Silba adipata McAlpine

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Home > Infestation > Larvae inside ripe figs: presence and visibility.

 

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

 

 

Larvae inside ripe figs: presence and visibility

 

 

 

According to the following plan: presence and visibility conditions, rarity of Black Fig Fly larvae infestation inside ripe figs, larvae species identification difficulty, emergence tests.

 

PRESENCE AND VISIBILITY CONDITIONS

 

I sometimes notice the presence of Silba adipata McAlpine larvae in ripe figs of the biferous and uniferous varieties, but it is quite rare. The Silba adipata McAlpine larvae present in a ripe fig result from egg-laying which took place when the fig was in the immature stage (hard and green). Silba adipata McAlpine does not attack ripe figs. To understand the presence of Silba adipata McAlpine larvae in a ripe fig, two parameters must be taken into account: the state of maturity of the fig, and the biological durations of the Silba adipata McAlpine's life cycle.

According to my observations of control figs, between the end of immature stage (start of softening and coloring) and the reaching of commercial maturity (just ripe fig), 3 to 4 days pass depending on the variety. For example: 3 days for 'Grise de la Saint-Jean', and 4 days for 'Bellone'. For a fully ripe fig, the time elapsed since the end of immature stage increases to 4 and 5 days. And the biological durations 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. The larva measuring a little less than 1 mm at birth, and between 7 and 8 mm at the end of development, we can consider with a reasonable approximation that it grows by 1 mm per day. Let us consider as an example the case of a fig having reached commercial maturity (during summer), and examine the visibility times of the Silba adipata McAlpine larvae in the fig.

Most favorable case: the larva is visible for as many days as possible.

For a fig that reaches commercial maturity in 3 days, this occurs under the following conditions: egg-laying 1 day before the end of immature stage; larva complete development indifferently in 6 or 7 days. In this case, when the fig reaches commercial maturity, the larva is 1 day old:  4 days (3 days for evolution towards maturity + egg-laying one day before), less 3 days for egg incubation. The larva then measures 2 mm (1 mm at birth, plus 1 mm for 1 day of growth). It will leave the fig 5 to 6 days later. The fig continues to ripen in the following way: 1 day later it reaches the fully ripe stage (larva 2 days old, 3 mm), then for 2 days it will continue to ripen (very ripe fig stage, called "figue penèque" in Provence; larva aged 3 to 4 days, length 4 to 5 mm). Beyond that, the fig is clearly overripe, and is no longer edible (the larva therefore completes its development for 1 to 2 days in the overripe fig, depending on whether the development duration is 6 or 7 days). Thus, in just ripe (commercially ripe) to very ripe figs, the larva is visible for 4 days, with a size of 2 to 5 mm. For a fig that reaches commercial maturity in 4 days, in the case of the shortest oviposition time before the end of the immature stage (1 day), the larva is 2 days old (length 3 mm) when the fig is ripe. As in the previous case, it is visible in the just ripe to very ripe fig for 4 days, but it measures 1 mm more at the different stages of development (i.e. 3 to 6 mm).

Most unfavorable case: the larva is visible in the ripe fig for only 1 day.

For a fig which reaches commercial maturity in 3 days, this occurs under the following conditions: egg-laying 5 days before the end of immature stage, and larva complete development in 6 days (larva aged 5 days; length 6 mm); or, egg-laying 6 days before the end of immature stage, and larva complete development in 7 days (larva aged 6 days; length 7 mm). For a fig which reaches commercial maturity in 4 days, this occurs under the following conditions: egg-laying 4 days before the end of immature stage, and larva complete development in 6 days (larva aged 5 days; length 6 mm); or, egg-laying 5 days before the end of immature stage, larva complete development in 7 days (larva aged 6 days; length 7 mm). In these cases of presence for only one day in the fig at the maturity stage, the larva is very visible because it is large (6 to 7 mm). Note: to count the most unfavorable cases, the following formula must be applied: DM + DPAV - 3 = DDC -1 (in days; and in which DM is the time for reaching maturity after the end of immature stage, DPAV the egg-laying time before the end of immature stage, 3 the egg incubation time, DDC the time for larva complete development). To determine the age of the larva we must consider: DM + DPAV - 3 (in days). For the length of the larva (in mm), the following approximation must be used: 1 mm at birth + 1 mm per day of age (growth of 1 mm per day).

Conclusions.

It appears from the above considerations that for a larva of Silba adipata McAlpine to be present in a ripe fig, egg-laying must have taken place between 1 to 6 days before the end of immature stage. And that the larva is only visible in the ripe fig for 1 to 4 days. Being emphasized that the values of these two ranges are not applicable to all cases, but depend on the combination of the time for reaching maturity (from the end of immature stage) and the time for larva complete development.
 

Silba adipata McAlpine larva inside a ripe fig.

Silba adipata McAlpine larva inside a ripe fig.
(note the larva damage within the infructescence).

 

Silba adipata McAlpine: larva and larva damage inside a ripe fig.

Silba adipata McAlpine: larva and larva damage inside a ripe fig.

 

Silba adipata McAlpine: larva and larva damage inside a ripe fig.

Silba adipata McAlpine: larva and larva damage inside a ripe fig.

 

RARITY OF BLACK FIG FLY LARVAE INFESTATION INSIDE RIPE FIGS

 

The presence of Silba adipata McAlpine larvae in a ripe fig is a fairly rare phenomenon. This is explained by the conjunction of three factors.

Firstly, the immature figs infested by Silba adipata McAlpine which reach maturity without falling to the ground are few in number: 2 to 5% of the tree's immature figs production, according to my observations.

Secondly, according to the three-phase Silba adipata McAlpine attack pattern (see chapter), the majority (75 to 97%) of ovipositions occurs during the short phase 1 (intense attacks), which occurs several weeks before the infested immature figs eventually reach the maturity stage. Knowing that the larvae resulting from the ovipositions occurring  during the intense attacks phase leave the fig 9 to 10 days after egg-laying in summer (3 days for egg incubation + 6 to 7 days for larva complete development), or 32 days after egg-laying for breba figs (8 days for egg incubation + 24 days for larva complete development). This considerably reduces the number of attacked figs eligible for the presence of larvae.

Thirdly, among these eligible figs, only a small part will be attacked at least 6 days before the end of immature stage. As indicated in the previous subchapter, egg-laying carried out in summer more than 6 days before the e,d of immature stage does not give rise to the presence of larvae in a ripe fig (the larvae having then completed their complete development, and left the fig before it is ripe).

According to my observations, almost all of the figs attacked by Silba adipata McAlpine that I detect at the end of the immature stage (i.e. when they begin to soften and to turn yellow) show one or several larvae exit holes, which indicate that the larvae have already left the fig.
 

Fig evolving towards maturity, with Silba adipata McAlpine larvae exit holes.

Fig evolving towards maturity (softened and yellowed), with Silba adipata McAlpine larvae exit holes.
('Grise de la Saint-Jean' breba fig; June 4, 2021; egg-laying occurred at least 32 days previously).

 

LARVAE SPECIES IDENTIFICATION DIFFICULTY

 

The presence of larvae in ripe figs raises the issue of the species to which these larvae 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 end of the abdomen.

To distinguish the larva of Ceratitis capitata Wiedemann from that of Silba adipata McAlpine in a ripe fig, we can use the morphology of the posterior spiracle (visible only when examined with a stereomicroscope), as well as, in certain cases, the indications provided by the larva size. We can also use emergence tests.

 

EMERGENCE TESTS

 

Imagos emergence tests carried out from ripe figs confirm the rarity of the Silba adipata McAlpine larvae presence in ripe figs. Knowing that emergence may come from a pupa which began to form at the fig immature stage.

 

TEST OF B. I. KATSOYANNOS

B. I. KATSOYANNOS studied in 1981 and 1982 significant black fig flies populations on the island of Chios (Greece). Reference: KATSOYANNOS B. I., 1983, Field observations on the biology and behavior of the black fig fly Silba adipata McAlpine (Diptera, Lonchaeidae), and trapping experiments, Z. ang. Entomol. 95, pp. 471-476.

He reports the following experiment which shows that Silba adipata McAlpine larvae are rare in ripe figs. Between September 10 and September 30, 1981, he collected 158 ripe figs (on the tree or fallen to the ground), and counted the pupae obtained from them: a single Silba adipata McAlpine pupa, for 660 Ceratitis capitata Wiedemann pupae. During the same period, he also collected 30 immature figs and he only obtained Silba adipata McAlpine pupae (more than 50).

 

MY TEST ON  HEALTHY-LOOKING RIPE FIGS

Inspired by the experience of B. I. KATSOYANNOS, I carried out an emergence test the results of which point in the same direction. Between September 15 and 20, 2016, I picked 25 ripe figs from my fig tree of the 'Col de Dame Noire' variety, and placed them each in a plastic box closed with transparent cling film held by an elastic band (after having checked that they did not show larvae exit holes).

I then followed for a month the release of larvae, the pupations, and the births of the imagos. Just one of these ripe figs released a Silba adipata McAlpine imago. 20 of them gave birth to Ceratitis capitata Wiedemann imagos (up to 15 per fig), and 4 did not give birth to any flies (no release of larvae...).

 

MY TESTS ON RIPE FIGS CONTAINING LARVAE

The emergence test of B. I. KATSOYANNOS and the one that I carried out, as reported above, show that the release of a Silba adipata McAlpine imago from a ripe fig, therefore the presence of larvae of the species in this one, is of rare occurrence. They were carried out by blindly harvesting ripe figs, without prior detection of larvae (identified or not) in them. I came to the same observation through the emergence tests that I carry out during the season, when I detect larvae in a ripe fig.

Indeed, until now, I have only seen emergences of Ceratitis capitata Wiedemann imagos. The number of emergence tests on ripe figs containing larvae that I have carried out (around thirty) may seem low, even if these relate to a panel of several varieties ('Bourjassotte Noire', 'Bellone', 'Col de Dame Noire', in particular). But it is not surprising, given that I only carry out these tests after the detection of larvae (and not blindly), and that the Ceratitis capitata Wiedemann larvae are rarely visible in ripe figs (they are mainly detected in overripe figs, when they have reached a certain size).
 

Ripe fig in emergence box having released Ceratitis capitata Wiedemann imagos.

Ripe fig in emergence box having released Ceratitis capitata Wiedemann imagos.
(note the presence of empty pupae at the bottom of the box).

 

Ripe fig in emergence box having released Ceratitis capitata Wiedemann imagos.

Ripe fig in emergence box having released Ceratitis capitata Wiedemann imagos.

 

 

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