Oligolectism and phenological shift between host plant and pollinator: study of two psammophilic spring species, Colletes cunicularius (L.) (Hymenoptera, Colletidae) and Andrena vaga (Panzer) (Hymenoptera, Andrenidae). - Colletes cunicularius (L.) and Andrena vaga (Panzer), both vernal solitary ground nesting bees, have been reported to feed strictly on Salix (Salicaceae) (oligolectic behavior). However a recent study has shown that Colletes cunicularius is able to shift on different plants, at least in the studied sites in Germany. In this study we investigated the extent to which this behavior applies to other regions within the distribution range of these bee species. We analysed the host-plant association of two populations in Belgium. Pollen loads and brood cells of both species have been investigated by optical microscopic analysis to estimate the proportion of willow pollen. Andrena vaga females collect pollen only on Salix but pollen loads of Colletes cunicularius sometimes contain other pollen types in agreement with a previously study. It is observed that the first nest cells made by C. cunicularius in early spring contain exclusively willow pollen. Towards the end of willow blooming, Colletes cunicularius females change their floral choice by foraging on other pollen types.