CIRCADIAN-RHYTHMS OF POLLEN AND NECTAR COLLECTION BY BEES ON THE FLOWERS OF LUDWIGIA ELEGANS (ONAGRACEAE)

The visits of Tetraglossula ventralis (Hymenoptera: Colletidae) and He terosarellus sp (Hymenoptera: Andrenidae) to collect pollen and nectar on the flowers of Ludwigia elegans (Onagraceae) were visually monitor ed from November 1989 to October 1991, in two localities of the State of Sao Paulo, Brazil. These localities are approximately at the same l atitude (Mairinque 23 degrees 52'S and Campos do Jordao 22 degrees 45' S), but with a difference of 1000m in the altitude between them. As s ame latitudes ensure similar photoperiods and the difference in altitu de, a persistent step in the temperature, the field work was conducted in environmental conditions varying within a known and limited range. Circadian rhythms are described for pollen and nectar harvesting beha viours, which are finely adjusted to the flower's anthesis and witheri ng. The environmental light/dark cycle is suggested as the main zeitge ber and temperature cycle either as a secondary zeitgeber or as a mask ing agent. Since these bee species are non-social, the possibility of social synchonization was discarded. We concluded that temporal adapta tion plays a central role in the interaction of flowers and its visito rs. In addition to specialized structures to collect the pollen of L.e legans, the acrophases of pollen and nectar collection rhythms coincid e with the rime when the stigma is most receptive and consequently the possibility of pollination increases highly.