Seasonal population changes of five parasitoids attacking the scale insect Nipponaclerda biwakoensis on the common reed, with special reference to predation by wintering birds

Authors


Shuji Kaneko, Shizuoka Prefectural Agricultural Experiment Station, 678-1 Tomigaoka, Iwata, 438-0803 Japan. Email: kaneshu@ja2.so-net.ne.jp

Abstract

Seasonal changes in the abundance of five species of hymenopterous parasitoids (four species of Encyrtidae and one species of Eulophidae) attacking the scale insect Nipponaclerda biwakoensis on the common reed were investigated for 2 years in Lake Biwa, with special reference to predation by the reed bunting, Emberiza schoeniclus, during winter. The scales settled on reed shoot stems under sheath leaves, passing through three discrete generations per year. The abundance of adult female scales increased exponentially from July (first generation) to December (third generation). Adult female scales of the third generation overwintered on reed shoots. During winter, female scale abundance dramatically declined, whereas the number of predation marks made by reed buntings using their bills on reed sheath leaves increased. The generations of all five parasitoids were synchronized with the host scale generations, and the five parasitoids overwintered as larvae inside the scale bodies. The abundance of parasitized scales and parasitoid adults emerging from the scales also increased from July to December, but greatly decreased during winter. The overall parasitism rate of the female scales remained at relatively low levels (less than 40%) throughout the year, including before and after winter. A bird exclusion experiment revealed that the dramatic winter decrease of the abundance of the scale and its five parasitoids was due to intensive and non-selective predation by the buntings on unparasitized and parasitized scales. Additionally, the proportion of immature parasitoids removed by birds varied between the five parasitoid species. Thus, seasonal population changes of the five scale parasitoids are considerably affected by bird predation on overwintering immature parasitoids.

Ancillary