Silba adipata McAlpine

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Home > Infestation > Multiple ovipositions in the same fig (Observing the egg-laying females).

 

Author: François DROUET.
Photographs / videos: F. DROUET
and Bernard PEYRE.
All rights reserved.

 

 

Multiple ovipositions in the same fig

 

(Observing the egg-laying females)

 

 

 

I have highlighted the existence of multiple Black Fig Fly ovipositions in a same fig by three means: direct observation of the egg-laying females on the fig trees (this chapter), examination of the larvae contained in the figs (see chapter), study of the eggs deposited under the ostiolar scales and/or in the ostiolar canal (see chapter).

 

OBSERVING THE EGG-LAYING FEMALES

 

2020 OBSERVATIONS CAMPAIGN

In June 2020, I carried out an observation campaign of egg-laying Silba adipata McAlpine females on a fig bush (uniferous variety 'Bellone') partially stripped beforehand, which allowed me to observe 182 ovipositions.

During this campaign, I was able to note that during its successive ovipositions sequence on the same fig tree (up to twenty figs, see chapter), the female lays eggs in figs in which other females have previously laid eggs . And it also happens that a female lays eggs in a fig in which it has already itself laid eggs a few minutes before, during the same successive ovipositions sequence.
 

Silba adipata McAlpine: egg-laying under an unripe fig ostiolar scale.

Silba adipata McAlpine: egg-laying under an unripe fig ostiolar scale.
 

I recorded over time on my daily observation sheets the occurrences of multiple ovipositions in the same fig, using four processes to detect them.

First process: I easily counted multiple ovipositions in figs labeled with an oviposition number. In fact, to study the increase in the immature fig diameter between egg-laying and abscission,  I was able to label a control batch of about forty attacked figs (after having located them by breaking the petiole of the leaf closest, immediately after oviposition).

Second process: during the successive ovipositions sequence of certain females, I was able to spot figs in which the female had already laid eggs during the same sequence.

Third process: I detected multiple ovipositions in figs located at positions which had become familiar to me from the observation posts (after several hours of observation...), and for which I remembered from a previous oviposition.

Fourth process : for certain immature figs which were still entirely green, I was able to identify the previous egg-laying infestation by noticing that the ostiole in which the female was laying eggs had the characteristic appearance of an ostiole attacked by ants having detected Black Fig Fly eggs presence. It is an ostiole with a whitish zone in the center, consequence of the reddish surface scales shredding by ants.

Taking into account traces of purplish red color on the immature fig for which I observed an egg-laying, which would have indicated that the fig was already infested, was not a possible process for detecting multiple ovipositions in the same fig. Indeed, I have noticed that Silba adipata McAlpine does not attacks suchs figs, perhaps because they do not appear to it to be unripe figs. And, anyway, the first traces of purplish red color on immature figs infested by Silba adipataMcAlpine (that begin to appear, for the crop of an uniferous variety, at least 10 days after the first oviposition in the fig) was really perceptible only after the intense attacks period (phase 1 of the Black Fig Fly general attacks pattern, see chapter).

It is important to note that multiple ovipositions in the same fig constitute an attenuation factor of the Black Fig Fly female individual nuisance: the number of new figs lost for the crop during the female successive ovipositions sequence on the same fig tree is equal to the total number of ovipositions minus the number of ovipositions on figs already infested.
 

Black Fig Fly female laying eggs in a fig ostiole attacked by ants.

Black Fig Fly female laying eggs in a fig ostiole attacked by ants.
(note the whitish color of the ostiole center, which indicates that ants have shredded the uppermost ostiolar scales).

 

EXCEPTIONAL OBSERVATION

Bernard PEYRE, former fig producer and consultant-trainer for commercial fig cultivation, reported to me of an exceptional observation testifying to Silba adipata McAlpine multiple ovipositions in the same fig.

On September 22, 2020, around 7 p.m., on one of his potted fig trees: two females landed simultaneously on the same immature fig, and began to lay eggs together in its ostiole. The two simultaneous ovipositions lasted about 4 minutes, and the two females were each able to carry out their repetitive sequences of comings and goings above the ostiole and egg deposits.
 

Silba adipata McAlpine : two females simultaneously laying eggs in the ostiole of a same fig.

Silba adipata McAlpine: two females simultaneously laying eggs in the ostiole of a same fig.
 Credit: Bernard PEYRE.
 

To my knowledge, on the date of communication of the information by Bernard PEYRE (September 22, 2020), this is an unprecedented observation.

For my part, while observing Silba adipata McAlpine egg-laying, I only saw (once) an individual of the species land for 2 seconds near a female laying eggs in the ostiole, then fly away. And, only once also, an individual of the species to make a rebound on the side (in the upper part) of a fig in the ostiole of which a female was laying eggs (by "rebound", I mean: arrival in rapid flight, touch of the legs on the surface of the fig without landing, and immediate removal).

Illustration video.

 

Silba adipata McAlpine: two females simultaneously laying eggs in the ostiole of a same fig.
 Credit: Bernard PEYRE.

 

Video analysis.

The analysis of the video (duration equivalent to a third of the time counted for the simultaneous ovipositions) makes it possible to appreciate more finely the two females behavior. To facilitate the analysis, it is better to observe separately (therefore successively) the behavior of each of the females.

When the video starts, the bottom female is laying eggs in the ostiole, while the top one is outside the ostiole, on the edge of the fig top.

Top female behavior: two egg-laying sequences.

First sequence. The female goes towards the ostiole, crosses it and undertakes a fairly long oviposition under an ostiolar scale. Shortly before the oviposition end, the female moves slightly to the right. It then frees its ovipositor from the ostiole and moves away slightly (end of the first egg-laying sequence). The female then engages in rubbing its hind legs together for a short time.

Second sequence. The female carries out an almost complete reversal on itself (nearly 180°) on its left, and initiates a crossing of the ostiole by placing itself behind the other female, under the wings, as if to push it. It bypasses it to the right to complete its crossing of the ostiole and begin oviposition. During oviposition, it engages in repelling the other female using its second and third legs on the left side (the other female doing the same on its right side). The female then gradually moves to the right without the ovipositor leaving the ostiole. Then it frees its ovipositor from the ostiole (end of the second egg-laying sequence), and moves away slightly. The female then engages in rubbing the hind legs and wings (in progress when the video stops).

Bottom female behavior.

After a few pushes to complete the egg deposit in progress when the video begins, the female moves away from the ostiole by pivoting to its left and moves away slightly. It then carries out a complete reversal on itself (180°) on its right, goes around the other female from behind, crosses the ostiole and undertakes egg-laying. During egg-laying, it pivots slightly to its left without the ovipositor coming out of the ostiole. Then, while continuing egg-laying, it repels the other female by spreading its wings horizontally, or with the help of its legs on the right side. Egg-laying lasts a long time and is still in progress as the video ends, with the female then having its wings open at 45° to prevent the other female from approaching.

Remarks.

The decomposition of a female Silba adipata McAlpine oviposition into several repetitive sequences is usual (see chapter). And it is also common for two Silba adipata McAlpine individuals in close proximity to each other to engage in mutual pushback.

 

PERCENTAGE OF MULTIPLE OVIPOSITIONS IN THE SAME FIG

 

PERCENTAGE DEDUCED FROM THE EGG-LAYING FEMALES OBSERVATION

According to the overall analysis of my observation records (2020 observation campaign), the proportion of ovipositions in figs already infested would be 15% (1 fig out of 6). But I realized a few weeks later the campaign that I had made a confusion in my observations. I had considered figs with the top reddened by the sun as being figs with the top beginning to redden consecutively to a Silba adipata McAlpine attack. I did not think, during the detection of multiple ovipositions in a same fig, of the significant stripping I had practiced around the immature figs groups to facilitate observation, which allowed an abnormally high insolation, causing a slight reddening of the figs tops. I therefore think that the percentage of 15% cannot be retained for the exact count of the cases of multiple ovipositions in a fig.

But this value of 15% gives us the indication of the ceiling value of the real percentage. Even if this is less satisfactory, it can be noted that for large direct egg-laying females observation (182 ovipositions), the percentage of multiple ovipositions in a fig is less than 15%. Thus, the observations did not make it possible to determine in how many infested figs on average a female lays eggs during a sequence of 20 successive ovipositions on the same fig tree, but they allow us to establish that this average does not exceed 3 figs. Let us insist on the fact that 15% does not constitute an order of magnitude, but a ceiling value that the real percentage of multiple ovipositions in a fig cannot exceed.

 

CEILING VALUE TO BE RETAINED FOR THE PERCENTAGE

The larvae examination led us to a ceiling value of 10% for the percentage of Black Fig Fly multiple ovipositions in a same fig (see chapter)..

And there is no interest in trying to determine a ceiling value for this percentage from eggs study, because it would be of high uncertainty (for figs containing 2 to 5 eggs, we do not know how to attribute eggs distributed under two or three ostiolar scales to one or more females).

So we only consider the ceiling values for the percentage of multiple ovipositions in a fig obtained by directly observing the egg-laying females, and by examining the larvae contained in the fig. And, of these two, as these are maximum values that cannot be exceeded, we retain the lowest, i.e. that obtained by examining the larvae: 10%.

It is important to point out that this value is not an order of magnitude, nor the maximum of a range resulting from counts during observations. It is simply a ceiling value that the percentage of multiple ovipositions in a fig will not exceed when we will have quantified observation elements to apprehend it more precisely.
 

Silba adipata McAlpine: egg-laying under an unripe fig ostiolar scale.

Silba adipata McAlpine: egg-laying under an unripe fig ostiolar scale.

 

Silba adipata McAlpine: egg-laying under an unripe fig ostiolar scale.

Silba adipata McAlpine: egg-laying under an unripe fig ostiolar scale.

 

 

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