Forum Entomologi Italiani

Ho visto questi giorni che centinaia di ditteri, imenotteri, blattoidei, formiche, raramente api, volano incessantemente sulle foglie di cespugli di lauroceraso, soprattutto se le foglie sono esposte al sole, senza mostrare reciproco interesse anche se si incrociano continuamente. Sembrano attirati da qualche cosa (un odore?) e almeno per alcuni vespidi noto la tendenza a palpare foglie e rametti ed a suggere o leccare velocemente soprattutto le foglie più giovani. Molte vespe rimangono immobili, come se stessero assaporando qualche cosa o fossero "stordite" da qualche sostanza.
Ho letto nel forum di raccolte di imenotteri su lauroceraso (Cerceris, Sceliphron, Psenulus...), ma mi sembra che nessuno abbia spiegato il perchè di questo comportamento, tenendo conto che le foglie di lauroceraso contengono acido cianidrico con le quali si possono uccidere anche grossi insetti. E' l'evaporazione dell'acido cianidrico che crea una azione "stupefacente" ed esercita un potente richiamo su di loro?

Hello Paolo,
I've seen a similar accumulation of insects on Ailanthus here in Bulgaria. This happens on a particular places from the middle of Sept. till the end of Oct. every year with different intensity. Ailanthus leaves contain many kinds of phenols and other bioactive oils. I didn't find data that hydrocyanic acid is present there,too.

Georgi

Thank you Georgi for your suggestion. Yes probably there is a substance, which I think is the hydrogen cyanide. Today there were hundreds of Dyptera, wasps, even few Lycaenidae and one Cassidae (on the image). The plant therefore seems very useful for an abundant collection of some species of insects.
On this site: https://www.piantedasiepe.it/piante-da- ... eraso.html
they report that some species of prunus laurocerasus "attract insects".

Dear Paolo, I get interested in the topic so I checked for more information. I think you were on the right path that the atractant is connected to the HCN production of the plant. But HCN is not contained in "free" form there. It is produced after the decomposition of the glucosides in the plant.A similar one is the glucoside amygdalin in apple seeds and other. Glucosides contain one or more saccharides and carry the CN group. After a plant is damaged glucosides are decomposed to glucose, benzaldehyde and/or acetone and HCN.
So I think the main attractant is the byproducts of production of HCN. And these are of course: glucose , benzaldehyde and acetone.

Dear Georgi, very interesting your hypothesis. I have only a doubt. The leaves in the garden are living and fresh, there is not a decomposition process.
I have searched again and have found that prunus laurocerasus (togheter to other trees) produces phytoncides. This substance in some papers is recognized to have a strong bactericidal effect (i.e.: L Potapenko et al 2021 IOP Conf. Ser.: Earth Environ. Sci. 723 022063; Li Q, Kawada T. Effect of forest environments on human natural killer (NK) activity. Int J Immunopathol Pharmacol. 2011 Jan-Mar;24(1 Suppl):39S-44S.).
Perhaps prunus laurocerasus is recognized by the insects as a therapy against their parasites. I remember that I have seen for few days a very similar inexplicable gathering in April-May on the top leaves of a young Pinus Pinea, by a single species of hymenoptera.
If I can, today I will try to cut some leaves and see if the wound on the leaves attracts them.

Please I want add new details.
Yesterday afternoon I have cut some leaves and have observed for a couple of hours if they could attract any insect. Only 1 ant and 1 diptera walked on the leaves, while nothing happened on the cut stumps of the plant. In the meantime I went to observe other laurocerasus bushes, on which dozens of insects flew, mainly hymenoptera, and I observed a Coenonympha pamphilus (Linnaeus, 1758). It was sucking long and avidly under the underside of many leaves, as if they were flowers. I was able to take some photos with my mobile phone and to film. From this film it is very clear that the butterfly was suking someting. I have observed even a bee sucking for at least 10 minutes under many leaves.
At nine o'clock, being completely into the dark (with the light of my mobile phone only), I have been to check the cut leaves, but there was nothing. Instead I have seen few diptera flying around and on the leaves. I have been very surprised when I have seen again a small butterfly sucking the underside of a leaf (it seemed Pterophorus pentadactyla or something similar). Unfortunately it was impossible to take a photo or film.

I found that Cherry laurel has extrafloral nectaries that produce nectar. There are reports it attracts bees, wasp, ants and flies.

Dear Georgi, this is a very good explanation. I will look for what other plants might have this unusual production. If I can, I will try to photograph a "sucking" butterfly at night.

Sono riuscito a caricare un breve filmato, che ha perso leggermente di qualità.
Eccolo:
https://youtu.be/zkQvT_76N24

Questo è stato ripreso alle 22, buio completo, 12° C, 600 metri, sempre su Prunus Laurocerasus. Noctuidae.
https://youtu.be/l9r-oREJjeA