Abstract
Psacothea hilaris (PASCOE) adults were collected in mulberry fields in Tsukuba City, Japan, in 1990 and 1991. In early summer of both years, overwintered (mainly diapaused) individuals emerged, and the same number of adults were collected as in autumn when non-diapaused individuals emerged. Adults with a broken yellow stripe on the pronotum accounted for about 50% of the males and more than 30% of the females collected in both years. These results indicate that the western ecotype which displays larval diapause and a broken-stripe pattern on the pronotum has invaded Tsukuba City, the dominant area of the eastern ecotype which displays no larval diapause and has a continuous-stripe pattern. The results also confirmed previous hypotheses that: (1) the sex ratio would decrease in early summer and rise in autumn in the hybridizing area between these two ecotypes; and (2) the prime indicator of the adaptive trends of the populations would not be the pronotum pattern but the diapausal response.
