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Author: François DROUET.
Photographs: François DROUET.
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Ovipositions observation campaign (2020)
According to the following plan: purposes, dates and conditions, observation device, ovipositions count, figs count, existence of an intense attacks phase.
PURPOSES
It is difficult to be able to observe a Silba adipata McAlpine oviposition. A fortiori to observe the sequence of successive ovipositions of a female on the same fig tree. In 2020, I decided to carry out a campaign to directly observe the Silba adipata McAlpine ovipositions on the fig tree.
Main objectives: determine the number of figs successively attacked by a female on the same fig tree; estimate the number of egg-laying females frequenting a fig tree (simultaneously and successively); confirm the existence of an intense attacks phase concentrated over a few days, and understand its modalities.
Secondary objectives: confirm that the fig continues to develop between the Silba adipata McAlpine attack and the fall to the ground, and measure the increase in diameter; know for the month of June the cumulative duration of the egg incubation phase and that of the complete larva development; evaluate the percentage of figs concerned by multiple ovipositions in the same fig; appreciate the time between the attack of a fig and the appearance of significant reddening of the fig.
Silba adipata McAlpine laying eggs under an immature fig ostiolar scale ('Bellone' variety).
DATES AND CONDITIONS
I started the ovipositions observation campaign on May 18 and ended it on the evening of June 26. So it lasted 40 days. It being specified that the period during which I was able to observe ovipositions was only 14 days (I observed the first oviposition on the sixteenth day of the campaign and I did not observe oviposition during the last 9 days of the campaign; two days were neutralized by rain).
Total observation hours: 125. Of which 47 hours for the period preceding the ovipositions, 64 hours for the ovipositions period, and 14 hours for the period following the ovipositions. Several hours of observation per day, all at once or split between morning, mid-day and afternoon. Variable observation hours depending on the day, between 6:15 a.m. and 9 p.m. (falling night).
I did not observe any Black Fig Fly oviposition from May 18 to June 1 inclusive, nor from June 18 to June 26 (closing date of the ovipositions observation campaign). With the exception of a single oviposition on June 19 (during an observation session that lasted 4 hours). The period during which I attended ovipositions lasted from June 2 to June 17 inclusive, i.e. 16 days. But 2 days (June 4 and 12) having been neutralized by the rain, the number of days during which I was able to observe ovipositions is 14.
In order not to make the statistics less significant, I do not take into account in the ovipositions period the day of June 19 with a single oviposition, nor the previous day (June 18), during which I did not observe any oviposition. But I include in the results of the observation campaign the oviposition of June 19.
OBSERVATION DEVICE
MPLEMENTATION
I chose as an observation field an old fig bush of the 'Bellone' uniferous variety, in my garden (Toulon region, French southeastern Mediterranean coast). This fig bush is very strongly attacked every year by Silba adipata McAlpine (crop losses of 95% ... ). Bush dimensions: 3 m in height; 4 m in diameter. Annual production: approximately 450 figs.
For facilitating the observation, I carefully removed the leaves (more or less widely) at the end of the branches.
'Bellone' variety fig bush with partial leaves removal for facilitating Black Fig Fly ovipositions observation.
Twigs at the fig bush top after leaves removal for Black Fig Fly ovipositions observation.
More precisely, I removed some or all of the leaves accompanying figs and left only one or two leaves at the very top of the twig, above the figs groups that had already formed. On the other hand, I kept the leaves located below the figs groups, except those obstructing the vision of other figs groups. Thus all the figs groups (without exceptions) were from the observations start clearly visible during a full turn of the fig bush.
Immature figs group in the center of the bush after leaves removal for Black Fiig Fly ovipositions observation.
(note the leaves kept at the bottom, and the one left at the top).
Peripheral immature figs group after leaves removal for Black Fig Fly ovipositions observation.
(note the preserved leaves on one side and on the back; a fig is marked with a label dating the oviposition).
Observation method: attentive observation of the figs groups and of the flies flights around and inside the fig bush, from the observation posts in a seated position (from each of which I could see without moving at least half of the bush). In addition, detailed inspections of the figs groups spaced a quarter of an hour apart by very slowly walking around the fig bush.
OBSERVATION DEVICE EFFICIENCY
I was able to see that the observation device is very effective. I am certain that without the drastic but thoughtful leaves removal in the fig bush, I would have observed very few ovipositions, and I would not have been able to follow any long sequence of successive ovipositions from a same female.
I think that the observation device and method could not have given rise to errors on the number of egg-laying females present on the tree, and on the detection of an egg-laying female arrival on it (immediately or at the latest within a quarter of an hour following this arrival).
The lower density of the fig bush may have made it less attractive to Silba adipata McAlpine. But I could not determine it and, in any case, the very high crop losses level reached again in 2020 (98%) shows that the fig bush has remained significantly attractive.
Silba adipata McAlpine laying eggs under an ostiolar scale of an immature fig visible from afar.
(the fly is on the highest fig, on the right; see enlargement below).
Silba adipata McAlpine laying eggs under an ostiolar scale of an immature fig visible from afar.
ABSENCE OF INFLUENCE OF THE FULL SUN CONDITIONS
I noticed that females did not hesitate to lay eggs in figs in full sun as a result of leaves removal, despite the species' preference for shade.
Silba adipata McAlpine laying eggs in an immature fig exposed to full sun as a result of leaves removal.
Silba adipata McAlpine laying eggs in an immature fig exposed to full sun as a result of leaves removal.
(note, between the legs, the ovipositor slipped under an ostiolar scale).
OVIPOSITIONS COUNT
During 14 days, I observed 181 ovipositions, realizing 60 hours of observation. But I include in the results of the observation campaign the single oviposition of June 19 (during an observation session of 4 hours). This brings the observed ovipositions number to 182, and the corresponding number of observation hours to 64.
Silba adipata McAlpine laying eggs under an immature fig ostiolar scale ('Bellone' variety).
Silba adipata McAlpine laying eggs under an immature fig ostiolar scale ('Bellone' variety).
(note the ovipositor slipped under an ostiolar scale).
FIGS COUNT
On the date of June 30, 2020, all the attacked figs have been collected, and the figs count is the following.
Fig tree total production: 434 figs (excluding physiological drop figs).
Figs attacked by Silba adipata McAlpine: 425 (98 %).
Healthy immature figs continuing to develop: 9 (2 %).
INTENSE ATTACKS PHASE EXISTENCE
The ovipositions period duration (14 days) leading to the destruction of 98% of figs production confirms the existence of an intense attacks phase in the season, concentrated over a few days (as detected in the year above, without egg-laying observation, by applying to all the attacked figs the method I have developed for dating the attack of a fig). More generally, this intense attacks phase is part of an attack pattern in three phases specific to the Black Fig Fly, reflecting a high attacks variability over the season for a given fig tree, of a given variety (see chapter).
The ovipositions observations of the 2020 campaign made it possible to identify the main characteristics of the intense attacks phase: ovipositions distibution (per days, during the day, discontinuity during the day), and egg-laying females flow on the fig tree (simultaneous presence and succession on the fig tree, egg-laying females average daily number and total number, non-increase of flies population on the fig tree). For details, see the chapter.
The description of the consecutive ovipositions sequence of a female on the same fig tree is given in a specific chapter.
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