Silba adipata McAlpine

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Author: François DROUET.
Photographs: Margaux ALLIX and François DROUET.
All rights reserved.

 

 

Infested figs reaching maturity

 

 

 

According to the following plan: crops observations, detection difficulty on the fig tree, egg-laying date, the various cases of infested ripe figs, curious observation of the CIVAMBIO 66.

 

CROPS OBSERVATIONS

 

According to my observations, all figs attacked by Silba adipata McAlpine continue to develop, despite the presence of larvae inside them. A few days after the larvae have abandoned the immature fig (appearance of larvae exit holes), the latter normally falls to the ground.

But I remarked that for most fig varieties, a little part of the attacked figs remains on the tree after the larvae exit holes appearance, and reaches the ripe stage, then the overripe and desiccation stages. Inside these figs, I notice larvae damage and partial rotting, and, sometimes, I observe larvae, pupae, and even young dried-up dead imagos. Figs that show Silba adipata McAlpine attack symptoms while they are ripe (or while they have just started to evolve towards maturity), are figs that have been attacked in the immature stage (see below the subchapter dedicated to egg-laying date).

I observe every year the phenomenon of ripe figs infested by Silba adipata McAlpine, mainly on my fig tree of the 'Grise de la Saint-Jean' biferous variety (both for breba figs and for second crop figs). Some years, I also detect a few cases on my fig trees of the 'Col de Dame Noire' and 'Bellone' uniferous varieties.

Depending on the variety and the year, the phenomenon affects 2 to 5% of the total number of immature figs carried by the tree during the season. Some examples below.

In 2015, I noted only a small number of infested ripe figs (5 to 6 figs), only of the 'Grise de la Saint-Jean' variety (breba and second crop figs). I detected them at the stage of barely ripe figs.

In 2019, on a production of 101 'Grise de la Saint-Jean' breba figs (excluding physiological drop), I detected 5 infested figs which had started to evolve towards the maturity, or which were fully ripe (i.e. 5% of the production). 

In 2021, on a production of 181 breba figs 'Grise de la Saint-Jean' (excluding physiological drop), I observed the passage of 3 infested figs from the immature stage to the start of softening and color change, then to full maturity (i.e. 2 % of tree production).

 

DETECTION DIFFICULTY ON THE FIG TREE

 

I have noticed that a fig attacked by Silba adipata McAlpine which has reached the stage of maturity is difficult to detect on the tree. In fact, it remains attached to the tree, continuing its evolution towards overripeness, and it seems completely healthy. Its appearance (shape, color, size) is similar to that of an unattacked fig. However, in my experience, in almost all cases this fig shows one or more larvae exit holes. There are cases of infested ripe figs not having larvae exit holes, therefore containing Silba adipata McAlpine larvae, but they are very rare.

These are the larvae exit holes that allow us to detect on the tree that a ripe fig is not healthy. But these are only seen if we have a trained eye, and we are near the infested ripe fig. The attack is less difficult to identify by the larvae exit holes if maturity is not very advanced (fig still firm with smooth skin). If the stage of maturity is more advanced, the wrinkling of the fig epidermis can prevent the detection of the larvae exit holes. For me, the infested figs are best spotted on the tree during the period separating the end of the immature stage from the maturity stage.

Hereafter, examples of infested figs detected by the larvae exit holes.

Example 1 (fig having just started to evolve towards maturity).
 

Black Fig Fly: infested fig starting to ripen.

Black Fig Fly: infested fig starting to ripen.

 

Black Fig Fly: larva exit hole on an infested fig starting to ripen.

Black Fig Fly: larva exit hole on an infested fig starting to ripen.
 

Example 2 (ripe fig).
 

Black Fig Fly: fig attacked in the immature stage and having reached maturity.

Black Fig Fly: fig attacked in the immature stage and having reached maturity.
(note the larva exit hole).

 

Black Fig Fly: larva exit hole on a fig attacked in the immature stage and having reached maturity.

Black Fig Fly: larva exit hole on a fig attacked in the immature stage and having reached maturity.
(note the small darker area above the larva exit hole).
 

Example 3 (ripe fig attacked by a magpie).

Black Fig Fly: fig attacked in the immature stage and having reached maturity.

Black Fig Fly: fig attacked in the immature stage and having reached maturity.
(at the bottom of the fig, a larva exit hole; on the left, magpie attack).

 

Black Fig Fly: larva exit hole on a fig attacked in the immature stage and having reached maturity.

Black Fig Fly: larva exit hole on a fig attacked in the immature stage and having reached maturity.
(on the left, magpie attack).

Note.

I have noticed that the small purple area which often surrounds the Silba adipata McAlpine larva exit hole on the epidermis of the immature fig (hard and green) turns blackish when the attacked fig reaches maturity. This can help a trained eye detect on the tree that the ripe fig is not healthy. In some cases, the blackish area around the larva exit hole may widen as the fig ripens. I even observed a case where the extension and color variation of the area around the larvae exit holes resulted in a blackish stripe visible from quite a distance for an experienced person (see photographs below).
 

Fig attacked by the Black Fig Fly in the immature stage, having reached the maturity stage (summer).

Fig attacked by the Black Fig Fly in the immature stage, having reached the maturity stage (summer).
(for a trained eye, a blackish stripe allows to detect that the fig is infested).

 

Fig attacked by the Black Fig Fly in the immature stage, having reached the maturity stage (summer).

Fig attacked by the Black Fig Fly in the immature stage, having reached the maturity stage (summer).
(note the 3 larvae exit holes, all along the blackish stripe, that allow to determine that egg-laying occurred at least 9 days earlier - immature stage).

 

EGG-LAYING DATE

 

According to my field observations, and my method for dating the ovipositions by the fig examination (exit holes presence; larvae size, this second criterion being rarely used for ripe figs), Silba adipata McAlpine never attacks ripe figs. The attack symptoms found in ripe figs result from ovipositions made in the immature stage, most often several weeks before the maturity stage. On the occasion of an experiment in the field, I was even able to follow the evolution of a 'Grise de la Saint-Jean' breba fig which reached the maturity stage 2.5 months after egg-laying and fell to the ground, completely dried out, 4 months after it (see details of this observation in a specific chapter).

The oviposition in the immature stage is easy to verify, by comparing the fig state of maturity to the biological durations of the Silba adipata McAlpine's life cycle.

The biological times indicated by F. SILVESTRI for summer are: 3 days for egg incubation; 6 to 7 days for the larva complete development (abandonment of the fig by the larva). 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. This puts the duration between egg-laying and the fig abandonment by the larva at a total of 9 to 10 days (appearance of a larva exit hole).

According to my observations of control figs, between the end of the immature stage (beginning of softening and color change) and the reaching of commercial maturity (just ripe fig), it takes 3 to 4 days, depending on the variety. For example: 3 days for 'Grise de la Saint-Jean' and 4 days for 'Bellone'. For a ripe fig, the time elapsed since the end of immature stage increases by 1 day, respectively becoming 4 and 5 days.

Thus, for a just ripe fig, egg-laying took place at least 5 to 7 days before the end of immature stage (possible cases: 9 days between egg-laying and abandonment of the fig - 3 days between end of immature stage and maturity; 9 - 4; 10 - 3; 10 - 4). And for a ripe fig, egg-laying occurred at least 4 to 6 days before the end of immature stage (possible cases: 9 - 4; 9 - 5; 10 - 4; 10 - 5).

It should be emphasized that these are minimum times of low occurrence. All the attacked figs that I detect on the tree at the end of immature stage (softened and yellowed fig) are so because they already show larvae exit holes, which places egg-laying before the aforementioned minimum times. In fact, according to my observations, the attacks detected in ripe figs most often took place several weeks before reaching the ripe stage. This follows from the 3-phase attack pattern of Silba adipata McAlpine, according to which a very high percentage of attacks is concentrated during the short phase 1 (intense attacks phase), this one occurring several weeks before the figs reach the maturity stage).
 

Black Fig Fly female laying eggs under an immature fig ostiolar scale.

Black Fig Fly female laying eggs under an immature fig ostiolar scale.

 

THE VARIOUS CASES OF INFESTED RIPE FIGS

 

CASE 1:  SILBA ADIPATA McALPINE ATTACK WITHOUT CRYPTOGAMIC INFECTION

An example of this case: a just ripe fig of the 'Bourjassote Noire' uniferous variety. The fig infructescence shows significant damage from larvae, but we note the total absence of cryptogamic attack traces which could have been added to it. This case is found on early season figs.
 

Silba adipata McAlpine: larvae damage in the infructescence of a fig attacked in the immature stage and having reached maturity.

Silba adipata McAlpine: larvae damage in the infructescence of a fig attacked in the immature stage and having reached maturity.
(note the absence of traces of cryptogamic infection which could have added to the larvae damage).

 

Silba adipata McAlpine: larvae damage in the infructescence of a fig attacked in the immature stage and having reached maturity.

Silba adipata McAlpine: larvae damage in the infructescence of a fig attacked in the immature stage and having reached maturity.

 

CASE 2: SILBA ADIPATA McALPINE ATTACK + WEAK CRYPTOGAMIC INFECTION

An example of this case: a ripe breba fig of the 'Grise de la Saint-Jean' variety showing Silba adipata McAlpine larvae exit holes.
 

Silba adipata McAlpine: ripe fig showing larvae exit holes (attack in the immature stage).

Silba adipata McAlpine: ripe fig showing larvae exit holes (attack in the immature stage).
(the fig appearance is that of a healthy fig, but we can see on the fig side a larva exit hole).

 

Silba adipata McAlpine: ripe fig showing larvae exit holes (attack in the immature stage).

Silba adipata McAlpine: ripe fig showing larvae exit holes (attack in the immature stage).
(note, by the latex on the side of the peduncle, that this fig was still attached to the tree).

 

Silba adipata McAlpine: two larvae exit holes on a ripe fig (attack in the immature stage).

Silba adipata McAlpine: two larvae exit holes on a ripe fig (attack in the immature stage).
 

Inside, the ripe breba fig shows yellowish to brownish clusters within the infructescence, resulting from damage by Silba adipata McAlpine larvae at the immature stage, as well as blackish rot traces. The latter are due to the action of a pathogenic fungus (mold type), which infects the infructescence weakened by the larvae damage (photograph below).
 

Interior of a ripe fig showing traces of Silba adipata McAlpine attack (immature stage) and a weak cryptogamic infection.

Interior of a ripe fig showing traces of Silba adipata McAlpine attack (immature stage) and a weak cryptogamic infection.

 

CASE 3: SILBA ADIPATA McALPINE ATTACK + MAJOR CRYPTOGAMIC INFECTION

An example of this case: a ripe second crop fig of the 'Grise de la Saint-Jean' variety showing Silba adipata McAlpine larvae exit holes.
 

Silba adipata McAlpine: ripe fig showing larvae exit holes (attack in the immature stage).

Silba adipata McAlpine: ripe fig showing larvae exit holes (attack in the immature stage).

 

Silba adipata McAlpine: a larva exit hole on a ripe fig (attacked in the immature stage).

Silba adipata McAlpine: a larva exit hole on a ripe fig (attack in the immature stage).
 

By opening the ripe fig showing larvae exit holes, we see that the interior is partially but significantly rotten, following the action of a pathogenic fungus (mold type) which has infected the infructescence weakened by the damage of Silba adipata McAlpine larvae. Due to the significant degree of mold (entire central cavity affected), larvae damage in the infructescence is no longer visible.
 

Interior of a ripe fig with Silba adipata McAlpine larvae exit holes, showing a major cryptogamic infection.

Interior of a ripe fig with Silba adipata McAlpine larvae exit holes, showing a major cryptogamic infection.
 (the attack occurred in the immature stage).
 

It is important not to confuse a ripe fig the interior of which shows larvae damage and rotting which has been added to it, with a ripe fig which only contains mold (cryptogamic attack), and which did not suffer a Silba adipata McAlpine attack. In the case where the larvae damage is no longer visible in the infructescence, in addition to larvae exit holes presence on the epidermis, traces of larvae galleries in the softened parenchyma are clues which make it possible to detect a Silba adipata McAlpine attack. As a last resort, examination under a stereomicroscope of the ostiolar scales and the ostiolar canal makes it possible to detect a Silba adipata McAlpine attack by the presence of empty egg envelopes (chorions).

 

CURIOUS OBSERVATION OF THE CIVAMBIO 66

 

Margaux ALLIX, in charge of technical support in organic arboriculture at CIVAMBIO 66 (and who is one of the agronomists part of our small team), reported to me in September 2019 a curious and unprecedented observation.This is a fig of the 'Bourjassotte Noire' variety, externally not yet ripe but approaching maturity, which shows larvae exit holes and which reveals after opening a dry infructescence of white color with a red base.
 

Interior of a 'Bourjassotte Noire' fig approaching maturity, showing a dry infructescence of white colour with red base.

Interior of a 'Bourjassotte Noire' fig approaching maturity, showing a dry infructescence of white colour with red base.
Credit: Margaux
ALLIX.
 

On close examination (see the two photographs below), I note on the small portion of visible epidermis that it is dark in color. I also notice that the ostiolar scales are not browned by internal mold.
 

Interior of a 'Bourjassotte Noire' fig approaching maturity, showing a dry infructescence of white colour with red base.

Interior of a 'Bourjassotte Noire' fig approaching maturity, showing a dry infructescence of white colour with red base.
(note that the epidermis is dark in colour, on the right, and that the ostiolar scales are not affected by mold).
 
Credit: Margaux
ALLIX.
 

And I especially observe internal symptoms of Silba adipata McAlpine attack (brown and shredded fruits ends, large gallery dug in the whitish parenchyma under the epidermis), which corroborate the larvae exit holes presence on the surface of the epidermis.
 

Internal Silba adipata McAlpine attack symptoms.

Internal Silba adipata McAlpine attack symptoms.
(note the brown fruits ends in the fig central cavity, and the larva gallery in the whitish parenchyma, at the bottom). 
Credit: Margaux
ALLIX.
 

How can we explain the presence of a dry infructescence (white in color with a red base) in a fig the external appearance of which suggests that it is approaching maturity?
 

Dry infructescence of white colour with red base of a fig approaching maturity, after a Black Fig Fly attack in the immature stage.

Dry infructescence of white colour with red base of a fig approaching maturity, after a Black Fig Fly attack in the immature stage.
(note that the reddish colour only concerns the inner wall of the parenchyma and the base of the fruits).
Credit: Margaux
ALLIX.
 

I interpret this phenomenon as follows: the fig began to ripen by becoming dark at the level of the epidermis, then the ripening continued at the level of the internal parenchyma wall and of the the fruits base, which have turned red; but the fruits could not continue to ripen, retaining their original white color on most of their surface. I think this is explained by the quantity of endogenous ethylene (synthesized following larvae damage) which was insufficient for the fruits to ripen other than at their base alone.

 

 

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