Silba adipata McAlpine

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

 

 

Size of attacked figs

 

 

 

I provide my observations of the figs size at the time of the Silba adipata McAlpine attack (fig diameter measurement with a caliper just after direct observation of the oviposition). Orchard located in the Toulon region, French eastern Mediterranean coast, USDA zone 9b.

These are not figs showing signs of infestation collected from the ground or from the tree. The size of such figs is not the size at the time of egg-laying. Indeed, all attacked figs continue to develop during the period between oviposition and abscission, despite the larvae activity inside them. According to my measurements, the diameter increase of an attacked fig can reach during this period up to 1.3 cm for the 'Bellone' uniferous variety, and up to 1.8 cm for the breba crop of the 'Grise de la Saint-Jean' variety (see chapter).

 

OBSERVATIONS SYNTHESIS

 

The smallest fig diameter I have measured just after an oviposition has been 1.1 cm (on several occasions), and the largest 2.3 cm (twice).

We conclude from these observations that the diameter from which Silba adipata McAlpine attacks immature figs is 1.1 cm. The figs size measurements at the time of egg-laying provide another important information for triggering certain control methods: a significant percentage of attacks concern tiny figs (for the 'Bellone' and 'Bécane' varieties, I observed that approximately 40% of ovipositions occur in figs with a diameter between 1.1 and 1.4 cm).

 

OBSERVATIONS DETAIL

 

OBSERVATION 1 (2016)

On September 18, 2016, at 6:15 p.m., in windy weather, I observed a Silba adipata McAlpine egg-laying in a fairly large green immature fig from a young bush of the 'Col de Dame Noire' uniferous variety, entirely in the shadow. Measuring the fig with a caliper immediately after egg-laying revealed a diameter of 2.3 cm. Only two immature figs and four ripe figs remained on the fig bush. This is also the latest egg-laying that I have witnessed in my orchard.

 

OBSERVATION 2 (2020)

During the 2020 ovipositions observation campaign, I was able to measure with a caliper the diameter of 38 control figs of the 'Bellone' uniferous variety, immediately after their attack (oviposition) by Silba adipata McAlpine.

The smallest diameter measured was 1.1 cm (for 3 figs), and the largest 2.3 cm (for 1 fig). Average diameter of figs at the time of egg-laying: 1.6 cm (60.8 / 38).

Distribution of the 38 control figs into three ranges of diameters at the time of egg-laying: 1.1 to 1.4 cm: 37% (14 figs); 1.5 to 1.9 cm: 45% (17 figs); 2 to 2.3 cm: 18% (7 figs). It being specified that the attacked figs with a diameter of 1.5 cm represent 18%.

It appears that tiny figs with a diameter less than or equal to 1.5 cm constitute 55% of the attacked figs.
 

Silba adipata McAlpine laying eggs under an immature fig ostiolar scale (

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

 

OBSERVATION 3 (2020)

This observation is interesting because it shows that, even for breba figs, egg-laying can take place in very small diameter figs.

On April 12, 2020, I picked up from a tree of the 'Grise de la Saint-Jean' variety a batch of tiny reddened immature breba figs. They were very close to the abscission stage, and seemed to me to be part of the physiological drop phenomenon.

When I opened them, I noticed that two of them were infested with Silba adipata McAlpine larvae although they had diameters of 1.2 cm and 1.3 cm. The fig measuring 1.3 cm in diameter contained 1 larva (length: 1.5 mm), and that of 1.2 cm in diameter (photographs below) contained 4 larvae of two different sizes (lengths: 3 mm and 1.5 mm). The presence of larvae of clearly different sizes in the fig reflects successive ovipositions spaced a few days apart. In the considered case, the fig was the subject of at least two Silba adipata McAlpine attacks (two sizes of larvae).
 

Immature fig with a diameter of 1.2 cm attacked by Silba adipata McAlpine.

Immature fig with a diameter of 1.2 cm attacked by Silba adipata McAlpine.

 

Diameter measurement for the tiny fig attacked by Silba adipata McAlpine: 1.2 cm.

Diameter measurement for the tiny fig attacked by Silba adipata McAlpine: 1.2 cm.

 

Immature fig with a diameter of 1.2 cm containing 4 Silba adipata McAlpine larvae (in fragment of the left).

Immature fig with a diameter of 1.2 cm containing 4 Silba adipata McAlpine larvae (in fragment of the left).

 

Silba adipata McAlpine larvae of two different sizes inside an immature fig with a diameter of 1.2 cm.

Silba adipata McAlpine larvae of two different sizes inside an immature fig with a diameter of 1.2 cm.
 

Egg-laying can be dated to 16 days previously (March 27) for the two 3 mm long larvae, and to 10 days previously ( April 2) for the two 1.5 mm long larvae. Taking into account that, for the month of April the duration of egg incubation is 8 days, and that of larva complete development is 24 days. And considering a larva average increase in size of 1 mm every 4 days, so that it reaches 7 mm during these 24 days (+ 6 mm compared to its birth size of approximately 1 mm).

I noticed that the fig had started to dry out before I collected it. To obtain the diameter of the fig before this start of drying, it can be reasonably applied an adjustment of 2 mm to the collection diameter of the fig (1.2 cm). Which gives a value of 1.4 cm for the reevaluated diameter. According to my measurements in the field, the diameter of a breba fig increases after egg-laying by 0.50 mm per day (the value retained is that observed for the first 20 days after egg-laying). If we take into account this average daily increase, and apply it since the oldest egg-laying, the fig diameter would have increased by 0.8 cm (0.050 cm x 16 days). And at the date of egg-laying it would then have been 1.4 - 0.8 = 0.6 cm. In the current state of the whole of my observations, it seems unlikely to me that a fig with a diameter of 0.6 cm could be attacked by Silba adipata McAlpine.

It should be borne in mind that the fig belonged to a batch of figs relating to physiological drop, doomed to a progressive cessation of development and to abscission after complete reddening (only a very small part of the reddened figs batch showed signs of Silba adipata McAlpine attack). Therefore, we can reasonably argue that this fig developed after egg-laying less quickly than a fig not weakened by the physiological drop phenomenon. So, my hypothesis is that the fig diameter at the time of egg-laying was 1.1 cm, and that the fig grew only 0.3 cm in diameter (not 0.8 cm) during the 16 days that followed.

 

OBSERVATION 4 (2021)

On March 31, 2021, I was able to observe a Silba adipata McAlpine ovipositions sequence, which involved 7 immature breba figs on my fig tree of the 'Grise de la Saint-Jean' variety. During the ovipositions sequence, I managed to mark six of the seven attacked figs with a green plastic tie (previously cut to 10 cm length).

Immediately after the end of the ovipositions sequence, I measured with a caliper the diameters of the marked figs, which were as follows (cm): 1.5, 1.6 (2 figs); 1.8; 1.9; 2.
 

Immature fig marked with a plastic tie immediately after Silba adipata McAlpine egg-laying.

Immature fig marked with a plastic tie immediately after Silba adipata McAlpine egg-laying.

 

OBSERVATION 5 (2022)

On June 13, 2022, late in the afternoon, my attention was caught by the sustained egg-laying activity of two Silba adipata McAlpine females on the immature figs of varying sizes of a young fig tree of the 'Bécane' uniferous variety. This one, planted two years previously in my orchard to compare its fruits with those of the 'Figue de Langres' variety, only bore around thirty fruits, some of which were very small.

I was able to observe ovipositions for about half an hour before the two females disappeared, and I noticed that the latter did not limit themselves to the largest figs. Suspecting that other attacks might have occurred during the day or in previous days, I decided to collect all the figs from the concerned tree, to study the ovipositions.
 

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

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

 

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

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

For each fig: diameter measurement using a caliper; examination under a stereomicroscope of the ostiolar scales and the ostiolar canal.

Total figs collected: 33. Attacked figs: 16 (48%). Unattacked figs: 17 (52%).

For 14 of the 16 attacked figs, I found one or more empty egg envelopes (accompanied or not by the presence of fresh eggs). For one fig, I only found fresh eggs, and for another fig, I found under an ostiolar scale 3 larvae which had just been born and which had not yet started their journey towards the fig central cavity.
 

Silba adipata McAlpine: two eggs discovered under two intersecting ostiolar scales.

Silba adipata McAlpine: two eggs discovered under two intersecting ostiolar scales.
(the scale which completely covered the eggs has been removed).
 

Diameters of the 16 attacked figs, measured on June 13: 1.2 cm (2 figs); 1.3 (2); 1.4 (1); 1.5 (2); 1.6 (1); 1.7 (2); 1.8 (2); 1.9 (2); 2 (1); 2.3 (1).

Knowing that the incubation period is at least 3 days in June, and that during these 3 days the diameter of the attacked fig increased by at least 1 mm, we can determine the figs maximum size at the time of Silba adipata McAlpine egg-laying. It is appropriate to reduce by 1 mm the diameter measured for the 14 attacked figs containing solely or in part empty eggs envelopes, as well as for the fig containing 3 larvae under an ostiolar scale (1.2 cm), and to keep the measured diameter for the fig containing only fresh eggs (1.8 cm).

It therefore results that the figs attacked on the young fig tree of the 'Bécane' variety had the following maximum diameters at the time of egg-laying: 1.1 cm (2 figs); 1.2 (2); 1.3 (1); 1.4 (2); 1.5 (1); 1.6 (2); 1.7 (1); 1.8 (3); 1.9 (1); 2.2 (1). Being noted that the actual size at the time of egg-laying could have been smaller.

We can point out that we find the critical attack diameter of 1.1 cm, identified during the observation 2 reported above. And it should be emphasized that 44% of the attacked figs (7 out of 16) had a maximum diameter of between 1.1 and 1.4 cm at the time of egg-laying.

 

 

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