Abundances of five parasitoids attacking the scale insect <Emphasis Type="Italic">Nipponaclerda biwakoensis</Emphasis> on morphologically changed reed shoots due to damage by a stem-boring caterpillar

Abundances of the scale insect Nipponaclerda biwakoensis and its five parasitoids per shoot of the common reed, Phragmites australis, were compared between shoots damaged by a stem-boring caterpillar and undamaged shoots. Reed shoots that were damaged by the stem-borer in spring change morphologically during summer, inducing tillers from the nodes beneath the damaged part. The number of female scales per shoot did not differ significantly between damaged and undamaged shoots in the second scale generation (September), but was significantly lower on damaged shoots in the third generation (November). Three parasitoid species attacking the scale exhibited different responses to the shoot damage, with the response by each parasitoid being constant in the two scale generations: the parasitism rate by Aprostocetus sp. per shoot was higher on damaged shoots, whereas that by Astymachus japonicus was lower on damaged shoots, and no difference was detected for Boucekiella depressa. In the third scale generation, the parasitism rate by Encyrtidae sp. 1 showed no difference, with respect to shoot damage, whereas that by Encyrtidae sp. 2 was lower on damaged shoots. In three dominant parasitoids, shoot damage had no effect on the number of emerging adults per host, and the sex ratio and body size of the adults. The number of emerging adults per shoot differed significantly between damaged and undamaged shoots for four parasitoids, except B. depressa. These results suggest that shoot damage by the stem-borer exerts a delayed negative impact on the scale numbers and affects the parasitism rate of the scales by three parasitoids and the emerging adult numbers of four parasitoids.