THE TRANSITION TO A TRIVOLTINE LIFE-CYCLE AND MECHANISMS THAT ENFORCE THE VOLTINISM CHANGE IN HYPHANTRIA-CUNEA DRURY (LEPIDOPTERA, ARCTIIDAE) IN KOBE

The fall webworm, Hyphantria cunea Drury has a trivoltine life cycle in Kobe. This was determined by a seasonal survey for the larval head width in the field and the critical photoperiod delineated in the laboratory. H. cunea was bivoltine when it was first introduced into Japan, but the annual number of generations has increased recently in the southwestern part of the distribution. The first generation of the present Kobe population required 686 degree-days (> 10°C), as calculated from regression lines for the seasonal change in head width, which suggests that heat units required for one generation decreased from those of the original population (800 degree-days). Required heat units for the second generation, 870 degree-days, were considerably larger than those for the first one. The photoperiodic response based on individuals reared either on an artificial diet or on a host plant showed the critical photoperiods were 14 hr 23 min and 14 hr 26 min at 20°C, respectively. © 1990, JAPANESE SOCIETY OF APPLIED ENTOMOLOGY AND ZOOLOGY. All rights reserved.