Silba adipata McAlpine

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

 

 

Intense attacks phase characteristics

 

 

 

According to the following plan: existence of an intense attacks phase in the season, ovipositions distribution, egg-laying females flow on the fig tree, characteristics synthesis.

 

EXISTENCE OF AN INTENSE ATTACKS PHASE IN THE SEASON

 

In 2020, I decided to carry out an ovipositions observation campaign on a fig bush of the 'Bellone' uniferous variety. Main objectives: determine the number of figs successively attacked by a female in the same fig tree, evaluate the number of egg-laying females simultaneously or successively present on the fig tree, study the ovipositions distribution (several secondary objectives were also defined) . For details about the 2020 campaign (purposes, observation device, dates, conditions, ovipositions count, figs count), see the specific chapter.

The ovipositions period duration (14 days) leading to the destruction of 98% of figs production of the concerned fig tree (425 figs out of 434) confirms the existence of a Black Fig Fly 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 variability of the attacks over the season for a given fig tree, of a given variety (see chapter).

The 2020 ovipositions observation campaign made it possible to identify the intense attacks phase main characteristics, that I report in this chapter. 
 

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).

 

OVIPOSITIONS DISTRIBUTION

 

OVIPOSITIONS DISTRIBUTION PER DAYS

Below, the ovipositions distribution per days, the corresponding observation hours number, contiguous or not, and the average ovipositions number per observation hour.

June 2 (16 ovipositions for 6 observation hours; average: 2.66 ovipositions per observation hour); June 3 (31 for 10 h; 3.1); June 4 (rain);

June 5 (3 for 3 h 30; 0,86); June 6 (29 for 4 h; 7.25); June 7 (13 for 5 h; 2.60)

June 8 (10 for 3 h 30; 2.86); June 9  (8 for 4 h 30; 1.77); June 10 (8 for 3 h 30; 1.86);

June 11 (4 for 2 h; 2); June 12 (rain); June 13 (28 for 3 h; 9.33); June 14 (2 for 2 h 30; 0.80);

June 15 (5 for 2 h 30; 2); June 16 (19 for 6 h 30; 3.02); June 17 (5 for 4 h; 1.25);

June 19 (outside period; 1 oviposition for 4 h; 0.25).

These observations results show that the black fig flies attacked the figs each day of the intense attacks period (14 days). And that, from one day to another, the attack was of an unequal intensity.

 

OVIPOSITIONS DISTRIBUTION DURING THE DAY

For the analysis of the ovipositions distribution during the day, I consider five time slots: early morning (6:15 a.m. to 8 a.m.), morning (8 a.m. to 12 p.m.), midday (12 p.m. to 2 p.m.), afternoon (2 p.m. to 6 p.m.), and end of the day (6 p.m. to 9 p.m.). It being specified that the five time slots did not give rise to the same volume of observation hours.

The ovipositions distribution according to the five time slots is as follows.

6:15 a.m. to 8 a.m: 0 oviposition (0%); for 4 hours of observation (6%) spread over 3 days

8 a.m. to 12 p.m: 34 ovipositions (19%); for 13 hours of observation (21%) spread over 10 days.

12 p.m. to 2 p.m: 37 ovipositions (20%); for 11 hours of observation (17%) spread over 8 days.

2 p.m. to 6 p.m: 68 ovipositions (37%); for 20 hours of observation (31%) spread over 13 days.

6 p.m. to 9 p.m: 43 ovipositions (24%); for 16 hours of observation (25%) spread over 13 days.

That is 182 ovipositions for 64 hours of observation.

The average number of ovipositions during one observation hour is 2.8 (182 / 64) for the entire ovipositions period (14 days, and taking into account an oviposition outside the period).

Average numbers of ovipositions during one hour of observation.for each of the time slots (see below).

6:15 a.m. to 8 a.m.: 0.

8 a.m. to 12 p.m.: 2.6.

12 p.m. to 2 p.m.: 3.4.

2 p.m. to 6 p.m.: 3.4.

6 p.m. to 9 p.m.: 2.7.

The above results show that Black Fig Fly ovipositions have no particular occurrence periods during the day. They are randomly distributed in all the time slots of the day, with an average number of ovipositions per observation hour varying from 2.6 to 3.4. This a great difference with the feeding activity of the species which is almost exclusively observed during two periods of the day (beginning of the morning and end of the afternoon). To make this difference even more concrete, I point out that 20% of the ovipositions were observed during the short slot between noon and 2 p.m., with an average number of ovipositions per observation hour of 3.4. Note: I exclude from the analysis the early morning slot (6:15 a.m. to 8 a.m.), for which the number of observation hours is insufficient (4 hours, 6%). 
 

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).

 

OVIPOSITIONS DISCONTINUITY DURING THE DAY

Considering the average numbers of ovipositions during one hour of observation for certain axes of analysis can distort the view of the reality of what daily happened during the ovipositions period. In fact, according to my observations, every day of the ovipositions period, the fig bush remained for hours without any egg-laying female. Even if it was daily frequented by at least one egg-laying female.

This reflects a clearly marked discontinuity in egg-laying activity of the Black Fig Fly on the fig tree during the day. In a general context of regular attacks (ovipositions) each day of the week, during the ovipositions period.
 

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).
(the left hind leg is placed on the scale, the withish beginning of the ovipositor is slipped under the scale).

 

EGG-LAYING FEMALES FLOW ON THE FIG TREE

 

The observation of the egg-laying females flow on a fig tree is only possible under demanding conditions: fig tree leaves severely (but selectively) removed, continuous presence during several hours on the observation post each of the days of the intense attacks period, and, in addition to careful observation from the post, slow walk aroud the fig tree every quarter of an hour with a meticulous inspection of the figs groups.
 

Silba adipata McAlpine: egg-laying female flying towards an immature fig ('Bellone' variety).

Silba adipata McAlpine: egg-laying female flying towards an immature fig ('Bellone' variety).

 

Silba adipata McAlpine laying eggs under an immature fig ostioloar scale ('Bellone

Silba adipata McAlpine laying eggs under an immature fig ostioloar scale ('Bellone' variety).

 

LOW OCCURRENCE OF EGG-LAYING FEMALES SIMULTANEOUS PRESENCE

Among the 14 days during which I witnessed ovipositions, I observed during 10 days a single egg-laying female present at a time on the fig tree (regardless of the times and the number of observations hours during the day ). I only observed during 4 days the simultaneous presence on the fig tree of several egg-laying females: June 6 (2 females, then 4 females in a different time slot), and June 3, 7 and 13 (2 females).

On the contrary, I observed almost every day from 3 to 6 consuming black fig flies simultaneously present on the fig tree, independently of the presence or not of an egg-laying female on this one.

These observations show that the number of black fig flies simultaneously present on the fig tree is not an indicator of overall nuisance. The mutually exclusive character of the females' activities (nutrition versus sequence of successive ovipositions) reinforces the lack of correlation between the number of back fig flies present on the fig tree during the egg-laying period and the height of the crop losses observed.

 

EGG-LAYING FEMALES SUCCESSION ON THE FIG TREE

The observation of the presence of a single egg-laying female on the fig tree does not mean that it is the only one that visits it during the day. I have observed that several egg-laying females follow one another on the fig tree during the day.

Indeed, during the same day my observations were often distributed between the morning and the end of the afternoon. The observations of a single egg-laying female were then spaced several hours apart, wheras the longest ovipositions sequences duration usually does not exceed 1 h 30.

Example : on June 10, I divided my observations into 3 time slots spaced 2.30 h to 3 h apart (10:30 a.m. - 11:30 a.m.; 2:30 p.m. - 4 p.m.; 6:30 p.m. - 7:30 p.m.). I observed ovipositions during each of the time slots: 3 ovipositions between 10:36 am and 10:53 am; 3 ovipositions between 2:50 p.m. and 3:35 p.m.; 2 ovipositions between 6:35 p.m. and 6:44 p.m. In all three cases, it was a single female (no presence of other egg-laying females on the fig tree during the observation time slot).

This example shows that 3 single egg-laying females followed one another on the fig tree during my observations of the day.
 

Black Fig Fly female laying eggs under an ostiolar scale of an unripe fig.

Black Fig Fly female laying eggs under an ostiolar scale of an unripe fig.

 

EGG-LAYING FEMALES AVERAGE DAILY NUMBER

First of all, it is important to keep in mind that egg-laying females have a high nuisance individual capacity on a same fig tree: a same female carries out an uninterrupted sequence of successive ovipositions involving up to twenty figs, free of any other activity (notably nutrition). The nuisance of this ovipositions sequence being slightly reduced due to ovipositions in already infested figs (for details, see the specific chapter).

I could not determine by my observations the precise number of egg-laying females succeeding each other on the fig tree in one day.

A realistic estimate of the average number of egg-laying females having daily frequented (during the ovipositions period) the fig tree on which the observations took place can be made from the attacked figs total number. Total number of figs attacked: 425. Ovipositions period: 14 days. Average daily number of attacked figs: 425 / 14 = 30.

If we consider the longest successive ovipositions sequence of a female on the same fig tree (20 figs - which is not rare), and subtracting about 10% to take into account the ovipositions carried out in an already infested fig, we obtain a maximum nuisance capacity for a female of 18 figs. Average number of egg-laying females having daily frequented the fig tree: 30 / 18 = 1.7, rounded up to 2.

This illustrates the few egg-laying females that would be enough to destroy the entire crop, but the reality of the egg-laying females daily flow on the fig tree is rather a mix of females performing short sequences (most frequently observed: from 3 to 5 ovipositions) and long sequences (most frequently observed: from 9 to 13 ovipositions).

To determine the real length of the observed successive ovipositions sequences, it is necessary to add an estimate of the number of ovipositions not seen at the beginning of the sequence (which is almost always taken in progress). This estimate is a range of 1 to 5 ovipositions, if we use the rigorous observation method followed during the 2020 ovipositions observation campaign (for the method, see the chapter "Female behavior on a fig tree"). Note that this estimate has no meaning for anyone observing a successive ovipositions sequence at random when passing through the orchard.

If we consider the most frequently observed short sequences of successive ovipositions by a same female, the average ovipositions real number in a sequence of a female is:  (4 +  6) / 2 + (8 + 10) / 2 = 14 / 2 = 7 ovipositions. And for observed long ovipositions sequences, it is: (10 + 14) / 2 + (14 + 18) / 2 = 28 /2 = 14 ovipositions. Thus for a mix of short and long ovipositions sequences, it is (7 +14 ) / 2 = 11. If we subtract about 10%  to take into account the ovipositions carried out in a fig already infested, the average nuisance capacity for a mix of most frequently observed short and long ovipositions sequences is 10 figs.

Average number of egg-laying females having daily frequented the fig tree: 30 / 10 = 3.

The result of this calculation is consistent with my general feeling of what I have observed on the fig tree, and of the egg-laying time related to the observations sessions amplitudes and positioning during the day.

 

EGG-LAYING FEMALES TOTAL NUMBER

Given the average daily number of egg-laying females (3), and the ovipositions period duration (14 days), the total number of egg-laying females having visited the fig tree is: 3 x 14 = 42.

If we assume that an egg-laying female visits the fig tree only once during the egg-laying period, there are 42 different egg-laying females. But, among the egg-laying females having frequented the fig tree during the ovipositions period, given its duration, it is very likely that some made at least two visits. The number of different egg-laying females having frequented the fig tree would then be less than 42, in a proportion that my observations do not allow to estimate. This number could be further reduced if it is established that an egg-laying female can visit a fig tree more than once in the same day, which I do not know.
 

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).

 

NON-INCREASE OF FLIES POPULATION DURING THE INTENSE ATTACKS

During the intense attacks phase in the season, which, in the studied case, was concentrated over two weeks and resulted in the loss of almost the entire crop, one would have expected a rain of egg-laying females on the fig tree. But, in fact, there was not a visible increase of the black fig flies population on this one. And I permanently observed more feeding females on the fig tree than egg-laying females.

Knowing that the number of black fig flies that frequent a fig tree to feed is low. I regularly observe 1 to 3 black fig flies on my fig trees, whatever their variety (dimensions of the considered fig trees: 4 m high by 4 m wide). At certain periods in the season, and depending on the time of day, the number of black fig flies can reach 4 or 5. Exceptionally, I observed 6 individuals of Silba adipata McAlpine on the same fig tree, once only 7 individuals, and another time 8 individuals.

The paradoxical observation of a non-increase of the fig tree black fig flies population during the intense attacks period is explained by the nature of the egg-laying females flow. As exposed in the above sub-chapters, during this period, there was generally only one egg-laying female at a time on the fig tree. And, altough the egg-laying activity was regular (egg-laying females could be observed each of the days for the considered fig tree), it was discontinuous during the day (observation of time slots of up to several hours in the day without any egg-laying female on the fig tree). And it was only an average number of 3 egg-laying females that daily passed over the fig tree to destroy the whole figs crop during the egg-laying period.

 

CHARACTERISTICS SYNTHESIS

 

According to the observations reported above, the intense attacks phase of Silba adipata McAlpine on a given fig tree has the following characteristics.

It is concentrated over a few days (14 in the studied case). The attacks (ovipositions) are regular during the ovipositions period (ovipositions counted every day), but of unequal intensity from one day to another. During the day, they are randomly distributed in all the time slots (no particular occurrence period), and they are discontinuous (observation of time slots lasting up to several hours without the presence of egg-laying females on the fig tree).

The destruction of almost the entire crop results from the action of an average daily number of 3 egg-laying females, present on the fig tree most often only one at a time.

The intense attacks phase does not result in an increase in the number of black fig flies usually present on the fig tree. And the number of non egg-laying females present on the tree (indulging exclusively in feeding activities) is permanently much higher than that of egg-laying females (indulging exclusively in egg-laying activity).

Additional observations from the 2020 campaign, reported in the other chapters of the "Infestation" section of this site, allow us to add the following two notable characteristics.

The beginning of the intense attacks phase is conditioned by the reaching of the critical diameter (1.1 cm) by the immature figs, and this phase therefore begins on a variable date from one fig tree variety  to another, and from one year to another for the same variety.

Egg-laying females have a significant individual nuisance capacity: uninterrupted sequence of successive ovipositions on the same fig tree involving up to twenty figs, free of any other activity (notably nutrition). The nuisance of this ovipositions sequence being slightly reduced due to egg-laying in already infested figs.

Note: I do not know if the observations presented in this synthesis can be transposed to fig trees commercial orchards. In all rigor, I think it is not certain.
 

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).

 

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).

 

 

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