Silba adipata McAlpine

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

 

 

Female behavior on a fig tree

 

 

 

Below, personal observations (no information reported from the literature). According to the following plan: uninterrupted sequence of ovipositions, figs selection criteria with incidence on control methods implementation.

 

UNINTERRUPTED SEQUENCE OF OVIPOSITIONS

 

MAIN CHARACTERISTIC OF THE FEMALE BEHAVIOR

The main characteristic of the Black Fig Fly egg-laying female behavior on a fig tree is that it performs an uninterrupted sequence of ovipositions on the same fig tree.

According to my observations, when the female Black Fig Fly arrives on a fig tree with egg laying need, it directly lands on a fig and lays eggs under its ostiolar scales. Then it immediatly performs an uninterrupted sequence of ovipositions which can involve up to 20 figs.

The sequence duration is variable: generally from 20 mm to 1 h 30, with exceptions that are shorter or longer. It depends on the number of visited figs and on the average time spent on each fig by the female. This time may be surprisingly different from one female to another. And it usually varies for the same female from one visited fig to another.

During the sequence, the female often lays eggs in figs in which other females have previously laid eggs. And I observed that it happens that it lays eggs twice in the same fig during a same sequence.

It is very important to point out the exclusive and uninterruptible character of the ovipositions sequence.

During the sequence, the female is completly absorbed in its egg laying task. It never rests and it is not interested in any food source, even by fresh latex oozing near the fig, of which it is usually very avid. It is not disturbed either by the other black fig flies which may come on the fig near it, or brush against it while flying, what usually would have made it fly off the tree in a swirling duo.

From these observations of the female Black Fig Fly behavior, I draw the conclusion that a trap baited with a food-attractant (whatever it is) never captures a female of this species which arrives on the fig tree with egg laying need, therefore never prevents it from carrying out its harmful activity on the crop. The action of such a trap is only at the level of the overall pressure of the black fig flies population in the region, as it captures a part of the females and the males which are feeding on the fig tree.

At the end of the sequence, the female leaves the fig tree. It does not remain on it to feed. So, I am inclined to consider that a Black Fig Fly female which has performed its ovipositions sequence has only touched unripe figs during its visit on the fig tree.
 

Silba adipata McAlpine female during its uninterrupted sequence of ovipositions on the same fig tree.

Silba adipata McAlpine female during its uninterrupted sequence of ovipositions on the same fig tree.

 

Silba adipata McAlpine female during its uninterrupted sequence of ovipositions on the same fig tree.

Silba adipata McAlpine female during its uninterrupted sequence of ovipositions on the same fig tree.

 

Silba adipata McAlpine female during its uninterrupted sequence of ovipositions on the same fig tree.

Silba adipata McAlpine female during its uninterrupted sequence of ovipositions on the same fig tree.

 

OVIPOSITIONS SEQUENCES LENGTH

During the 2020 ovipositions observation campaign, I was able to establish that the female Silba adipata McAlpine can reach a number of 20 successive ovipositions on the same fig tree. But the number of successive ovipositions of the same female that I counted was most frequently 3 to 5 for the short sequences, and 9 to 13 for the long sequences.

We have to keep in mind that these are counts of observed ovipositions, which do not include any oviposition that was not seen at the beginning of the sequence or during it. When I observe an ovipositions sequence from the observation post, if I have not seen the female arrive on the fig tree (it is very rare to be able to do this), I must add 1 to 5 ovipositions for the beginning of the sequence not seen. And also add 3 to 5 ovipositions if I lose sight of the egg-laying female during the sequence to find it 10 to 20 minutes later.

Thus, the short ovipositions sequences most frequently observed (3 to 5 ovipositions) are in reality (except rare cases where I could have seen the female arrive on the fig tree) sequences which count at least 4 to 6 ovipositions, and which can count up to 8 to 10 ovipositions. And the long ovipositions sequences most frequently observed (9 to 13 ovipositions) are in reality (except rare cases where I could have seen the female arrive on the fig tree) sequences which count at least 10 to 14 ovipositions, and which can count up to 14 to 18 ovipositions. Knowing that, since these are the most frequently observed long ovipositions sequences, and not the longest sequences that can be observed, the value 18 does not constitute the maximum number of observable or real successive ovipositions of a female on the same fig tree.

To evaluate the entire sequence of ovipositions it is also necessary to take into account the ovipositions that may occur during periods when I lose sight of the egg-laying female, only to find it 10 to 20 minutes later. During this period of time, 3 to 5 ovipositions may have taken place, depending on the average time spent on each of the figs by the female (which is specific to it).

To explain the values used to pass from the number of successive ovipositions observed to the number of real ovipositions of the sequence, we must refer to my observation method. This is the sitting position 2 meters from the fig tree, in a place allowing the maximum view of the immature figs groups (only half of these, due to the volume of the fig tree, although the leaves were selectively removed), interspersed every fifteen to twenty minutes with a very slow walk around the fig tree (7 minutes), during which I devote myself to a meticulous examination of all the figs groups.

In fact, during the 2020 ovipositions observation campaign, I found myself in two cases of observing ovipositions sequences: either when I arrived at the observation post (during an immediate first walk around the fig tree), or during the observation session, once installed on the observation post.

In the first case, I probably almost always observed an ovipositions sequence in progress. My arrival may have coincided with the first oviposition of the sequence, but the probability is low. In this first case, of very low occurrence compared to the second, I cannot estimate the number of ovipositions possibly not seen since the start of the sequence. In the second case, the probability of taking an ovipositions sequence in progress is also very high, because it is very rare to be able to observe the arrival of an egg-laying female on the fig tree. But I can estimate the number of ovipositions possibly missed at the beginning of the sequence.

When I am seated, an egg-laying female may show up in my observation field after having started its ovipositions sequence in the figs groups not visible from the observation post. I can also detect it during a walk around the fig tree, but it then began its ovipositions sequence (between two successive walks on my part) in the part of the figs groups not visible from my observation post. Given the regularity of the walk around the fig tree that I compel myself to respect, and the duration separating two successive walks (fifteen to twenty minutes), I consider that I can miss at most the first 5 ovipositions of the sequence (therefore from 1 to 5 ovipositions).

It being underlined that the number of not seen ovipositions cannot be estimated for anyone observing an ovipositions sequence at random when passing in the orchard. (i.e. outside an observation post with the method explained above).
 

Silba adipata McAlpine: female visiting a fig during its ovipositions sequence.

Silba adipata McAlpine: female visiting a fig during its ovipositions sequence.

 

OVIPOSITIONS SEQUENCES DURATION

The duration of a Silba adipata McAlpine ovipositions sequence on the same fig tree depends on the number of ovipositions constituting the sequence, and on the female presence duration on each of the figs concerned by an oviposition (egg-laying time + time of non-laying presence on the fig: walking, grooming, or motionless presence, sometimes long).

In the sequence duration is also included the total duration of the "fig tests" (figs visited without laying eggs) between two figs giving rise to ovipositions: residence time on the fig (generally, from 2 to 20 seconds per tested fig). Keeping in mind that, according to my observations, between two successive ovipositions, an egg-laying female tests 1 to 5 figs without laying eggs. And the sequence duration also incorporates the time of the flights from one fig to another (visited figs and tested figs), representing a maximum of 2 minutes for a sequence of 10 ovipositions.

There are no other intermediate times to consider. During an ovipositions sequence of a female, I never observed feeding activity (even in the presence of oozing points of fresh latex), nor resting time on the underside of the leaves or on the current year wood, nor diversion of the egg-laying activity by the proximity of consuming black fig flies, posed on the tree or passing very close in flight (in the latter case, a non-laying female most often joins its congener in flight in a swirling duo...).

I have noticed that the time spent on a fig significantly varies from one egg-laying female to another, in terms of egg-laying time, non-laying time spent on the fig, or both. As a result, for an equal number of visited figs, it is not uncommon to observe an ovipositions sequence duration which varies from simple to double from one egg-laying female to another.

According to my observations, I retain 1 h to 1 h 30 as the most frequent duration for the long ovipositions sequences, and 20 to 40 min for that of the short ovipositions sequences. This is the duration of the observed ovipositions sequences, to which we must add the duration of the ovipositions at the beginning of the sequence that I may have missed (therefore the duration of 1 to 5 ovipositions, very variable, that can be estimated at 5 to 30 mm).

It being emphasized that the range of 1 to 5 not seen ovipositions can only be used for an observation from a fixed position with the rigorous method set out above; the number of not seen ovipositions and their duration cannot be estimated for anyone observing an ovipositions sequence at random when passing in the orchard. The duration of the ovipositions carried out during the periods when I lost sight of the egg-laying female during the ovipostions sequence is not to be added, as it is included in the overall timing of the sequence.

Examples of ovipositions sequences observed in June 2020 (fig bush of the uniferous variety 'Bellone'): 16 ovipositions in 1 hour 25 minutes (June 2, 12:45 p.m. to 2:10 p.m.); 12 ovipositions in 1 hour 20 minutes (June 3, 5:35 p.m. to 6:55 p.m.); 11 ovipositions in 1 hour 24 minutes (June 16, 4:36 p.m. to 6 p.m.); 13 ovipositions in 1 hour (June 13, 6 p.m. to 7 p.m.); 9 ovipositions in 26 mm (June 6, 2:40 p.m. to 3:06 p.m.); 7 ovipositions in 50 minutes (June 7, 8:50 a.m. to 9:40 a.m.); 6 ovipositions in 21 min (June 16, 12:44 p.m. to 1:05 p.m.).

Examples of ovipositions sequences observed in April 2021 (fig tree of the 'Grise de la Saint-Jean' variety): 10 ovipositions in 45 minutes (April 13, 5 p.m. to 5.45 p.m.); 7 ovipositions in 45 minutes (March 31, 2:30 p.m. to 3:15 p.m.); 6 ovipositions in 40 minutes (April 23, 10:50 a.m. to 11:30 a.m.).
 

Silba adipata McAlpine: female rubbing its ovipositor with its hind legs at the end of an oviposition.

Silba adipata McAlpine: female rubbing its ovipositor with its hind legs at the end of an oviposition.

 

FIGS TESTS

According to my observations of the ovipositions sequences, between two figs in which it successively lays eggs, the Silba adipata McAlpine female briefly visits one or more figs without laying eggs in them, seeming to test these figs. This behavior is almost systematic (only very short ovipositions sequences can be devoid of it), and the number of tested figs between two successive ovipositions varies from 1 to 5.

I was able to establish the following typology for these fig tests.

Type 1: simple rebound of the female on the top of the fig, without its landing there.

Type 2: the female lands in the ostiolar region, performs a short walk there approaching the ostiolar scales, sometimes until it touches them, but suddenly flies away after 2 to 5 seconds.

Type 3: the female lands in the ostiolar region, walks there longer, sometimes crossing the ostiole and even, in some cases, going so far as to press it abdomen against the ostiole in the laying position (but without pull out the ovipositor), before suddenly flying away. The presence time in the fig ostiolar region is then of the order of 10 to 20 seconds, sometimes more.

The following video shows a female Silba adipata McAlpine at the end of an oviposition in an immature fig, which then performs two figs tests (type 2) in 6 seconds.

 

Silba adipata McAlpine: female performing two figs tests (in 6 seconds), after an oviposition.

 

This other video below shows a female Silba adipata McAlpine performing a type 3 fig test (in 8 seconds), knowing that its actual presence time on the fig is longer because the video does not include the landing and the start of the course on it.

 

Silba adipata McAlpine: female performing a fig test, after an oviposition.

 

I do not know the reason for carrying out figs tests between two successive ovipositions. The female may be evaluating the ostiolar scales configuration, and does not retain figs with scales that do not meet the length and arrangement criteria required for oviposition. Or it may need some recovery time after the effort of expelling 1 to 5 eggs during the oviposition it comes out of, and it forgoes egg laying on the next visited figs as long as it is not yet ready for carrying out it.

It should be noted that this cannot be a question of ruling out figs that are already infested. Indeed, direct observations of ovipositions sequences, examination of the sizes of the larvae contained in a fig, and eggs study under the ostiolar scales and in the ostiolar canal, show the existence of multiple ovipositions in a same fig. According to my observations, multiple ovipositions in a same fig can occur during the same ovipositions sequence (one female), in two ovipositions sequences of the same day, or on the occasion of ovipositions sequences spaced several days apart.

 

TWO MUTUALLY EXCLUSIVE ACTIVITIES DURING A STAY ON A FIG TREE

I never observed that a female feeding on the fig tree (underside of the leaves, twigs of the year, ripe figs, latex oozing...) suddendly gives up its activity to start laying eggs. As previously indicated, I observed that an egg-laying female never interrupts for feeding activities its sequence of ovipositions on a fig tree (up to twenty visited figs...), and that it always leaves the fig tree at the end of the sequence.

Taking into account these observations, for a Black Fig Fly female, egg laying and feeding are two mutually exclusive activities during a stay on a fig tree. And I consider that the black fig flies population on a fig tree at a given time is composed of two strictly distinct categories: the "feeding flies" (females not in egg laying need, and males) and the "egg-laying flies" (females performing their ovipositions sequence), and that a black fig fly does not change category during its stay on the fig tree.

In addition, the feeding activity is permanent throughout the season, while, according to my observations, the egg laying activity is concentrated over a short period of the season, variable depending on the variety and the location of the fig tree (and starting as soon as the unripe figs reach the critical diameter for being attacked by the Black Fig Fly - 1,1 cm). After the intense attacks phase, the attacks are weak, then zero until the end of the season, even though the fig tree still bears many unripe figs of all sizes (see chapter).

In a rigorous approach, I invite you to check if my observations and my overall interpretation are transposable in the specific environment of your own orchard.

 

FIGS SELECTION CRITERIA WITH INCIDENCE ON CONTROL METHODS IMPLEMENTATION

 

I have noticed three figs choice crtiteria, guiding the egg-laying female during its stay on the fig tree, which must be taken into account in control methods implementation: exclusive attacks on figs at immature stage, fig critical diameter, and fig saving size.

 

EXCLUSIVE ATTACKS ON FIGS AT IMMATURE STAGE

According to my observations, the Black Fig Fly exclusively lays eggs in green hard immature figs. It never attacks ripe figs. I nevertheless regularly find in ripe figs larvae (alive or dead), pupae, and even desiccated remains of young imagos. These figs are infested immature figs which reach the maturity stage (and usually the overripe stage, then the desiccation phase on the tree), and, according to my observations, they represent 2 to 5 % of the global production of the fig tree, depending on the fig variety and the year.

With a method for determining the fig attack date using the exit holes appearance or the larvae size, I have always verifed for these ripe figs (in my own cases and in those that were submitted to me) that the attack occurred at the fig immature stage (often several weeks before the maturity stage).
 

Silba adipata McAlpine only attacks unripe figs.

Silba adipata McAlpine only attacks unripe figs.

 

FIG CRITICAL DIAMETER

The Black Fig Fly attacks surprisingly tiny figs. The smallest diameter that I observed (several times) for an attacked fig is 1.1 cm. I call it the critical diameter. And, according to my measurements, for certain figs varieties like 'Bellone', about 40 % of Silba adipata McAlpine attacks occur on immature figs with a diameter varying between 1.1 and 1.4 cm.

I give the fig diameter at the egg-laying date, not the one when it is possible to detect that the fig is infested. In fact, the diameter of all the infested figs continue to increase after the egg-laying, although the larvae activity inside them. According to my measurements, the attacked figs diameter increase from egg laying to abscission is 0.5 to 2 cm, depending on the egg-laying period in the year, the fig variety and, for a given variety, the figs individualy considered in the same crop.

Knowing and using the fig critical diameter is very important. For most of the control methods, it is not a question of triggering the implementation on a given date. To act effectively against Silba adipata McAlpine, it is necessary to act precisely, according to the egg-laying females behavior. It is therefore necessary to only take into consideration the diameter from which Silba adipata McAlpine can attack the fig (critical diameter: 1,1 cm). Knowing that the date of reaching the critical diameter varies according to the variety and the location of the fig tree, and, for the same fig tree, from one year to another as the result of climatic conditions.

In practice, I start the control method (for instance the unripe figs individual bagging) when the five largest figs on the tree have reached the critical diameter. Using, a Vernier caliper to exactly measure the figs diameter (the sycone is not a regular sphere, so I usually measure the largest of its diameters). Even if, with experience, the visual appreciation of the fig critical diameter can be acquired.

Note: the fig critical diameter is the triggering factor for all the control methods which directly target the Black Fig Fly egg-laying female, or which prevent it from reaching the fig or the fig ostiole (insecticides spraying, protective nets, clay barriers, deterrence by odors, figs bagging, ostioles sealing...). It is not, of course, the triggering factor for mass trapping, the implementation of which depends on the global strategy defined for this control method.
 

Silba adipata McAlpine attacks surprisingly tiny figs.

Silba adipata McAlpine attacks surprisingly tiny figs.

 

Silba adipata McAlpine attacks surprisingly tiny figs.

Silba adipata McAlpine attacks surprisingly tiny figs.

 

FIG SAVING SIZE

I observe that when a fig has reached a certain size at the immature stage, the Black Fig Fly does not attack it. The fig can be considered as saved. It exists exceptions, but they are not numerous for a given crop.

The "fig saving size" depends on the variety, and on the crop for the biferous varieties. For instance, according to my measurements, it is a 2.8 cm diameter for the 'Bellone' and 'Col de Dame Noire' uniferous varieties, and it is a 3.8 cm diameter for the breba crop of the 'Grise de la Saint-Jean' variety. 

It should be noted that the explanation of the "fig saving size" deserves further investigation. It could be that it is simply the size that the fig reaches the last days of the phase 2, or at the beginning of the phase 3, of the Black Fig Fly general attack pattern (see chapter).

 

 

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