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Author: François DROUET.
Photographs: François DROUET.
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Correlation study larvae exit holes number / eggs number
According to the following plan: study purposes and conditions, analysis summary, detailed study results, explanation of the differences larvae exit holes number / eggs number.
STUDY PURPOSES AND CONDITIONS
During the summer of 2021, I decided to examine with a stereomicroscope the ostiolar region of a large number of immature figs from my old fig trees infested by Silba adipata McAlpine, and having reached the stage of abscission ('Grise de la Saint-Jean' biferous variety, 'Bellone' uniferous variety, 'Col de Dame Noire' uniferous variety). My objective was to consolidate the observations made over the course of the previous six years, by studying eggs volumetry and eggs physical distribution in figs, and trying to deduce the number of ovipositions per fig. I set myself as another objective to study the correlation between the number of larvae exit holes carried by a fig and the number of eggs laid by Silba adipata McAlpine inside it.
So, for two months (July and August), I collected the figs on the ground by performing a daily inspection under the fig trees. For each of the infested figs, I carried out the following operations: rmeasurement of the diameter with a Vernier caliper; recording of the larvae exit holes number; ostiolar scales and ostiolar canal examination with the stereomicroscope. Total figs examined: 143. The parasitized figs having been collected on the ground after abscission, I found no unhatched eggs when examining the ostiolar region of all the figs; only empty eggs envelopes (chorions). The results, fig by fig, of the ostiolar regions examinations and of the larvae exit holes counts are provided at the end of the chapter relating to the study of infestation by eggs.
The study of the correlation between the number of larvae exit holes shown by the figs and the number of eggs deposited in them was carried out on 139 figs. Indeed, 4 figs were excluded from this study: 3 'Grise de la Saint-Jean' figs (collected from the tree, without larvae exit holes), and 1 'Bellone' fig (collected from the ground, and on which it was not possible to detect a larva exit hole). Distribution by varieties of the 139 figs included in the study: 77 of the 'Grise de la Saint-Jean' biferous variety (second crop), 43 of the 'Col de Dame Noire' uniferous variety, and 19 of the 'Bellone' uniferous variety.
Immature figs parasitized by Silba adipata McAlpine, having fallen to the ground.
('Grise de la Saint-Jean' biferous variety, second crop).
Immature fig parasitized by Silba adipata McAlpine, bearing 1 larva exit hole.
('Grise de la Saint-Jean' biferous variety, second crop).
ANALYSIS SUMMARY
In summary, it appears from the study that a little more than half of the figs attacked by Silba adipata McAlpine show a number of larvae exit holes lower than the number of eggs deposited in the fig. This phenomenon concerns all figs containing 5 eggs or more, and approximately 70% of figs containing 2 eggs, 3 eggs, or 4 eggs. It should be noted that it does not concern figs containing 1 egg.
Concerning the observed differences: for figs containing 2 eggs, the difference can only be 1; for figs containing 3 to 5 eggs: significant occurrence of all possible differences for the considered category (except the difference of 1 for figs with 5 eggs, an exception that could be explained by an insufficiently large number of figs of this type in the studied batch).
Immature fig parasitized by Silba adipata McAlpine, showing 2 larvae exit holes.
('Grise de la Saint-Jean' variety, second crop; 3 chorions under an ostiolar scale: difference of 1 compared to the larvae exit holes).
Silba adipata McAlpine: 3 chorions under an ostiolar scale of a reddened immature fig; no other infestation.
('Grise de la Saint-Jean' variety, second crop; 2 larvae exit holes: difference of 1 compared to the number of chorions).
DETAILED STUDY RESULTS
OVERALL VOLUMETRY
Figs with a number of larvae exit holes equal to the number of eggs laid: 61, or 44% (61 / 139). Figs with a number of larvae exit holes less than the number of eggs laid: 78, or 56% (78 / 139).
DISTRIBUTION OF FIGS WITH A NUMBER OF LARVAE EXIT HOLES EQUAL TO THE NUMBER OF EGGS
The distribution of the batch of 61 figs according to the number of eggs laid in the fig is as follows: 1 egg: 52% of the 61 figs (32 figs); 2 eggs: 30% (18 figs); 3 eggs: 11% (7 figs); 4 eggs: 7% (4 figs). Note: none of the figs containing 5, 6 or 7 eggs have a number of larvae exit holes equal to the number of eggs laid in the fig.
DISTRIBUTION OF FIGS WITH A NUMBER OF LARVAE EXIT HOLES LESS THAN THE NUMBER OF EGGS
The distribution of the batch of 78 figs according to the number of eggs laid in the fig is as follows: 2 eggs: 45% of the 78 figs (35 figs); 3 eggs: 24% (19 figs); 4 eggs: 13% (10 figs); 5 eggs: 14% (11 figs); 6 eggs: 3% (2 figs); 7 eggs: 1% (1 fig). Note: figs in which only one egg has been laid are not included in this lot because they have only one larva exit hole, i.e. a number of larvae exit holes equal to the number of eggs laid in the fig.
DIFFERENCES OBSERVED WHEN THE NUMBER OF LARVAE EXIT HOLES IS LESS THAN THE NUMBER OF EGGS
Figs containing 2 eggs: difference of 1 for 100% of cases (these figs have only one larva exit hole); 3 eggs: difference of 1 (2 larvae exit holes) for 42%, difference of 2 (1 larva exit hole) for 58%; 4 eggs: difference of 1 (3 larvae exit holes) for 40%, difference of 2 (2 larvae exit holes) for 10%, difference of 3 (1 larva exit hole) for 50%; 5 eggs: difference of 1 (4 larvae exit holes) for 0%, difference of 2 (3 larvae exit holes) for 27%, difference of 3 (2 larvae exit holes) for 46%, difference of 4 (1 larva exit hole) for 27%; 6 eggs: the two figs of this type in the analyzed batch have 3 larvae exit holes, i.e. a difference of 3; 7 eggs: the only fig of this type in the analyzed batch has 2 larvae exit holes, i.e. a difference of 5.
BREAKDOWN OF FIGS BETWEEN THE TWO CATEGORIES
Breakdown according to the number of eggs. Figs containing 1 egg: 100% with a number of larvae exit holes equal to the number of eggs laid in the fig, 0% with a number of larvae exit holes lower than the number of eggs laid in the fig; 2 eggs: 34% (equal number), 66% (lower number); 3 eggs: 27%, 73%; 4 eggs: 29%, 71%; 5 eggs: 0%, 100%; 6 eggs: 0%, 100%; 7 eggs: 0%, 100%.
EXPLANATION OF THE DIFFERENCES LARVAE EXIT HOLES NUMBER / EGGS NUMBER
The first explanation is the mortality rate of the larvae in the fig. It is not uncommon to find dead larvae in immature figs: under the ostiolar scales, in the ostiolar canal, in the central cavity, and in the galleries dug in the whitish parenchyma (under the epidermis).
The second explanation is the use by some of the larvae of the exit hole pierced by a larva that left the fig before them. This phenomenon is almost impossible to observe on the fig tree. But it is regularly visible when storing in a plastic box figs attacked by Silba adipata McAlpine that do not yet have larvae exit holes. I thus happened to directly observe the exit of several larvae through the same hole in a plastic box containing several attacked figs. And if, for a dozen of attacked figs, one fig is placed per box, it is easy to count a few hours later the larvae present in each of the boxes, and to compare their number to that of the larvae exit holes that are detected on the fig in the same box. Very often, the larvae present in the box are more numerous than the exit holes detected on the fig.
Black Fig Fly larva leaving an immature fig.
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