THE EFFECTS OF CROP MICROCLIMATE AND ASSOCIATED PHYSIOLOGICAL CONSTRAINTS ON THE SEASONAL AND DIURNAL DISTRIBUTION PATTERNS OF RASPBERRY BEETLE (BYTURUS-TOMENTOSUS) ON THE HOST-PLANT RUBUS-IDAEUS

1. The occurrence of raspberry beetles (Byturus tomentosus) on the hos t plant Rubus idaeus is extremely variable between seasons, between da ys and within days, with occupancy of the available raspberry flowers (the feeding and oviposition sites) varying from 0 to 60%. 2. This var iation 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 constr aints acting via the insects' physiological constraints. 3. Initial as cent into raspberry canes from soil emergence sites was limited by the three-fold higher water loss rates from recently eclosed young adult beetles as compared with mature beetles. Young adults reduced their hy gric stress by remaining in the humid microclimate of tightly furled p rimocane leaftips. 4. Mature beetles spread upwards over the plant, bu t showed a preference for insolated sites (tops of canes, east or west facing according to time of day). In such sites their body temperatur es could rise above the threshold for flight (requiring a T-b of 15 de grees C in laboratory studies). Flight activity was therefore common o nly in the early afternoon of warm days. Later in the day, beetles mov ed down and sometimes off the plants, starting to return at around daw n. 5. Thus physiological constraints, even on adult beetles (relativel y well-protected insect stadia), can be important components in predic ting insect movements and locations on a host plant; they are likely t o be even more crucial to less highly sclerotized plant-feeding adult insects and many larval herbivorous pests.