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Author: François DROUET.
Photographs: Alain COSTA and François DROUET.
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Larvae inside ripe figs: species identification
The presence of larvae in ripe figs raises the issue of the species to which these belong: Silba adipata McAlpine or Ceratitis capitata Wiedemann (Mediterranean fruit fly). Knowing that the larvae present in ripe beba figs (biferous varieties) before the appearance of Ceratitis capitata Wiedemann in my orchard (June 30) are larvae of Silba adipata McAlpine. And that Drosophila suzukii Matsumura larvae, which I also encounter in ripe figs, are easily recognizable by the shape of the abdomen end (strong magnifying glass, stereomicroscope, close-up photographs). Chapter plan: morphological differences, chosen identification criterion, indications provided by the larva size, emergence tests.
MORPHOLOGICAL DIFFERENCES
I noticed that to the naked eye the Ceratitis capitata Wiedemann larva has an identical morphology to that of Silba adipata McAlpine, but that it is a little larger and a little thicker than the latter. However, it is almost impossible to distinguish in a ripe fig, with the naked eye or when examined with a magnifying glass, the larva of one of the two species compared to that of the other species.
Ceratitis capitata Wiedemann larva.
Silba adipata McAlpine larva.
F. SILVESTRI provides Ceratitis capitata Wiedemann larva descriptives figures on page 141 of his masterful study devoted to the Black Fig Fly. 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. I reproduce these figures below. They should be compared with those relating to Silba adipata McAlpine provided by F. SILVESTRI on pages 128 and 129 of his study, and partially integrated into the chapter describing the larva.
Ceratitis capitata Wiedemann larva descriptive figures.
Credit: F. SILVESTRI.
CHOSEN IDENTIFICATION CRITERION
By looking for the differences in the figures of F. SILVESTRI relating to the larva of one and the other species, I determined that it is the posterior spiracle which is the easiest criterion to use during the examination under the stereomicroscope. According to the representations of F. SILVESTRI, that of Ceratitis capitata Wiedemann is kidney-shaped, and only presents the three air passage slits, which are roughly parallel to each other. While that of Silba adipata McAlpine is circular, and has a dark spot in addition to the three air passage slits, one of which is perpendicular to the other two which are an extension of one another. Another clearly visible difference: the posterior spiracle of Silba adipata McAlpine is protruding (carried by a short protuberance), while that of Ceratitis capitata Wiedemann is pressed against the rear surface of the larva.
Larva posterior spiracle: Ceratitis capitata Wiedemann (two spiracles, on the left), and Silba adipata McAlpine (a single spiracle represented, on the right).
Credit: F. SILVESTRI (figure on the left, figure on the right).
My stereomicroscope examinations having confirmed the drawings provided by F. SILVESTRI for the larva posterior spiracle of the two species, I regularly use the appearance of this to identify the species to which a larva found in a ripe fig belongs, if in doubt. See examples below.
SILBA ADIPATA McALPINE LARVA
Black Fig Fly larva: posterior spiracles.
Credit: A. COSTA.
Black Fig Fly larva: posterior spiracles.
Credit: A. COSTA.
Silba adipata McAlpine larva: posterior spiracles.
Credit: A. COSTA.
Silba adipata McAlpine larva posterior spiracles.
(note: the spiracle is circular, carried by a short protuberance, and one of the respiratory slits is perpendicular to the other two).
Credit: A. COSTA.
CERATITIS CAPITATA WIEDEMANN LARVA
Ceratitis capitata Wiedemann larva posterior spiracles.
Ceratitis capitata Wiedemann larva posterior spiracles.
Ceratitis capitata Wiedemann larva posterior spiracles.
Ceratitis capitata Wiedemann larva posterior spiracles.
(note: the spiracle is kidney-shaped, not protruding, and the three respiratory slits are roughly parallel to each other).
INDICATIONS PROVIDED BY THE LARVA SIZE
The size of the larvae in relation to the stage of development of the fig constitutes in certain cases an interesting clue to determine the species they belong to. A ripe fig that you want to eat is just ripe or ripe (it is not overripe). At these two stages of the fig development, the Ceratitis capitata Wiedemann larvae are tiny. For a just ripe fig: length of 1 or 2 mm, depending on the variety; for a ripe fig: length of 2 or 3 mm, depending on the variety. Thus, if the larvae have a length of 4 to 5 mm (medium-sized larvae), or 6 to 7-8 mm long (larvae very visible to the naked eye), we can deduce that they are not larvae of this species and that they are Silba adipata McAlpine larvae.
Let us explain why the Ceratitis capitata Wiedemann larvae are tiny in a just ripe or ripe fig.
It should first be remembered that Ceratitis capitata Wiedemann can only attack figs at the end of immature stage (when the fig starts to soften and color), or later (during the evolution towards maturity, and at different stages of maturity and overripeness). It is in fact impossible for it to lay eggs in immature figs, as Silba adipata McAlpine does. Whether through the epidermis (too hard), or in the ostiole (too closed, knowing that Ceratitis capitata Wiedemann deposits its eggs in the vertical ostiolar canal and not under the horizontal scales surmounting it). Due to this limit on egg- laying and taking into account the egg incubation time (3 days), a Ceratitis capitata Wiedemann larva cannot be born earlier than 3 days after the end of immature stage.
It should also be considered that the duration of reaching the stage of maturity after the end of immature stage depends on the variety. According to my observations, if the harvested fig is just ripe, the maturity stage is reached 3 days after the end of immature stage for a 'Grise de la Saint-Jean' fig, and 4 days after this for a 'Bellone' fig. For a ripe fig, this time is 1 day longer (4 days for the biferous variety ‘Grise de la Saint-Jean’, and 5 days for the uniferous variety ‘Bellone’). Thus, in a just ripe ‘Grise de la Saint-Jean’ fig (end of immature stage + 3 days), the larva has just been born and is 1 mm in size. And in a just ripe 'Bellone' fig (end of immature stage + 4 days), the larva is 1 day old and has a size of 2 mm (1 mm: size at birth + 1 mm: increase in size in 1 day). In a ripe 'Grise de la Saint-Jean' fig (end of immature stage + 4 days), the larva is 1 day old and 2 mm long (1 mm: size at birth + 1 mm: increase in size in 1 day). And in a ripe 'Bellone' fig (end of immature stage + 5 days), the larva is 2 days old and is 3 mm long (1 mm: size at birth + 2 mm: increase in size in 2 days).
Case of larger larvae.
I better detect the Ceratitis capitata Wiedemann larvae (because they are larger) if I leave the figs on the tree beyond the usual maturity for consumption (figs approaching overripeness, called “penèques” in Provence). And I see them more often in rotten figs that have fallen to the ground.
Ceratitis capitata Wiedemann larvae in a fig at the overripe stage.
For example, if we consider a 'Bellone' fig 4 days after the ripe stage, the aforementioned larva, which measured 3 mm long at this stage, measures 7 mm long (+ 4 mm in 4 days ...). Note: to determine the length of a larva, I measure it on the graduated side of a caliper with the help of a strong magnifying glass and dissecting forceps.
Measuring a Ceratitis capitata Wiedemann larva extracted from an overrripe 'Bellone' fig.
(the larva is 7 mm long, and has exceeded the size of 3 mm which it was in the fig at the ripe stage).
EMERGENCE TESTS
The discrimination process via the examination of the larva posterior spiracles under a stereomicroscope is reliable and produces immediate results, but it is only applicable to larvae that can be detected. The emergence tests only provide results (imagos release) ten to fifteen days later, but they allow us to take into account the tiny larvae buried in the infructescence which may have escaped visual examination (for details about emergence tests, see chapter).
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