McIntosh M. E., Pollination of two species of Ferocactus: interactions between cactus-specialist bees and their host plants, Functional Ecology, 2005, 19(4):727-734, doi: 10.1111/j.1365-2435.2005.00990.x
A comparison of the effectiveness of flower-visiting insects in two Ferocactus populations in Arizona. Single-visit experiments link insect behaviour with fruit formation and seed number. The paper analyses the role of solitary bees, nectar and pollen collection, and the importance of visit number. The full text describes the observation hours and the restriction to one population of each species.