In field studies in the UK in 1990-93, the occurrence of Byturus tomentosus on the host plant Rubus idaeus was extremely variable between seasons, between days and within days, with occupancy of the available raspberry flowers (the feeding and oviposition sites) varying from 0 to 60%. This variation could not be explained by plant chemistry or food quality (leaf nitrogen, carbon or water levels, or floral nectar reward); however, beetle distributions were in part attributable to microclimatic constraints acting via the insects' physiological constraints. Initial ascent into raspberry canes from soil emergence sites was limited by the 3-fold higher water loss rates from recently eclosed young adults as compared with mature beetles. Young adults reduced their hydric stress by remaining in the humid microclimate of tightly furled primocane leaftips. Mature beetles spread upwards over the plant, but showed a preference for isolated sites (tops of canes, east or west facing according to time of day). In such sites their body temperatures could rise above the threshold for flight (requiring a Tb of 15C in laboratory studies). Flight activity was therefore common only in the early afternoon of warm days. Later in the day, beetles moved down and sometimes off the plants, starting to return at around dawn. Thus physiological constraints, even on adult beetles (relatively well-protected insect stadia), can be important components in predicting insect movements and locations on a host plant; they are likely to be even more crucial to less highly sclerotized plant-feeding adult insects and many larval herbivorous pests.