Biology of a new bee species that nests in sandstone, Anthophora pueblo (Hymenoptera: Apidae)

Bees nest in a wide variety of substrates, from garden soil to the ash beside volcanic craters. Trade-offs between the costs and benefits of available options theoretically drive substrate choice, but these factors are often difficult to discern. Through field and lab studies on a new sandstone-nesting species, Anthophora (Anthophoroides) pueblo, we sought to understand how and why bees use challenging nest substrates. We repeatedly found nests aggregated in softer sandstone, indicating that hardness is a limiting factor. Nesting so closely may incur additional costs, as high nest densities lead to nest-site competition and parasite attraction. However, evidence for benefits was found through rearing adults from two sandstone samples. Where most bees develop over winter and emerge the following year, A. pueblo emerged over the course of four years. As sandstone nests are more durable, they likely provide better protection against flash floods and other damage over time, making delayed emergence less costly. The parasite load of A. pueblo was also telling. Though the dominant parasite, Tricrania stansburii, inhabited nearly 50% of all cells in 1980, it accounted for <10% in the 1982 sample. As most beetles died in their cells, unable to emerge, their population seems controlled by the hardness of sandstone. Further research is necessary to confirm and quantify the costs and benefits of nesting in sandstone. Nonetheless, there are many ways in which the benefits of nesting in sandstone may outweigh the costs.