Colletes halophilus are "solitary bees", but they nest in aggregations, which can sometimes number in the thousands.
They nest in bare soil—for example, where vegetation has been removed by land slips—in soil heaps, and even in and around the burrows of European rabbits.
The nesting sites are located at the edge of saltmarshes and may occasionally be flooded by the highest spring tides; bees have been observed emerging from recently submerged mud.
Each female digs her own burrow, creates the cells, and provisions them on her own.
Each nest consists of a short, curved burrow which terminates in a cluster of around half a dozen cells which radiate around the end of the shaft.
The males sometimes form roosts; such roosts may consist of as many as a dozen individuals resting on grass stems.

They are active from the late summer, sometimes as early as July, with the males emerging first and stay close to the emergence site where they scout for females to mate with.
When a female emerges, the males swarm towards her and attempt to mate; they may form a "mating ball" with many males surrounding a single female.
Once mated, the females begin to excavate their burrows, lining the walls of the burrows with a glandular secretion which hardens and acts as an anti-fungal defence.
Each cell is provisioned with pollen and nectar, mainly collected from the flowers of the sea aster, and the bee's flight period is timed to coincide with the flowering period of sea aster.

The late summer emergence of the adults of C. halophilus mean that this species is univoltine, and the bees overwinter in their natal cells to emerge the following summer as adults to begin the life cycle again.
The bees feed on a limited range of plants, mostly in the Asteraceae, including weld (Reseda luteola), but sea aster is especially important.
Both males and females need to feed on pollen to power their flight, and it is important that the nesting aggregations are located close to sources of food.
C. halophilus is parasitized by the cuckoo bee Epeolus variegatus which takes over the nests of the mining bee, including the food provisions, and times its emergence to coincide with C halophilus.
Another parasite is the sarcophagid fly Miltogramma punctata  which has been reared from puparia taken from nests of C. halophilus and observed flying around nest entrances.
Colletes halophilus are "solitary bees", but they nest in aggregations, which can sometimes number in the thousands. They nest in bare soil—for example, where vegetation has been removed by land slips—in soil heaps, and even in and around the burrows of European rabbits. The nesting sites are located at the edge of saltmarshes and may occasionally be flooded by the highest spring tides; bees have been observed emerging from recently submerged mud. Each female digs her own burrow, creates the cells, and provisions them on her own. Each nest consists of a short, curved burrow which terminates in a cluster of around half a dozen cells which radiate around the end of the shaft. The males sometimes form roosts; such roosts may consist of as many as a dozen individuals resting on grass stems.
They are active from the late summer, sometimes as early as July, with the males emerging first and stay close to the emergence site where they scout for females to mate with. When a female emerges, the males swarm towards her and attempt to mate; they may form a "mating ball" with many males surrounding a single female. Once mated, the females begin to excavate their burrows, lining the walls of the burrows with a glandular secretion which hardens and acts as an anti-fungal defence. Each cell is provisioned with pollen and nectar, mainly collected from the flowers of the sea aster, and the bee's flight period is timed to coincide with the flowering period of sea aster.
The late summer emergence of the adults of C. halophilus mean that this species is univoltine, and the bees overwinter in their natal cells to emerge the following summer as adults to begin the life cycle again. The bees feed on a limited range of plants, mostly in the Asteraceae, including weld (Reseda luteola), but sea aster is especially important. Both males and females need to feed on pollen to power their flight, and it is important that the nesting aggregations are located close to sources of food. C. halophilus is parasitized by the cuckoo bee Epeolus variegatus which takes over the nests of the mining bee, including the food provisions, and times its emergence to coincide with C halophilus. Another parasite is the sarcophagid fly Miltogramma punctata  which has been reared from puparia taken from nests of C. halophilus and observed flying around nest entrances.
Colletes halophilus