The pollen component of the larval food of Colletes halophilus, studied in four nature reserves in the Netherlands, was strongly dominated by Aster tripolium. The liquid component of the larval food contained high concentrations of sugars (glucose and fructose), far more than could be supplied by A. tripolium pollen present in the food. This indicates that in addition to pollen, considerable amounts of nectar were collected from this plant. The larval food showed hydrogen peroxide production. We did not, however, find this in pollen collected directly from A. tripolium flowers. We conclude that females of C. halophilus produce the enzyme glucose oxidase and add this to larval food. This is the first such finding for solitary bees. The apparent dependency of C. halophilus on A. tripolium has implications for the conservation management of this solitary bee.