scale and deposits the egg there horizontally. It is the larva born under the scale that will travel to the central cavity of the fig via the ostiolar canal.
 

Silba adipata McAlpine: egg laying under an ostiolar scale (immature fig, 'Bellone' variety).

Silba adipata McAlpine: egg-laying under an ostiolar scale (immature fig, 'Bellone' variety).
(note the ovipositor inserted under the scale).

 

Silba adipata McAlpine: eggs laid under an horizontal ostiolar scale of an immature fig.

Silba adipata McAlpine: eggs laid under an horizontal ostiolar scale of an immature fig.
(note that eggs are not laid in the ostiolar canal, composed of vertical scales).
Credit: Alain COSTA 
 

 

OVIPOSITION SUBDIVISION INTO REPETITIVE SEQUENCES

 

According to my observations and deductions, the female lays from 1 to 5 eggs in each visited fig. And it is easy to observe with the naked eye on the fig trees and, more precisely, on videos that the laying of all the eggs does not take place in one go.

The egg deposit action itself results in ovipositor pushing movements in the ostiole, visible to the naked eye for an experienced observer. When the female has finished its first ovipositor pushing movements in the ostiole, it changes position in order to be able to cross the ostiole again with its head forward, and present its ovipositor facing the ostiole, for making another egg deposit.

I have noticed that, to change position, the female uses three usual ways: it either rotates its body 90°, or rotates it 180°, or it withdraws its ovipositor from the ostiole and performs a short walk in the ostiolar region. Knowing that, in any case, it again crosses the ostiole to put itself in egg-laying position.

In fact the female consecutively perforrms several times an oviposition sequence: changing position, crossing the ostiole, inserting ovipositor under an ostiolar scale, depositing the egg by ovipositor pushing movements in the ostiole.

 

Oviposition example - video.

 

Silba adipata McAlpine: oviposition in an immature fig.

 

Video analysis: in this video, the oviposition is subdivided into 3 sequences.

Sequence 1: the female crosses the ostiole, positions itself on the ostiole edge (head down on the screen) and performs ovipositor pushing movements in the ostiole. During this sequence, the female gradually rotates the body to its left without the ovipositor leaving the ostiole. At the end of the sequence, the female has rotated 90° (head to the right of the screen) and it withdraws the ovipositor from the ostiole. Outside the ostiole, it pauses briefly while moving to its left 90° (head up screen) and rubbing the hind legs against the ovipositor held out.

Sequence 2: as soon as the rubbing of the legs is finished, the female pivots the body to its left by 90°, crosses the ostiole, positions itself on the edge of the ostiole (head to the left of the screen) and performs ovipositor pushing movements in the ostiole. During this sequence, the female gradually rotates the body to its right without the ovipositor leaving the ostiole. At the end of the sequence, the female has rotated 45° and it withdraws the ovipositor from the ostiole.

Sequence 3: in the movement, out of the ostiole, the female rapidly pivots to its right by 45° (head facing the top of the screen), then by 90° to find itself facing the ostiole. It crosses the ostiole, positions itself on the edge of the ostiole (head to the right of the screen) and performs ovipositor pushing movements into the ostiole. During this sequence, the female gradually rotates the body to its right without the ovipositor leaving the ostiole. At the end of the sequence, the female has rotated 90° (head down on the screen) and it withdraws the ovipositor from the ostiole. Then it definitely moves away from the ostiole.

 

Note: grooming during oviposition.

I noticed that at the end of almost every oviposition, the female rubs its hind legs together for a few seconds, resting above or near the ostiole, before flying away. Rubbing is carried out against the ovipositor maintained exteriorized. It is also possible that it is the ovipositor which is cleaned using the hind legs. Or both...

It happens that rubbing of the hind legs also takes place between two oviposition sequences, and not only at the oviposition end. I noted that during some ovipositions the female also rubs its front legs together.

 

Silba adipata McAlpine: female rubbing the hind legs against the ovipositor, just after egg laying.

Silba adipata McAlpine: female rubbing the hind legs against the ovipositor, just after egg-laying.

 

OVIPOSITION DURATION

 

According to my observations, the oviposition duration is very variable, and most often clearly greater than 1 minute. As an example, here are the results of the duration measurements of 28 ovipositions, which I carried out on June 5 and 6, 2020.

29% of ovipositions lasted between 50 and 70 seconds. Details: 8 ovipositions; duration in seconds: 50, 51, 57, 58, 59, 60, 63, 69.

46% of ovipositions lasted significantly longer than a minute (between 75 and 134 seconds), with an exceptionally long oviposition of 278 seconds (four and a half minutes). Details: 13 ovipositions; duration in seconds: 75 (2 times), 77, 86, 88, 91, 94, 95, 101, 109, 116, 134, 278.

25% of ovipositions lasted significantly less than a minute (14 to 48 seconds). Details: 7 ovipositions; duration in seconds: 14, 20, 25, 30, 31, 40, 48.

To measure the oviposition duration, I considered as the oviposition start the first ovipositor insertion into the ostiole, and as the oviposition end the ovipositor withdrawal at the end of the last sequence of egg deposit. Are therefore excluded from the measurement the female circulations on the fig before and after egg-laying, as well as the legs rubbing times after egg-laying.
 

Silba adipata McAlpine: oviposition under an ostiolar scale of an immature fig.

Silba adipata McAlpine: oviposition under an ostiolar scale of an immature fig.
 

According to my observations, the presence times on a fig of Silba adipata McAlpine before and after oviposition are generally short, on the order of 10 to 20 seconds. But they can sometimes be very long.

For example, on June 3, 2020, for a presence time on the fig of 163 seconds, I only counted 58 seconds of egg-laying (i.e. 105 seconds of non-laying presence). Thus, during two-thirds of its time on the fig, before and after egg-laying, the egg-laying female walked over the fig or stopped for a long time on its surface, in both cases often out of sight. the region of the ostiole. I was able to note grooming activities on different parts of the body during the time of non-laying presence, but not continuously.

The record presence duration of a Silba adipata McAlpine egg-laying female on a fig was reported to me by Bernard PEYRE, former fig producer and consultant-trainer for the commercial figs cultivation. In September 2020, he observed an egg-laying female in his garden which remained for 17 minutes on the ostiole.

 

FALSE EGG-LAYING

 

NATURE

I was able to observe that, during the ovipositions sequence of a Silba adipata McAlpine female on the same fig tree, some of the figs giving rise to egg-laying behavior on the ostiole for a significant time are not infested: no larvae exit holes; no internal damage from larvae; on examination with a stereomicroscope: no detection of eggs, or empty eggs envelopes, under the ostiolar scales or in the ostiolar canal. I describe this phenomenon as "false egg-laying".

I personally observed false egg-laying on four figs (two figs each time, during two ovipositions sequences separated by several months, on two different fig trees (see observations below). The number of observations is too low to be able to estimate a percentage of oviposirtions sequences giving rise to false egg-laying, or a percentage of false egg-laying occurrences during an ovipositions sequence.

 

OBSERVATION 1 (2 FALSE EGG-LAYING OCCURRENCES)

On March 31, 2021, during a sequence of seven ovipositions in breba figs of my fig tree of the 'Grise de la Saint-Jean' variety, I was able to mark six of the seven attacked figs with a plastic tie, and I daily followed their evolution. Among these, two figs (noted no. 3 and no. 6) gave rise to the observation of false egg-laying.

Fig no. 6 dried out during the 21 days following the ovipositions sequence, because it was affected by physiological drop. On April 21: yellowed fig with red marks; diameter 1.4 cm (1 mm smaller than that it presented at the time of the ovipositions sequence); no larvae exit holes. After opening: no damage from larvae. On examination under a stereomicroscope: no eggs, and no empty eggs envelopes, under the ostiolar scales or in the ostiolar canal. There was therefore no eggs deposition in fig no. 6 during the egg-laying behavior of the female on the ostiole, observed at length on March 31. The fig was then entirely green, not withered, and had a diameter of 1.5 cm.

Fig no. 5 was still on the tree on June 3 (64 days after the ovipositions sequence): diameter 3.8 cm, entirely green, without larvae exit holes. I collected it to check if it was infested with Silba adipata McAlpine. After opening: no damage from larvae. On examination under a stereomicroscope: no eggs, and no empty eggs envelopes, under the ostiolar scales or in the ostiolar canal. There was therefore no eggs deposition in fig no. 5 during the egg-laying behavior of the female on the ostiole, observed at length on March 31.

 

OBSERVATION 2 (2 FALSE EGG-LAYING OCCURRENCES)

On June 23, 2021, during a sequence of 3 ovipositions on my fig tree of the 'Bellone' uniferous variety, I identified each of the visited figs with a red label. As soon as the sequence was over, I bagged the three figs to check the egg incubation duration at this time of the season. After unbagging the figs, 3 days later, two of them (figs no. 2 and no. 3 in the sequence) were found not to be infested. When opened: no damage from larvae. On examination under a stereomicroscope: no eggs, no empty eggs envelopes, under the ostiolar scales or in the ostiolar canal.

I had very precisely timed the egg-laying behavior of the female on the 3 figs: coming and going on the ostiole lasted 2 minutes and 5 seconds for fig no. 2, and 1 minute and 26 seconds for fig no. 3; for fig no. 1, in which I found an empty egg envelope, the egg-laying time was 3 minutes and 54 seconds.

 

FURTHER INVESTIGATIONS NECESSITY

I do not know the precise cause of the four false egg-laying occurrences revealed by my two observations.

It would be necessary to be able to check whether, during false egg-laying, the ovipositor comes out and is inserted under an ostiolar scale. We can imagine that the egg-laying female tests the ostiolar scales with its ovipositor, and considers that they are of insufficient length, or in an unsuitable position, for oviposition. This would then be a fig test of an exceptionally long duration (without oviposition).

In the case where the ovipositor has not come out, or is not inserted under a scale, it could also be an exceptionally long fig test, during which successive visual recognitions of the ostiolar scales carried out by the female during the back and forth on the ostiole do not encourage it to move on to oviposition.

It should also be checked that pushing movements, such as usually observed during egg-laying, are taking place. It could then be a temporary blockage of the eggs release (voluntary or not), for an undetermined reason.

 

MITIGATION OF THE OVIPOSITIONS SEQUENCE NUISANCE

It should be noted that false egg-laying constitutes a mitigation factor for the nuisance of the ovipostions sequence of a female on the same fig tree. In the same way as the plurality of ovipositions in the same fig, which is easier to observe (see chapter).

 

Silba adipata McAlpine egg-laying female on the ostiole of an immature fig.

Silba adipata McAlpine egg-laying female on the ostiole of an immature fig.

 

 

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