Seasonal Occurrence of Uroleucon nigrotuberculatum (Hemiptera: Aphididae) in Northern Kyushu and Mechanisms of Its Summer Disappearance.

Regulation mechanisms of insect population dynamics are important for conservation biology and insect pest management. The aphid Uroleucon nigrotuberculatum (Olive), native to North America, was introduced into Japan in the late 1980s. A previous study revealed that, in Japan, the aphid population densities increased in spring, but they suddenly disappeared from low-elevation areas in midsummer (called "summer disappearance"). However, the aphids were found continuously in a mountainous area throughout the season. Field investigations and field and laboratory experiments were conducted to clarify the mechanism of such population dynamics. Although the number of aphids increased in spring, they rapidly decreased in lowland and mountainous areas in early summer. Both top-down and bottom-up effects seemed to contribute to the rapid decline because the density of predators peaked in the growing season and the host plant height reached 60­80 cm, which suppressed the reproduction of aphids in our laboratory experiment. Then the aphids disappeared from lowland areas in midsummer when the maximum air temperature reached 35°C there. The laboratory experiment revealed that the aphid does not survive at 35°C. In addition, our field experiment in summer showed that U. nigrotuberculatum disappeared regardless of the presence or absence of an aphidophagous coccinellid, and the aphids did not disappear even on the tall plant treatment in the laboratory experiment. These results strongly suggest that high temperature in lowland areas causes the summer disappearance of U. nigrotuberculatum. Therefore, the population density of U. nigrotuberculatum is regulated by different factors between seasons and altitudes.