Silba adipata McAlpine

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

 

 

The egg

 

 

 

According to the following plan: description, maturation / incubation, eggs observations, empty eggs envelopes observations, difficulty in observing eggs without a stereomicroscope, number of mature eggs carried by a female, two curious observations.

 

DESCRIPTION

 

Filippo SILVESTRI provides a description and a figure for the Black Fig Fly's egg . According to his measurements, the egg is 0.9 mm long and 0.2 mm wide. Référence: SILVESTRI F., 1917, Sulla Lonchaea aristella Beck. (Diptera : Lonchaeidae) dannosa alle infiorescenze e fruttescenze del caprifico e del fico, Bollettino del Laboratorio di Zoologia Agraria in Portici, vol.12, pp. 123 -146. 

Silba adipata McAlpine: egg.

Silba adipata McAlpine: egg.
Dorsal view (1), lateral view; (2); enlarged part of the chorion (3).
 
(credit:
F. SILVESTRI).
 

My observations agree with those of F. SILVESTRI. The egg is elongated. Its outer envelope, called chorion, is thick and whitish.

The anterior pole is a little narrower than the posterior one. The dorsal side is convex, the ventral side is very slightly concave. On either side of the anterior pole, for a segment one-fifth of the length, the egg bears a linear cleft along which the chorion will open to let out the newly born larva.
 

Silba adipata McAlpine: 6 unhatched eggs under the same unripe fig ostiolar scale.

Silba adipata McAlpine: 6 unhatched eggs under the same unripe fig ostiolar scale.

 

Silba adipata McAlpine: 2 empty eggs envelopes (chorions).

Silba adipata McAlpine: 2 empty eggs envelopes (chorions).

 

MATURATION / INCUBATION

 

According to the observations of F. SILVESTRI, in the summer period and for individuals fed with water and honey, the egg maturation (before egg laying) takes place in about ten days.

The author also indicates that the egg incubation lasts 8 days in April, but only 3 days in summer. It therefore takes 3 to 8 days, depending on the season, for the larva to hatch from the egg.

Reference: SILVESTRI F., 1917, Sulla Lonchaea aristella Beck. (Diptera : Lonchaeidae) dannosa alle infiorescenze e fruttescenze del caprifico e del fico, Bollettino del Laboratorio di Zoologia Agraria in Portici, vol.12, pp. 123 -146. 
 

Silba adipata McAlpine: 1 larva and 3 chorions under an ostiolar scale.

Silba adipata McAlpine: 1 larva and 3 chorions under an ostiolar scale.
(the larva has just been born from one of the chorions; length: 0.80 mm).

 

EGGS OBSERVATIONS

 

EXAMPLE 1

June 2019 - naked eye and photographic observation - 3 eggs under a reddish colored ostiolar scale of an immature breba fig of the 'Grise de la Saint-Jean' variety. It should be remembered that the egg is deposited under a horizontal ostiolar scale. If the ostiole is clearly enlarged, eggs can sometimes be found in the ostiolar canal.
 

Silba adipata McAlpine: 3 eggs under an unripe fig ostiolar scale.

Silba adipata McAlpine: 3 eggs under an unripe fig ostiolar scale.

 

Silba adipata McAlpine: 3 eggs under an unripe fig ostiolar scale.

Silba adipata McAlpine: 3 eggs under an unripe fig ostiolar scale.

 

EXAMPLE 2

April 30, 2021 - naked eye and photographic observation - 2 eggs under a reddish colored ostiolar scale of an immature breba fig of the 'Grise de la Saint-Jean' variety.
 

Silba adipata McAlpine: 2 eggs under an unripe fig reddish ostiolar scale

Silba adipata McAlpine: 2 eggs under an unripe fig reddish ostiolar scale.

 

EMPTY EGGS ENVELOPES OBSERVATIONS

 

The empty eggs envelopes (chorions) remain under the ostiolar scales and are a valuable fig infestation indicator. For example, to check if a fig has been attacked by Silba adipata McAlpine, or to count the eggs that have been laid in the fig regardless of the number of larvae that could be found in it.

 

EXAMPLE 1

The photograph below was sent to me in the spring of 2020 by Alain COSTA, agricultural engineer-consultant and fig producer in the Albatera region (Spain). This is an immature fig of the 'Colar de Albatera' variety in longitudinal section. In the ostiolar region (top), a Silba adipata McAlpine empty egg envelope can be identified under a horizontal scale (right).
 

Silba adipata McAlpine: empty egg envelope under an unripe fig ostiolar scale.

Silba adipata McAlpine: empty egg envelope under an unripe fig ostiolar scale (longitudinal section).
Credit: Alain COSTA.
 

On the ostiole region enlargement (photograph below), we can see on the right the gap through which the female slipped its ovipositor to place the egg under the ostiolar scale.
 

Silba adipata McAlpine: empty egg envelope under an unripe fig ostiolar scale.

Silba adipata McAlpine: empty egg envelope under an unripe fig ostiolar scale.
Credit: Alain COSTA.

 

EXAMPLE 2

May 6, 2021 - naked eye and photographic observation - 5 empty eggs envelopes under a reddish ostiolar scale of an immature breba fig of the 'Grise de la Saint-Jean' variety.
 

Silba adipata McAlpine: 5 empty eggs envelopes under an unripe fig reddish ostiolar scale.

Silba adipata McAlpine: 5 empty eggs envelopes under an unripe fig reddish ostiolar scale.

 

EXAMPLE 3

On April 12, 2020, I picked up from the tree a batch of 92 partially reddened immature breba figs of the variety 'Grise de la Saint-Jean'. Inside almost all of these very small figs, I did not find any larvae or larval damage because it was a physiological drop. But I was able to find under the horizontal reddish ostiolar scales of some of them whitish empty eggs envelopes (chorions), proving the presence of Silba adipata McAlpine larvae (of undetectable size due to the very recent nature of the births).
 

Silba adipata McAlpine: empty eggs envelopes under an unripe fig reddish ostiolar scale.

Silba adipata McAlpine: empty eggs envelopes under an unripe fig reddish ostiolar scale.

 

EXAMPLE 4

During the summer of 2021, I collected from the ground and examined under a stereomicroscope the figs attacked by Silba adipata McAlpine from my three fig trees (on a daily basis, so just after the abscission). My goal was to study egg grouping and implantation, as well as the relationship between the eggs number and the larvae exit holes number. Here is an example of an observation of empty eggs envelopes carried out on this occasion.
 

Silba adipata McAlpine: a group of 3 empty eggs envelopes (chorions) under a first level ostiolar scale.

Silba adipata McAlpine: a group of 3 empty eggs envelopes (chorions) under a first level ostiolar scale.
(reddened immature fig; the scale has been turned over and almost completely detached from the ostiole edge).

 

Silba adipata McAlpine: a group of 3 empty eggs envelopes (chorions) under a first level ostiolar scale.

Silba adipata McAlpine: a group of 3 empty eggs envelopes (chorions) under a first level ostiolar scale.
(reddened immature fig).

 

DIFFICULTY IN OBSERVING EGGS WITHOUT A STEREOMICROSCOPE

 

During my annual Silba adipata McAlpine study campaigns, I realized that it is very difficult to be able to observe the eggs with the naked eye (in the field or on the table).

The first cause is the rarity of egg laying females direct observations, which would allow the fig collection immediately after oviposition. If we look for the eggs from infested figs that have been identified by the partial turning of the epidermis to a reddish color, we almost always find empty eggs envelopes (chorions). The second cause is the difficulty of access to the eggs under the ostiolar scales of the attacked figs. The reddish ostiolar scales are tiny, and if the eggs are inserted under a reddish scale which covers a whitish scale, the latter is strongly attached to it and it is very difficult to properly separate the two scales to bring the eggs to light. The need to separate the two scales most often results in their deterioration, and in the destruction or loss of the eggs (even using dissecting forceps and a strong magnifying glass). The search for Silba adipata McAlpine eggs is more a result of work under a stereomicroscope (binocular magnifying glass). With this device, the search for eggs is easy, whether under the horizontal ostiolar scales or in the ostiolar canal.

On the following photograph, on the left, we can clearly see the whitish ostiolar scale which has been removed to reveal the eggs (now empty) deposited by Silba adipata McAlpine under the reddish scale. On the fig, the two ostiolar scales were horizontal, the reddish scale above the other
 

Silba adipata McAlpine: empty eggs envelopes under an unripe fig reddish ostiolar scale.

Silba adipata McAlpine: empty eggs envelopes under an unripe fig reddish ostiolar scale.
(note, on the left, the whitish ostiolar scale which was attached to the reddish scale, and which has been pushed aside).

 

NUMBER OF MATURE EGGS CARRIED BY A FEMALE

 

OBSERVATION 1

On June 5, 2021, as I walked around a fig tree to try to spot ongoing Silba adipata McAlpine ovipositions, I managed to crush an individual of the species with a sharp flick against a torn off petiole base on which it was standing motionless, absorbed in its latex consumption task.
 

Silba adipata McAlpine: female crushed by a sharp flick against a torn off petiole base.

Silba adipata McAlpine: female crushed by a sharp flick against a torn off petiole base.
 

It was a female and, under the impact of the finger, a compact mass of eggs was extracted from the abdomen which split open.
 

Silba adipata McAlpine: mass of eggs extracted from the abdomen of a female crushed by a sharp flick.

Silba adipata McAlpine: mass of eggs extracted from the abdomen of a female crushed by a sharp flick.

 

Silba adipata McAlpine: mass of eggs extracted from the abdomen of a female crushed by a sharp flick.

Silba adipata McAlpine: mass of eggs extracted from the abdomen of a female crushed by a sharp flick.

 

OBSERVATION 2

In order to find out how many eggs at most a Silba adipataMcAlpine mature female can carry at a time, I started in the spring of 2021 to open the abdomen of captured females. During the first examinations of the abdomen contents (under stereomicroscope), I found no eggs, or I found a few eggs which seemed incompletely developed.

But on June 17, 2021, when examining a good-sized Silba adipata McAlpine female (5 mm long), my attention was drawn to two rounded and rather elongated reliefs, slightly lighter in color, located on the ventral side of the abdomen, on either side of the midline. I recognized the two ovaries, and associated their size with a mature state of the female. When I carefully opened the abdomen with two sewing needles, using maximum magnification (45X), I revealed two bulky eggs masses.

Still under maximum magnification, I succeeded in breaking these masses into several smaller clusters and I counted the well-formed eggs, which I most often removed one by one, sometimes two or three at a time, at the tip of a needle. My count was therefore complete and precise: 92 eggs. This one, however, included ten to fifteen well-individualized eggs, but which seemed to me to be much smaller than the others. They may have been eggs that would not have completed their formation until the next day.

I therefore think that only 80 eggs ready to be laid should be retained, which denotes the existence of 80 ovarioles for the 2 ovaries, i.e. 40 ovarioles per ovary (for the insects' ovaries and ovarioles, refer if necessary to the first two chapters of the "Biology of the insects development" course (L3 entomology) of the Mentouri Brothers University (Constantine, Algeria) - head: Dr AGUIB Sihem. It is possible that the number of well-formed eggs carried by a small female (3.5 mm) is lower than that which I found for the large female (5 mm). Note: the prior detection of well swollen ovaries seems to me a good process to avoid opening an immature female. I also noticed that the abdomen was relatively thick, but was not swollen. The swollen abdomens, more translucent than usual, are not specific to females, and result from an abundant consumption of sugary food.

 

TWO CURIOUS OBSERVATIONS

 

EGGS RELEASE ON AN ATTRACTIVE STICK

By examining an enlarged photograph of a female stuck on a yellow attractive stick, I found that this one had released two eggs before dying, the ovipositor having remained fully extended (photographs below). Intrigued, I went back to examine the stick on which I had taken the photograph a few moments before. The ovipositor was clearly visible to the naked eye, which was not the case for the eggs. Observed on the stick using a magnifying glass, I was able to see them better, but they were impossible to detail being less than 1 mm long.
 

Black Fig Fly: female stuck on a yellow stick, having released two eggs.

Black Fig Fly: female stuck on a yellow stick, having released two eggs.

 

Black Fig Fly: female stuck on a yellow stick, having released two eggs.

Black Fig Fly: female stuck on a yellow stick, having released two eggs.
(the ovipositor is clearly out; two whitish elongated eggs are visible).

 

OTHER SPECIES OVIPOSITIONS PROXIMITY

In the photograph of a Black Fig Fly feeding on the edge of an overripe fig dilated ostiole, I noticed the presence of an eggs group on the infructescence (in the central cavity),and that of a much smaller egg on the ostiole edge.
 

Black Fig Fly on the edge of an overripe fig dilated ostiole.

Black Fig Fly on the edge of an overripe fig dilated ostiole.
('Col de Dame Noire' variety; note the eggs group in the central cavity, and the smaller egg to the left of the head).

 

Black Fig Fly on the edge of an overripe fig dilated ostiole.

Black Fig Fly on the edge of an overripe fig dilated ostiole.
(note the 6 Medfly's eggs group in the central cavity, and an egg of spotted wing drosophila to the left of the head).
 

Photographic enlargement shows that the eggs on the infructescence are 6 in number, and that their appearance makes it possible to attribute them either to Silba adipata McAlpine, or to Ceratitis capitata Wiedemann (Mediterranean fruit fly).

The eggs of the two species have indeed a very similar appearance. They are practically the same size (a little less than 1 mm), although this varies slightly from one oviposition to another for the same species. The Silba adipata  McAlpine's egg is a little wider than that of Ceratitis capitata Wiedemann. But the difference is difficult to perceive on photographs, a fortiori with the naked eye, because of the eggs smallness. Considering that the eggs in the group are 6 in number, that the oviposition was carried out in the central cavity (and not under the horizontal ostiolar scales), and that it is an overripe fig, I can determine that these are Ceratitis capitata Wiedemann's eggs.

The egg to the left of the black fig fly's head is a spotted wing drosophila's egg (Drosophila suzukii Matsumura). It is recognizable by its size and the two joined white respiratory filaments which appear to be one. It was accidentally released on the fig epidermis, which I was able to observe several times on my photographic enlargements. Spotted wing drosophila's eggs are normally laid inside the fig, through the epidermis. The respiratory filaments remain outside the fig, protruding through the egg-laying hole.

 

 

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