Silba adipata McAlpine

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

 

 

Mass trapping

(experiment 1: transparent trap + ammonium sulfate)

 

 

 

In this chapter, I report an experiment on Silba adipata McAlpine mass trapping, carried out in 2019 in my garden (Toulon region, France, southeastern Mediterranean coast, USDA zone 9b). According to the following plan: experiment dates and conditions, figs count, catches count and distribution, analysis of catches versus attacks, comparison of the attractiveness of the transparent trap with that of a McPhail trap.

 

EXPERIMENT DATES AND CONDITIONS

 

MODALITIES

On June 13, 2019, I placed on a dense fig bush of the uniferous variety 'Bellone', for test purposes, a fully transparent wasp trap, with a holed and upturned bottom (ECOGENE brand), which I baited with ammonium sulfate. The fig bush, 20 years old, measured 3 m high and 3.5 m wide, due to a transplant at the age of 15 for which it had been cut down. It was chosen because of its very high susceptibility to Silba adipata McAlpine attacks (crop losses of 98% in previous years). The trap was suspended on the western periphery of the fig bush, 1.80 m from the ground, in an area usually frequented by Silba adipata McAlpine.
 

Silba adipata McAlpine: transparent trap with a holed and upturned bottom, in a fig bush ('Bellone' variety).

Silba adipata McAlpine: transparent trap with a holed and upturned bottom, in a fig bush ('Bellone' variety).
 

The trap was loaded with a 4% (i.e. 40 g/l) aqueous solution of powdered ammonium sulfate. Fig growers who use ammonium sulfate in commercial orchards usually dose the aqueous solution at 20 g/l, but I wanted to keep the same dosage as that which I used in my tests with diammonium phosphate, so as to compare the results of the two products with the same concentration. Ammonium sulfate is used in oenology as a yeast growth activator, but I was unable to find it in garden centers and agricultural cooperatives in my region. I had to order it online from one of the few suppliers that offers it.
 

Ammonium sulfate attractant.used in the transparent trap against the Black Fig Fly.

Ammonium sulfate attractant.used in the transparent trap against the Black Fig Fly.

 

DATES

The experiment lasted two and a half months, from June 13 to August 28, 2019.

I started the test on June 13 because that is the date on which some of the 'Bellone' figs reached 1,1 cm in diameter, which constitutes the critical size for the attack of Silba adipata McAlpine. For information, on June 1, the largest figs on the bush of the uniferous variety 'Bellone' were only 0.5 cm in diameter.

In fact, reaching critical size by figs is not the right criterion to trigger the mass trapping implementation. But I only observed the following year (especially during the 2020 ovipositions observation campaign) that food traps do not capture the egg-laying females. According to my observations (see chapter), the females complete their sequence of successive ovipositions on the same fig tree in an uninterrupted manner,  without being distracted from this task by their conspecifics, and without suspending it, even briefly, for moments of rest or feeding activities (they are not at all interested in latex oozing points, leaves underside, ripe figs, food bait traps, etc.). It would therefore have been preferable for me to set up the trap several weeks before, in order to capture as many females and males as possible coming to feed on the fig tree, with the aim of reducing the overall pressure of the population of Silba adipata McAlpine individuals on my fig trees.

The last inspection of the trap took place on August 28, when I collected the last ripe figs and ckecked that there were no immature figs left on the tree.

 

TRAP INSPECTIONS

I had not set myself any specific frequency of trap inspections, but I tried not to exceed the time limit of one week between two inspections. During each inspection, the trap was cleaned of captured individuals, and the attractant liquid was topped up or renewed.

Number of inspections carried out since the trap was set (on June 13): 18.

Inspections dates: June 21, 25, 28 and 30; July 1, 3, 5, 8, 15, 17, 19, 23 and 28; August 3, 10, 16, 23 and 28.

 

FIGS COUNT

 

On the date of August 28 (end of the experiment), the figs count was as follows.

Total figs produced by the fig bush (uniferous variety 'Bellone'): 465 (which is within the annual crop range of 450 to 500 figs observed on a regular basis).

Figs with a diameter greater than or equal to 1.1 cm having fallen before maturity (reddened but not attacked, or destroyed during other tests carried out on the fig bush): 25.

Number of figs (production) to be considered in the experiment: 440 (465 - 25).

Immature figs attacked by Silba adipata McAlpine: 334, i.e. 76% of production (334/440).

Attacked figs detected at the maturity stage: 0. Note: 3 ripe figs were detected rotten when opened, and placed in emergence boxes; they released Ceratitis capitata Wiedemann individuals.

Harvested ripe figs: 106, i.e. 24% of production (106/440).

 

CATCHES COUNT AND DISTRIBUTION

 

COUNT

On the date of the last inspection (August 28), i.e. after 77 days of trapping (trap installation on June 13), the catches count was as follows.

Black Fig Fly (Silba adipata McAlpine): 13 (9 females and 4 males).

Mediterranean fruit fly (Ceratitis capitata Wiedemann): 125 (mostly females).

Other flies and various insects (excluding insects of around one millimeter in size): 235.

Note: the test shows that ammonium sulfate is not selective (the trap captured 9 times more Mediterranean fruit flies, and 18 times more other insects, than black fig flies).
 

Silba adipata McAlpine: fully transparent trap inspection.

Silba adipata McAlpine: fully transparent trap inspection.

 

DISTRIBUTION

The catches were distributed throughout the season as follows: June 13 to July 8 (26 days): 1 female; July 9 to July 15 (7 days): 1 female; July 16 to August 3 (19 days): no capture; August 4 to 10 (7 days): 1 male; August 11 to 16 (6 days): 3 (1 female and 2 males); August 17 to 23 (7 days): 4 (3 females and 1 male); August 24 to 28 (5 days): 3 females.

We note the existence of two periods presenting a different catches level, although very low for both.

A first period of 8 weeks (from June 13 to August 10), during which only 3 individuals (2 females, 1 male) were captured, i.e. 1 catch every 20 days.

A second period of 2.5 weeks (from August 11 to 28), during which catches were regular, although remaining very low: 10 individuals (7 females, 3 males), i.e. 1 catch every 2 days.

 

ANALYSIS OF CATCHES VERSUS ATTACKS

 

On July 1st (i.e. after 19 days of trapping), I already realized that the transparent trap was really ineffective. Indeed, while the catches count was only 1 female, I had already collected on the tree 218 figs attacked by Silba adipata McAlpine. And these were only the attacked figs which could be detected on this date by the beginning of a color variation, or the presence of larvae exit holes. In fact, the analysis of the ovipositions distribution over the season showed that on July 1st the number of attacked figs was 301.

I decided not to use the transparent trap baited with ammonium sulfate in the fight against Silba adipata McAlpine, but it seemed interesting to me to continue testing it until the end of the figs season.

The study of Silba adipata McAlpine attacks over the 2019 season on the 'Bellone' fig tree showed that they obeyed the 3-phase attack pattern of Silba adipata McAlpine, for a given fig tree of a given variety (see chapter).

Phase 1 (intense attacks) took place from June 17 to 28 (12 days), with an average of 25 attacked figs per day. It concentrated 301 ovipositions, which represent 90% of the season attacks (334). Analysis of the catches distribution (see previous subchapter) shows that only one individual of Silba adipata McAlpine (a female) was captured in the period from June 13 to July 8, which encompasses phase 1 of the attacks.

Phase 2 (weak attacks) took place from June 29 to August 6 (5 weeks), with an average of 1 attacked fig per day. It concerns 33 figs, i.e. 10% of the total figs attacked in the season (334). Analysis of the catches distribution shows that during the period from July 9 to August 6, included in phase 2 of the attacks, the transparent trap only captured 2 Silba adipata McAlpine individuals (one female and one male). Note: the male captured between August 4 and 10 is considered to have been captured on August 6 at the latest; if it was captured between August 7 and 10, it increases the number of catches made during phase 3.

Phase 3 (total absence of attacks) took place from August 7 to 28 (3 weeks). Being emphasized that during the phase 3, immature figs with the critical size for the attack of the Black Fig Fly were still on the fig tree, and there was a regular presence of feeding black fig flies on it. Analysis of the catches distribution shows that during the period from August 11 to 28, included in phase 3 of the attacks, the transparent trap captured 10 Silba adipata McAlpine individuals (7 females and 3 males).

In summary, successively: 1 catch for 301 attacked figs, 2 catches for 33 attacked figs, 10 catches for 0 attacked figs.

The catches analysis covering the entire season confirms my decision of July 1st not to use the fully transparent trap baited with ammonium sulfate in the fight against Silba adipata McAlpine.

Note.

It seems important to me to remind that, as explained in a previous subchapter, the catches do not concern egg-laying females, but only females and males which come to feed on the fig tree. For example, the results above do not mean that the female captured during the intense attacks phase was an egg-laying female, taken from among thirty females which attacked the figs during this phase (I deduce from the average of 25 figs attacked per day that an average of 2 to 3 egg-laying females daily frequented the fig tree, and the phase lasted 12 days). It was a female that came to the fig tree to feed, and not to lay eggs.

The number of catches only measures the attractiveness of a trap (type of trap + bait), that can lead to a reduction in the overall pressure of the population of Silba adipata McAlpine individuals found on the fig tree, or passing near it (provided that there is no phenomenon of permanent renewal of this population). Without us being able to determine the exact correlation between the number of catches and the percentage drop in overall pest pressure, nor being assured that the latter will be sufficient to have an impact on the harvest level. Beyond the number of catches, to evaluate the effectiveness of a trap, it is necessary to determine the additional percentage of figs saved from Silba adipata McAlpine attacks that its implementation induces, compared to the harvests usually obtained in the absence of traps.

 

COMPARISON OF THE ATTRACTIVENESS WITH THAT OF A McPHAIL TRAP

 

On July 1, I placed next to the transparent trap a McPhail type trap with a yellow lower part, baited with the same attractant liquid (aqueous solution of ammonium sulfate at 40 g/l), to compare the attractiveness of one relative to the other.
 

 Fully transparent trap and McPhail trap in a fig bush, against the Black Fig Fly.

Fully transparent trap and McPhail trap in a fig bush, against the Black Fig Fly.
 

At the end of the experiment (August 28), I noted that the McPhail trap captured 79 Silba adipata McAlpine individuals (over a period of 59 days), while the fully transparent trap only captured 13 individuals (over a period of 77 days). The McPhail trap therefore allowed a number of catches 6 times greater than the transparent trap, over a setting period representing 3/4 of that of the latter.

And, by comparing the catches over the 59-day period of coexistence of the two traps, I deduced an average of 1.34 catches per day for the McPhail trap, and an average of 1 catch every five days for the fully transparent trap.

These results are clearly unfavorable for the fully transparent trap, and support the decision not to use this trap for the fight against Silba adipata McAlpine.

They also show that it is not ammonium sulfate which is responsible for the very low number of catches of Silba adipata McAlpine individuals, but the transparent trap (in terms of color and/or shape). We could also put forward the hypothesis that the very poor results of the transparent trap are due to the low height of the lid and of the upturned part of the bottom, which would allow a large part of the captured individuals to escape from the trap. But the number of Ceratitis capitata Wiedemann individuals captured in the transparent trap, 9 times greater than the number of Silba adipata McAlpine individuals, appears to contradict this hypothesis.

 

 

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