Numbers of pollen grains produced per floret by three host plants (Asteraceae) and utilized by the leafcutter bee Megachile wheeleri Mitchell (Megachilidae) are reported. Achillea borealis ssp. arenicola, Erigeron glaucus, and Solidago spathulata produced 1,670 ±122, 1,870 ±171, and 4,580 ±236 pollen grains per floret respectively ( ±SEM). These species dehisced pollen from 1.15 ±0.01, 41.95 ±4.35, and 4.11 ±0.19 florets per head respectively.

Megachile wheeleri foragers gathered 489,000 ±25,000 pollen grains per foraging trip and provisioned 26,300,000 ±1,240,000 grains per offspring. Based on the mean forager load, 30-77 foraging trips were required to complete the smallest and largest provisions respectively. Numbers of florets and flower heads visited per foraging trip and nest provision were estimated for each host plant based on these provisions. Potential application of pollen counting techniques to measure effects of species composition and density on components of plant and bee fitness is discussed.