Is pollinator activity influenced by non-protein nectar aminoacids?

This study is focused on plant-pollinator relationship in an isolated population of Gentiana lutea subsp. symphyandra. G. lutea L. (Gentianaceae) is a long-lived species that mainly grows on calcareous (sub)- alpine pastures (800–2500 m a.s.l.). Flowering occurs between June and July. The study population is located on the eastern slope of Mount Grande (Northern Apennines; Bologna, Italy), between 1380 and 1460 m a.s.l. This is the only documented disjoint population occurring outside the known distribution range of the subspecies. The main visitors of the plant in this population are bees of the genus Bombus. Observations have been performed during flowering in the last 4 years; in 2011 and 2012 an unusual sluggish behaviour was observed in bumblebees feeding on plant nectar. The abundant nectar of the plant was analysed and it resulted remarkably rich in proline and β-alanine aminoacids; the hypothesis was that this composition could influences feeding choice and insect dynamism, exerting a narcotic effect on pollinators. In 2013 further investigations were conducted during two successive days and two day intervals (morning and afternoon). Nectar standing crop was recorded sampling nectar from 37 flowers open to insect visitation (12 flowering stems), and nectar production was estimated through samplings from 40 flowers protected by net (20 flowering stems). Furthermore, we analysed the aminoacidic and sugar profiles of nectar collected from i) not yet or just opened flowers with indehiscent anthers and ii) flowers at older anthesic stage with dehiscent anthers. In the same days and intervals, behavioural observations were performed on the visitors of the genus Bombus, for a total of 9 study hours. Insect movements within and among flowers and inflorescence, collected reward (nectar or pollen), and visitation time were recorded on 155 individuals; insects showing unusual behaviour were captured and identified. The results of nectar analysis shows that in 2013 samples amino acids concentration was 100 times lower than in 2011; in parallel, observations on visitors do not reveal a marked abnormal behaviour. This seems to confirm our hypothesis. On the other hand, the few collected Bombus individuals, showing the most sluggish activity, belonged mainly to the Psythirus subgenus, whose adults, especially the males, are known to perform slow movements on flowers. This suggest an alternative explanation to the observed behaviour.