Silba adipata McAlpine: McPhail trap inspection.

 

DISTRIBUTION

McPhail trap catches were distributed throughout the season as follows: July 1 to July 8 (8 days): 2 females; July 9 to July 15 (7 days): 7 (5 females and 2 males); July 16 to July 17 (2 days): 2 (1 female and 1 male); July 18 to 19 (2 days): 7 (4 females and 3 males); July 20 to 23 (4 days): 4 (1 female and 3 males); July 24 to 28 (5 days): 5 (2 females and 3 males); July 29 to August 3 (6 days): 7 (5 females and 2 males); August 4 to 10 (7 days): 5 (1 female and 4 males); August 11 to 16 (6 days): 19 (8 females and 11 males); August 17 to 23 (7 days): 14 (10 females and 4 males); August 24 to 28 (5 days): 7 (6 females and 1 male).

The analysis of the catches distribution reveals two periods with different catch levels, although low for both periods. A first period of 6 weeks (from July 1 to August 10), during which the catches were regularly distributed from one trap inspection to another and for which, at each inspection, I observed a result of 1 capture per day, except for two trap inspections for which I noted 1 capture for 4 days and 7 captures for two days. Daily average for the period: 1 catch. A second period of 2 weeks (from August 11 to 28), during which the catches were of a significantly higher volume (although remaining low), resulting in a daily average of 2 catches (double that of the first period). Considered in absolute value, the low level of catches was perhaps related to the low number of Silba adipata McAlpine individuals that I observed on the fig tree throughout the season.

 

ANALYSIS OF CATCHES VERSUS ATTACKS

 

The study of Silba adipata McAlpine attacks over the 2019 season shows that the attacks (ovipositions) on the 'Bellone' fig tree 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). The McPhail trap was placed on the fig tree after the phase 1 (on July 1st).

Phase 2 (weak attacks) took place from June 29 to August 6 (5 weeks), with an average of 1 attacked fig per day. It concerned 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 1 to August 10, which almost exactly covers the duration of phase 2 of the attacks, the McPhail trap captured 39 individuals of Silba adipata McAlpine (21 females and 18 males).

Phase 3 (total absence of attacks, while immature figs were still on the fig tree with the required size for the attack of Silba adipata McAlpine, with the daily presence of feeding black fig flies on the tree) took place from August 7 to 28 (3 weeks). Analysis of the catches distribution shows that during the period from August 11 to 28, included in phase 3 of the attacks, the McPhail trap captured 40 Silba adipata McAlpine individuals (24 females and 16 males).

In summary, successively: 39 catches for 33 attacked figs, 40 catches for 0 attacked figs. It appears from the above considerations that half of the captures were carried out during the period without any attack on figs (although immature figs having exceeded the critical size were present on the tree until August 24, with a regular presence of feeding black fig flies on the tree).

It seems important to me to specify that, according to my observations (see chapter), the catches do not concern egg-laying females, but only females and males who come to feed on the fig tree. The egg-laying females complete their sequence of successive egg-layings on the same fig tree in an uninterrupted manner, without being distracted from this task by their conspecifics, and without suspending it for moments of rest or feeding activities (latex oozing points, leaves underside, ripe figs, food bait traps, etc.). Thus, the above results do not mean that the 21 females captured during the weak attacks phase, and the 24 ones captured during the total absence of attacks, were egg-laying females. They were females who came on the fig tree to feed, and not to lay eggs.

 

EFFICIENCY EVALUATION

 

For the McPhail trap with a yellow lower part, baited with a 40 g/l aqueous solution of ammonium sulfate, the average catch over the installation period (59 days) is low:1.34 per day. It is even lower during the period of weak attacks (1 catch per day), while it increases to 2 catches per day during the period of absence of attacks.

However, it is not the daily average of catches during Silba adipata McAlpine attacks periods that determines the effectiveness of a trap. 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 this percentage will be sufficient to have an impact on the harvest level.

Beyond the number of catches, to evaluate the efficiency of a trap, it is necessary to know the additional percentage of figs preserved from Silba adipata McAlpine attacks that its implementation induces, compared to the harvests usually obtained in the absence of traps. We must therefore determine the gain in collected ripe figs that the presence of the McPhail trap baited with ammonium sulfate has generated. Knowing that the period to take into account is July 1 (intallation of the trap) to August 28 (end of the experiment).

The study of Silba adipata McAlpine attacks on the 'Bellone' fig tree during the 2019 season shows that the number of attacked figs on July 1 was 301, so that 139 immature figs were still healthy on this date (440-301). With the usual rate of crop loss on the 'Bellone' fig tree in the absence of traps (98%), the number of harvested ripe figs would have been 9 (440 x 2%). And the number of figs attacked from July 1st would have been 130 (139 -9), i.e. a crop loss rate of 94% (130/139).

The first ripe fig was harvested on August 4, and from August 4 to 28, 106 ripe figs were harvested. The last attack occurred on August 6, and from July 1 to August 6, 33 immature figs were attacked, i.e. a crop loss rate of 24% (33/139). These considerations show for the period starting July 1 a gain of 97 ripe figs (106-9) compared to the previous years, corresponding to an attacks decrease of  97 figs (130-33).

In all rigor, we have no evidence that the drop in the number of attacks is the result of the trap implementation rather than a lower activity of Silba adipata McAlpine during the 2019 season, compared to previous seasons. But, according to my feelings, the overall level of Silba adipata McAlpine activity in my garden was the same in 2019 as in previous years. Which leads us to consider that the McPhail trap was at the origin of the observed gain in harvested ripe figs. The following additionnal considerations show that it is a plausible hypothesis.

Taking into account a gain of 97 non-attacked figs, and considering an average of 10 attacked figs per egg-laying female passing on the fig tree (see chapter), we deduce a drop of 10 females in the flow of egg-laying females having visited the fig tree from July 1 to August 6. If we want to attribute this drop to the McPhail trap, it must have been the result of the catches during the same period. The latter concern 21 females, which came on the fig tree to feed. This number makes the correlation plausible, especially if we consider that a part of these captured females had probably mated before July 1st, and another part of them would have mated with males among the 18 ones captured during the same period (generating a large potential of ovipositions, even if we take into account the eggs maturation duration of 10 days).

And if, without the McPhail trap, the 97 figs would have been infested by Silba adipata McAlpine, the attacks would have been distributed over a period of 37 days (July 1 to August 6), with an average of 2,7 attacked figs per day (97/37). This average remains consistent with the attack pattern of Silba adipata McAlpine, because it does not remove the qualifier of "weak attacks phase" to the considered period, if we relate it to the average daily number of attacked figs during the intense attacks period (25).

Conclusion.

The McPhail trap with a yellow lower part, baited with an aqueous solution of 40 g/l of ammonium sulfate, is not fully effective in countering the actions of Silba adipata McAlpine, but we can reasonably attribute to it an appreciable increase of the number of non-attacked figs. We can therefore use it while waiting for a possible more effective mass trapping type control method.

For my part, in an amateur orchard (3 old fig trees, including a biferous variety, pruned to allow harvests without the use of a ladder; 4 annual crops of a total of 1500 figs), I chose to no longer use mass trapping, and to replace it with immature figs individual protection by organza bags. In my experience, this process ensures the preservation of 100% of each harvest, while requiring an implementation time that is completely acceptable for an amateur. See the chapter detailing this control method.