Patterns of feeding by Japanese beetles, Popillia japonica Newman, were studied in relation to time of day or night and to ambient temperature (T_a). We also measured beetles’ thoracic temperatures (T_th) at rest, during preflight warm up, and during flight, and observed their within-plant distributions in relation to sunlight or shade to gain insight regarding whether their in-field behavior involves physiological and behavioral thermoregulation. Finally, we studied startle response as affected by T_a and time of morning. Japanese beetles fed most extensively within trees from midmorning until late afternoon, but also substantially in evening (1800–2200 hours) and sparingly throughout the night so long as T_a was >15°C. Beetles in the field were capable of endothermy, maintaining elevated T_th independent of basking. At low to moderate T_a (19–32°C), nonflying beetles in shade or direct sunlight had T_th as much as 3.75 or 5.5°C higher, respectively, than T_a. Thoracic temperature was elevated during flight, ranging from 32 to 34°C when T_a was 26–29°C. The minimum T_th for take-off flight was ≈27°C. Despite their reputation as sun-loving insects, a high proportion of Japanese beetles was found in shaded microsites (i.e., shade patches, abaxial leaf surfaces), regardless of time of day. The proportion of beetles in sunlight tended to be highest in morning, although that pattern was more evident in soybeans than in linden trees. As T_a warmed from dawn through late morning, the beetles’ response to disturbance changed from falling at T_a <23°C, to about equal numbers falling or flying at 25°C, to all flying at >25°C. Implications of these findings for management of P. japonica are discussed.