Répartition de l'entomofaune pollinisatrice sur des fleurs de colza (Brassica napus L) et de navette (Brassica campestris L) : incidence du caractère apétale de la navetteE. Brunela, J. Mesquidaa, M. Renardb and X. Tanguyb
a INRA, laboratoire de zoologie, domaine de la Motte au Vicomte, BP 29, F 35650 Le Rheu, France
b INRA, station d'amélioration des plantes, centre de recherche de Rennes, domaine de la Motte au Vicomte, BP 29, F 35650 Le Rheu, France
Abstract - Distribution of pollinating entomofauna on rapeseed (Brassica napus L) and turnip rape (Brassica campestris L) flowers: the effect of the turnip rape's apetalous character
Foraging behavior and spatial distribution of insect pollinators depend on numerous parameters such as attractivity factors (nectar, flowers, color, aroma). Morphological modifications in flowers could induce changes in attractiveness to pollinators. An apetalous turnip rape was compared to rapeseed (var Hokkaïdo and Samouraï) and also turnip rape (var Chicon) varieties with well-developed petals. Comparisons were carried out on flowering and pollinating entomofauna. A 1.5 x 8.0-m plot size was used in a randomized complete block design with 4 replicates and 4 varieties. At full flowering and at optimal insect activity, various parameters were estimated: flower density (flower number per m2), nectar production per flower (pipetting method), sugar concentration (refractometer) and pollinator insect distribution (insect number on 0.5 x 0.8 m strips for each, plant variety and for each block). The results demonstrated that: 1) flower densities on rapeseed and turnip rape were equal; 2) nectar secretion was quantitatively and qualitatively similar in turnip rape varieties but quantitatively less when compared to rapeseed; 3) the pollinators on rapeseed and turnip rape varieties were mainly composed of Hymenoptera (in particular A mellifera) and Diptera; 4) the total number of pollinator insects was the same on rapeseed and turnip rape varieties; 5) honeybee density was significantly higher (3 to 7-fold higher; table I-1) on turnip rape than on rapeseed; 6) the distribution of other Hymenoptera (ie solitary bees and bumblebees) did not differ between rapeseed and turnip rape varieties (table I-2); 7) Diptera density was significantly higher on turnip rape than on rapeseed (2 to 7-fold higher; table I-3); 8) for the same total number of insects, the entomofauna was mainly composed of Hymenoptera (honeybees) on rapeseed and Diptera (Syrphidae) on turnip rape (table II); 9) apetalous turnip rape was as attractive to pollinators as the variety with petals; the results were the same for rapeseed varieties, between-species differences were significant. In conclusion, it was found that lack of petals in turnip rape did not seem to affect the foraging behavior of local pollinating insects.
Key words: Apis mellifera / Syrphidae / foraging / behavior / apetalous plant / nectar secretion / Brassica