Silba adipata McAlpine

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

 

 

Overwintering

 

 

 

According to the following plan: overwintering form, winter weather influence.

 

OVERWINTERING FORM

 

I have not been able to make personal observations of Silba adipata McAlpine overwintering. I found very little published information on this subject, and I report it below.
 

F. SILVESTRI

According to Fillipo SILVESTRI, the adult that appeared in late autumn of the previous year "should overwinter". His hypothesis of overwintering in the adult state (diapause) is based on the fact that he has never found larvae from December to early April, nor seen pupae remaining in the state for such a period. 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.

Without formulating it explicitly, F. SILVESTRI has certainly wondered if the Black Fig Fly does not overwinter in the mamme of caprifig trees. These are the well-developed figs that are observed in winter on the leafless branches of caprifig trees, and which shelter the larvae of the blastophage (Blastophaga psenes L.).

F. SILVESTRI insists on the fact that he has never found Black Fig Fly larvae in the mamme he examined in winter, in any condition whatsoever. He only found empty eggs envelopes, in significant numbers. He cites as examples the observations he made on December 6, 7, 9 and 29, 1916 at Portici, Resina, Lecce, Salemi and Cosenza (from 28 to 127 mamme examined per day). He also mentions the observations he made from January to March 1917 on mamme received from Algeria, Spain and Portugal, as well as on mamme from Portici.

 

N. SCHEWKET

Nihat SCHEWKET, who studied the Black Fig Fly in the early 1930s in Western Anatolia (Turkey), indicates that "overwintering takes place as a pupa in the soil, very rarely as a larva". Reference:  SCHEWKET N., 1934, Die Feigeninsekten und die wesentlichsten Ursachen der Feigenfruchtfaüle, Anzeiger für Schädlingskunde, 10 (10), 118-119. This assertion contradicts the hypothesis of overwintering in the adult state put forward by F. SILVESTRI.

N. SCHEWKET does not refer to any personal observation, and he does not provide any other element relating to overwintering in the article. But he uses an affirmative turn and the overall tenor of his article shows that he has extensively studied the Black Fig Fly in the field. Moreover, the precision relating to the overwintering in the larval form in very rare cases is unprecedented. We could therefore think that his indications result from personal observations, but, in all rigor, with the simple sentence contained in the article, this cannot be guaranteed.

 

R. PUSSARD

Roger PUSSARD, who was director of the Agricultural Zoology Station and the Insectarium of Antibes (Alpes-Maritimes department, France), specifies that "for SCHEWKET, in Turkey, overwintering would take place in the form of pupa, very rarely of larva". Reference: PUSSARD R., 1950, A propos de la mouche noire des figues Lonchaea aristella Beck., comptes rendus des séances de l'Académie d'agriculture de France, T. 36, pp. 144-145. He cites the following document as a reference: SCHEWKET N., 1934, Die Feigeninsekten und die wesentlichsten Ursachen der Feigenfruchtfaüle, Anzeiger für Schädlingskunde, 10 (10), 118-119, Berlin.

R. PUSSARD adds a personal contribution: on October 5, 1949 and January 20, 1950, he looked in vain for pupae in the first ten centimeters of soil under the fig trees in the region of Solliès-Pont (Var department, France). These had, however, been particularly affected by the Black Fig Fly the previous summer, when the loss in shipping tonnage was more than 20 % (the Solliès-Pont region was already an important figs production center with 300 hectares of mixed cultivation, and an annual shipment of 1500 tons of fresh figs).

 

J. GHESQUIERE

Jean GHESQUIERE, Belgian entomologist, declares that "the insect overwinters in the state of pupa, rarely in the larval state". And he quotes N. SCHEWKET among his sources for the evolutionary cycle, without mentioning in reference this author article. Reference: GHESQUIERE J., 1949, La mouche noire des figues Lonchaea aristella Beck. à la Côte d'Azur, comptes rendus des séances de l'Académie d'agriculture de France, T. 35, pp. 650-653.

 

EPHYTIA WEBSITE (INRAE)

I consulted the INRAE Ephytia website listing European pests, and, on the date of the last site consultation (July 11, 2023), I note that the Black Fig Fly is not referenced on it. For the Mediterranean fruit fly (Ceratitis capitata Wiedemann), this website indicates that overwintering is as a pupa, but that adults have been observed in southern Italy having successfully overwintered in late orange trees.
 

For me, on examining the above information, the question of overwintering as a pupa in the soil, or as an adult (diapause) in sheltered places, remains open for the Black Fig Fly... 
 

Black fig flies feeding on a latex ooze (fig tree twig of the year).

Black fig flies feeding on a latex ooze (fig tree twig of the year).

 

WINTER WEATHER INFLUENCE

 

LETHAL TEMPERATURE

My orchard is located in a place that is very susceptible to the Black Fig Fly, but which has not known in recent years significant variations in winter temperatures (usually mild), nor late frosts (non-existent in my region). I therefore do not have personal observations on the lethal temperature for the Black Fig Fly.

But I can report the observation of an unusually low temperature for ma region which had no influence on the Black Fig Fly activity. In January 2023, a cold snap in my garden lowered the temperature to -3.2 °C (26.24 °F) at the end of the night. This had no influence on the Black Fig Fly activity, which remained the same as in previous years (in particular intense attacks on my 'Bellone' fig tree during the first half of June).

None of my correspondents gave me indications on lethal temperature, and so far (2023) I have not read in the French or foreign specialized literature the mention of a precise, or even approximate, lethal temperature.

In France, Silba adipata McAlpine is very active in the Mediterranean region, from the Spanish border to that of Italy, in a coastal strip 30 km (18 miles) wide. But according to the observations reported by some of my correspondents, it is found in plains with a mild climate up to 150 km (90 miles) from the sea (Toulouse - Avignon line).

It is probably present in mild climate zones at much higher latitudes, with a low activity for the moment, as evidenced in July 2023 by an identification that I was led to make of eight Silba adipata McAlpine individuals trapped on a fig tree in a small town located at the latitude of more than 47° north...
 

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

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

 

WINTER CLIMATIC CONDITIONS EFFECT

According to the observations of Margaux ALLIX and Bernard PEYRE, who technically advise professional fig growers, the Black Fig Fly population variations in relation with the winter climate are the same as that observed for the other fruit flies (especially the medfly and the olive fly, with which the black fig flies population variations from one year to the next are synchronized). If the winter is mild and wet, the conditions are favorable for the appearance in the spring of large populations of black fig flies. If the winter is cold, with the presence of significant frosts, the populations appearing in the spring are reduced.

And it has been noted that the occurrence of severe late frosts considerably limits the black fig flies populations. A significant example is the very rare climatic accident that occurred in the southwest of France, which Margaux ALLIX and Bernard PEYRE reported to me. On April 20, 2021, an exceptional cold wave hit this region causing temperatures to drop from -3 to -6 °C (26.6 to 21.2 °F) in commercial fig orchards, depending on the spots. And the cold (of varying intensity) continued for two or three days. In certain orchards, all the shoots of the year froze and restarted, causing a fruiting delay of two to three weeks.

Following this climatic accident, Margaux ALLIX observed that in fig trees orchards usually moderately susceptible to the Black Fig Fly, damages due to the pest were almost zero, and in susceptible ones, they were low (< 5 %). But she noted that, in an orchard usually very susceptible to the Black Fig Fly, damages due to this pest were more important (for example, 1 to 2 figs per branch were attacked for the ‘Panachée’ and ‘Longue d’Août’ varieties). 
 

Black fig flies feeding on a cut fig tree leaf petiole.

Black fig flies feeding on a cut fig tree leaf petiole.

 

I provide in a specific chapter observations concerning the Silba adipata McAlpine activity start and end in the year (emerging from hibernation and first ovipositions, last ovipositions and last presence in my garden).

 

 

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