Silba adipata McAlpine

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

 

 

Size increase of attacked figs

 

 

 

According to my observations and measurements, all figs attacked by Silba adipata McAlpine continue to develop during the period between oviposition and abscission, despite the larvae activity inside them. See below my observations synthesis and detail. Orchard located in the Toulon region, French eastern Mediterranean coast, USDA zone 9b.

 

OBSERVATIONS SYNTHESIS

 

Observation of the size evolution of figs attacked by Silba adipata McAlpine (caliper measurements) shows for all of them an increase in diameter between oviposition and abscission, and a variability of this increase depending on the variety, and on a fig to another for the same variety.

For the studied figs of the 'Bellone' uniferous variety: between oviposition and abscission, the increases in diameter varied from 0.2 to 1.3 cm depending on the figs; the average increase in diameter being 0.7 cm; the average daily increase in diameter was 0.40 mm.

For the studied breba figs of the 'Grise de la Saint-Jean' variety: the figs diameter increased during the 40 days following egg-laying; the average increase in diameter was 1.80 cm; the average daily increase in diameter was 0.45 mm.

 

OBSERVATIONS DETAIL

 

OBSERVATION 1

During the ovipositions observation campaign carried out in June 2020 on a fig bush of the 'Bellone' uniferous variety, I was able to establish with caliper measurements on a 38 control figs batch that all the figs attacked by Silba adipata McAlpine continue to develop during the period between egg-laying and abscission.

 

CONTROL FIGS

I labeled 38 figs at the end of the ovipositions sequences during which they were attacked. To do this, I spotted them immediately after each egg-laying by breaking the petiole of the nearest leaf, and letting the latter hang on the broken petiole. I assigned a number to each of the labeled control figs by writing it down on the label.
 

Control fig labeled with an egg-laying number at the end of Silba adipata McAlpine ovipositions sequence.

Control fig labeled with an egg-laying number at the end of Silba adipata McAlpine ovipositions sequence.
 

And I measured (caliper) twice the diameter of the attacked fig: just after egg-laying, then at the fully reddened fig stage with a dried peduncle detaching without any resistance when very lightly pressed with a finger. I daily checked each of the control figs to verify if it had reached this stage.

This stage is very close to abscission (natural detachment of the dried out peduncle from the tree, causing the fig to fall to the ground). I did not wait for the abscission because this would have resulted, in certain cases, in the mixing on the ground of figs and labels separated from the figs, no longer making it possible to identify the ovipositions in relation to each other.

 

INCREASE IN DIAMETER BETWEEN EGG-LAYING AND ABSCISSION

The record of control figs study (see appendix below) shows that for all of them there was an increase in the diameter during the period between egg-laying and abscission, and that this increase is variable from one fig to another.

The two smallest diameter increases are 0.2 and 0.3 cm. The two strongest being 1.1 and 1.3 cm.

Distribution of the increases in diameter: 0.2 to 0.4 cm: 16%; 0.5 to 0.9 cm: 74%; 1 to 1.3 cm: 10%.

 

AVERAGE INCREASE IN DIAMETER BETWEEN EGG-LAYING AND ABSCISSION

Average increase in diameter during the period between egg-laying and abscission: 7 mm  (0.67 cm rounded to 0.7 cm).

Average daily increase in diameter during the period between egg-laying and abscission: 0.4 mm (0.039 cm rounded to 0.04 cm).

It is possible that the figs increase in size stops before the end of the period separating egg-laying from abscission, for example from a certain level of drying out of the fig peduncle. Since fig diameter measurements were not daily taken, this could not be verified.

 

INCREASE IN DIAMETER ACCORDING TO TIME BETWEEN EGG-LAYING AND ABSCISSION

The record of control figs study makes it possible to break down the increases in the diameter according to the time separating egg-laying and abscission, as follows.

Increases in diameter: 0.2 to 0.4 cm: 10 to 16 days; 0.5 to 0.9 cm: 15 to 21 days; 1 to 1.3 cm: 21 to 25 days.

It appears from this distribution  that the longer is the time between egg-laying and abscission, the more the fig increases in diameter (with a few exceptions, which justify the slight times overlap).

 

INCREASE IN DIAMETER DEPENDING ON INITIAL DIAMETER

The record of control figs study makes it possible to break down the increases in the diameter according to the diameter at the time of egg-laying, as follows.

Diameters 1.1 to 1.4 cm at the time of egg-laying: + 0.8 to 1.3 cm (exceptions: + 0.6 cm for 3 figs, 1 of 1.2 cm and 2 of 1.4 cm).

Diameters 1.5 to 1.9 cm at the time of egg-laying: + 0.5 to 0.7 cm (exceptions: + 0.3 cm for 1 fig of 1.9 cm and + 0.8 cm for 1 fig of 1.5 cm).

Diameters 2 to 2.1 cm at the time of egg-laying: + 0.2 to 0.4 cm (exceptions: + 0.6 cm for 1 fig of 2 cm).

It appears from the distribution above that, with a few exceptions, the larger is the fig diameter at the time of egg-laying, the less it increases afterwards.

And the analysis of the record of control figs study reveals that some of the smallest figs at the time of egg-laying double in volume between egg-laying and abscission, and that the largest figs at the time of egg-laying show a volume increase of 10 to 20% during the same period.

 

APPENDIX: RECORD OF CONTROL FIGS STUDY
 

Egg-laying 1: dated 2/06, fig with a diameter of 1.5 cm; at the stage very close to abscission (June 19), diameter of the fig: 2 cm; increase in diameter: 0.5 cm in 17 days.

Egg-laying 2: 2/06, 1.6 cm - accidental loss of the fig - Egg-laying 3: 2/06, 2 cm; 06/15, 2.2 cm; + 0.2 cm in 13 days.

Egg-laying 4: 2/06, 1.4 cm; 06/21, 2.3 cm; + 0.9 cm in 19 days - Egg-laying 5: 2/06, 1.5 cm; 06/19, 2.1 cm; + 0.6 cm in 17 days.

Egg-laying 6: 2/06, 1.1 cm; 06/27, 2.4 cm; + 1.3 cm in 25 days - Egg-laying 7: 2/06, 1.3 cm; 08/07, 2.2 cm; + 0.9 cm in 36 days.

Egg-laying 8: 2/06, 1.2 cm; 06/22, 2.1 cm; + 0.6 cm in 20 days - Egg-laying 9: 2/06, 1.9 cm; 06/17, 2.5 cm; + 0.6 cm in 15 days.

Egg-laying 10: 3/06, 1.5 cm; 06/22, 2.3 cm; + 0.8 cm in 19 days - Egg-laying 11: 3/06, 1.8 cm; 06/19, 2.3 cm; + 0.5 cm in 16 days.

Egg-laying 12: 3/06, 1.4 cm; 06/23, 2 cm; + 0.6 cm in 20 days - Egg-laying 13: 3/06, 2 cm; 06/15, 2.2 cm; + 0.2 cm in 12 days.

Egg-laying 14: 3/06, 1.5 cm; 06/23, 2.3 cm; + 0.8 cm in 20 days - Egg-laying 15: 3/06, 2 cm; 06/19, 2.6 cm; + 0.6 cm in 16 days.

Egg-laying 16: 3/06, 1.5 cm; 06/21, 2.2 cm; + 0.7 cm in 18 days - Egg-laying 17: 3/06, 1.5 cm; 06/21, 2.2 cm; + 0.7 cm in 18 days.

Egg-laying 18: 3/06, 1.4 cm; 06/24, 2.5 cm; + 1.1 cm in 21 days - Egg-laying 19: 3/06, 1.1 cm; 06/26, 2.1 cm; + 1 cm in 23 days.

Egg-laying 20: 3/06, 1.6 cm - accidental loss of the fig - Egg-laying 21: 3/06, 1.4 cm; 19/06, 2 cm; + 0.6 cm in 16 days.

Egg-laying 22: 3/06, 1.7 cm; 06/23, 2.1 cm; + 0.6 cm in 20 days - Egg-laying 23: 3/06, 1.5 cm; 20/06, 2.2 cm; + 0.7 cm in 17 days.

Egg-laying 24: 3/06, 2.1 cm; 06/19, 2.5 cm; + 0.4 cm in 16 days - Egg-laying 25: 3/06, 2 cm; 06/19, 2.3 cm; + 0.3 cm in 16 days.

Egg-laying 26: 3/06, 2.3 cm - accidental loss of the fig - Egg-laying 27: 3/06, 2.2 cm - accidental loss of the fig.

Egg-laying 28: 5/06, 1.1 cm - accidental loss of the fig - Egg-laying 29: 5/06, 1.4 cm; 06/21, 2.2 cm; + 0.8 cm in 16 days.

Egg-laying 30: 6/06, 1.3 cm; 06/23, 2.1 cm; + 0.8 cm in 17 days - Egg-laying 31: 6/06, 1.8 cm; 06/26, 2.5 cm; + 0.7 cm in 21 days.

Egg-laying 32: 6/06, 1.9 cm; 06/16, 2.2 cm; + 0.3 cm in 10 days - Egg-laying 33: 6/06, 1.3 cm - accidental loss of the fig.

Egg-laying 34: 6/06, 1.6 cm; 06/23, 2.1 cm; + 0.5 cm in 17 days - Egg-laying 35: 7/06, 1.7 cm - accidental loss of the fig.

Egg-laying 36: 7/06, 1.9 cm; 08/07, 2.6 cm; + 0.7 cm in 31 days - Egg-laying 37: 7/06, 1.4 cm; 06/25, 2.3 cm; + 0.9 cm in 19 days.

Egg-laying 38: 7/06, 1.4 cm; 06/25, 2.2 cm; + 0.8 cm in 18 days.

Notes.

Seven control figs were lost accidentally: figs fallen and not found, loss of labels, etc. Their increase in diameter could not be determined, but their size at the time of attack was recorded.

Figs 7 and 36 were not taken into account in the calculations of the diameters increase after egg-laying. On July 8, these two figs did not show larvae exit holes or traces of reddening (respectively 36 days and 31 days after egg-laying). I collected  and opened them. Fig 7 showed larvae damage in the central cavity, with no galleries in the parenchyma, but I detected no larvae, living or dead. The absence of galleries in the parenchyma could nevertheless indicate that the larvae died quite quickly in the central cavity, while they were very small (2 mm), therefore difficult to detect. Fig 36 showed no internal signs of larvae damage. This case could relate to the phenomenon of false egg-laying (see chapter), but  I cannot certify it, having omitted to check the absence of eggs envelopes under the ostiolar scales and in the ostiolar canal.
 

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

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

 

OBSERVATION 2

On March 31, 2021, I observed an ovipositions sequence of a Silba adipata McAlpine female which involved 7 breba figs from my fig tree of the 'Grise de la Saint-Jean' variety. Size measurements of attacked figs, regularly carried out during the period separating egg-laying from abscission, showed that they significantly increased in diameter.

 

MARKING OF ATTACKED FIGS

During the observation of the ovipositions sequence, I was able to mark six of the seven attacked figs with a green plastic tie (cut in advance to a length of 10 cm). The marked attacked figs were identified according to their position on the tree, and designated fig no. 1 to fig no. 6.
 

Immature fig marked with a plastic tie at the end of the Silba adipata McAlpine ovipositions sequence.

Immature fig marked with a plastic tie at the end of the Silba adipata McAlpine ovipositions sequence.
 

At the end of the observation of the evolution of the six marked figs, it appeared the need to consider two lots: one bringing together three figs which have been free of larvae activity, the other consisting of three figs having undergone larvae activity.

 

OBSERVATION OF LOT 1 (FIGS HAVING BEEN FREE OF LARVAE ACTIVITY)

These are figs no. 1, no. 5 and no. 6.

March 31: at the end of the ovipositions sequence, the diameters measured with a caliper of the 3 figs were as follows (in cm): 1.5 (fig no. 6); 1.9 (fig no. 1); 2 (fig no. 5).

April 20 (20 days after the ovipositions sequence): the diameters of the 3 figs were as follows. Fig no. 6: diameter 1.4 cm, decrease of 0.1 cm; fig no. 1: diameter 2.9 cm, growth + 1 cm; fig no. 5: diameter 3 cm, growth + 1 cm. The fig n° 6 being affected by physiological drop (see note below), it is appropriate to only retain figs no. 1 and no. 5 in the study of diameters evolution. For these two figs, average daily increase in diameter over 20 days: 0.05 cm (0.50 mm).

Note: fig no. 6, yellowed and showing red marks, had begun to dry out on the tree, with a loss of 1 mm in diameter. And, the following day (April 21), during the firmness check of the point of attachment of the peduncle, this fig detached itself from the branch under very light pressure with the finger, a sign that abscission was very close, and a confirmation of drying out. This shows that fig no. 6 was affected by physiological drop. It did not present any larvae exit holes. I opened it and noticed no larvae damage. I looked with a stereomicroscope for empty eggs envelopes under the ostiolar scales and in the ostiolar canal, but I found none. There had been therefore no eggs deposition during the egg-laying behavior of the female observed on March 31 on fig no. 6 (a phenomenon that I observed on a few occasions, and which I call "false egg-laying", see chapter).

April 30 (30 days after the ovipositions sequence): the diameters of the 2 figs were as follows. Fig no. 1: diameter 3.3 cm, growth + 0.4 cm; fig no. 5: diameter 3.5 cm, growth + 0.5 cm. Average daily increase in diameter over 10 days: 0.045 cm (0.45 mm).

And for the entire period separating the ovipositions sequence (March 31) from April 30, i.e. 30 days, average daily increase in diameter: 0.048 cm (0.48 mm).

May 10 (40 days after the ovipositions sequence): the diameters of the 2 figs were as follows. Fig no. 1: diameter 3.7 cm, growth + 0.4 cm; fig no. 5: diameter 3.7 cm, increase + 0.2 cm. Average daily increase in diameter over 10 days: 0.030 cm (0.30 mm).

And for the entire period separating the ovipositions sequence (March 31) from May 10, i.e. 40 days: average increase in diameter: 1.75 cm; average daily increase in diameter: 0.044 cm (0.44 mm).

May 20 (50 days after the ovipositions sequence: figs no. 1 and 5 had a diameter of 3.8 cm. That is a negligible increase of 1 mm over 10 days for each of them.

May 30 (60 days after the ovipositions sequence): the diameter of figs no. 1 and no. 5 had not increased (3.8 cm).

June 3 (64 days after the ovipositions sequence): collection from the tree of fig no. 5, to check if it was infested by Silba adipata McAlpine. Fig with a diameter of 3.8 cm, entirely green, without larvae exit holes. When opening the fig: no damage from larvae. On examination under a stereomicroscope: no eggs, or empty eggs envelopes, under the ostiolar scales or in the ostiolar canal. The fig was therefore healthy, not attacked by Silba adipata McAlpine. There was no eggs deposition during the egg-laying behavior of the female observed on March 31 on fig no. 5 (a phenomenon that I observed on a few occasions, and which I call "false egg-laying", see chapter).

June 9 (70 days after the ovipositions sequence): collection from the tree of fig no. 1, to check if it was infested by Silba adipata McAlpine. Fig with a diameter of 3.8 cm, entirely green, without larvae exit holes. When opening the fig: no damage from larvae. On examination under a stereomicroscope: 4 empty eggs envelopes grouped under a first level ostiolar scale. I think that they came from the egg-laying observed on March 31. And that the larvae died when they were of an undetectable size among the inflorescence, before being able to cause damage in it (a case I already observed several times previously). As a result, fig no. 1 increased in size like a healthy fig.

 

OBSERVATION OF LOT 2 (FIGS HAVING UNDERGONE LARVAE ACTIVITY)

These are figs no. 2, no. 3 and no. 4.

March 31: at the end of the ovipositions sequence, the diameters measured with a caliper of the 3 figs were as follows (in cm): 1.6 (fig no. 2 and fig no. 4); 1.8 (fig no. 3).

April 20 (20 days after the ovipositions sequence): the diameters of the 3 figs were as follows. Fig no. 2: diameter 2.6 cm, growth + 1 cm; fig no. 4: diameter 2.6 cm, growth + 1 cm; fig no. 3: diameter 2.8 cm, growth + 1 cm. Average daily increase in diameter: 0.05 cm (0.50 mm).

April 30 (30 days after the ovipositions sequence): the diameters of the 3 figs were as follows. Fig No. 2: diameter 3.2 cm, growth + 0.6 cm; fig no. 4: diameter 3.2 cm, growth + 0.6 cm; fig no. 3: diameter 3.3 cm, growth + 0.5 cm. Average daily increase in diameter over 10 days: 0.057 cm (0.57 mm).

And for the entire period between the ovipositions sequence (March 31) and April 30, i.e. 30 days, average daily increase in diameter: 0.052 cm (0.52 mm).

On April 30, fig no. 2, 3.2 cm in diameter and with 3 larvae exit holes, accidentally fell during handling. The diameter increase measurements continued with figs no. 3 and no. 4.

May 10 (40 days after the ovipositions sequence): the diameters of the 2 figs were as follows. Fig no. 4: diameter 3.4 cm, growth + 0.2 cm; fig no. 3: diameter 3.6 cm, growth + 0.3 cm. Average daily increase in diameter over 10 days: 0.023 cm (0.23 mm). Calculation of the average on 3 figs, considering that fig no. 2 would have had the same increase in diameter over the 10 days as fig no. 4.

And for the entire period between the ovipositions sequence (March 31) and May 10, i.e. 40 days, average increase in diameter: 1.80 cm; average daily increase in diameter: 0.045 cm (0.45 mm). It being specified that the two values were calculated on 3 figs, considering that fig no. 2 would have had during the last 10 days the same increase in diameter as fig no. 4.

May 11 (41 days after the ovipositions sequence): fig no. 3, 3.6 cm in diameter and presenting 2 larvae exit holes, naturally detached itself from the branch and fell to the ground (abscission), completely reddened and with the peduncle dried out. The diameter increase measurements continued with fig no. 4 alone.

May 20 (50 days after after the ovipositions sequence): the diameter of fig no. 4 had not increased (3.4 cm).

May 30 (60 days after after the ovipositions sequence): the diameter of fig no. 4 had not increased (3.4 cm).

June 9 (70 days after after the ovipositions sequence): the diameter of fig no. 4 had not increased (3.4 cm).

June 16 (77 days after the oviposions sequence): fig no. 4, showing 2 larvae exit holes since May 4, has reached the end of immature stage. Diameter: 4.2 cm (sudden increase of 0.8 cm, in two or three days). Then the fig continued its evolution towards the maturity stage. This fig is a case that I encounter every year on my 'Grise de la Saint-Jean' fig tree, for 2 to 5% of the total immature figs production (excluding physiological drop). Given the nature of this last increase in diameter, it was not taken into account in the calculations relating to the increase in diameter of the attacked figs. End of the diameters measurements campaign for the figs of the lot 2.

 

CONCLUSIONS

Lot 1.

Beyond 40 days after the ovipositions sequence, the diameter of the figs increased only negligibly (1 mm). During the 40 days of significant growth: the average increase in diameter was 1.75 cm; the maximum increase in diameter observed for a fig was 1.80 cm; the average daily increase in diameter was 0.44 mm.

During the month following the ovipositions sequence, the average daily increase in figs diameter was 0.48 mm, and during the following 10 days it was significantly less (0.30 mm).

 

Lot 2.

The figs diameter increased during the 40 days following the ovipositions sequence. During the 40 days of growth: the average increase in diameter was 1.80 cm; it is also the maximum increase in diameter observed for a fig, due to the equality of the increases in diameter for all figs; the average daily increase in diameter was 0.45 mm.

During the month following egg-laying, the average daily increase in diameter was 0.52 mm, and during the following 10 days it was significantly less (0.23 mm).

 

Note: over the 40-day growth period, the maximum increase in diameter observed for a fig is the same, and the average daily increase in diameter of figs is practically the same, for figs having undergone larvae activity and for those who have not known it. However, the number of studied figs is too low to be able to draw a general rule of identical development for healthy figs and figs attacked by Silba adipata McAlpine.
 

Silba adipata McAlpine laying eggs under an ostiolar scale of an immature breba fig ('Grise de la Saint-Jean' variety).

Silba adipata McAlpine laying eggs under an ostiolar scale of an immature breba fig ('Grise de la Saint-Jean' variety).

 

Silba adipata McAlpine laying eggs under an ostiolar scale of an immature breba fig ('Grise de la Saint-Jean' variety).

Silba adipata McAlpine laying eggs under an ostiolar scale of an immature breba fig ('Grise de la Saint-Jean' variety).

 

 

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