Biological control by two exotic parasitoids: eight-year population dynamics and life tables of the arrowhead scale

To determine the process of regulation of Unaspis yanonensis (Kuwana) (Hemiptera: Diaspididae) by the two introduced parasitoids, Aphytis yanonensis DeBach et Rosen and Coccobius fulvus (Compere et Annecke) (Hymenoptera: Aphelinidae), the temporal changes in the population density of U. yanonensis as well as the parasitism rates were monitored for eight years before and after the release of the two parasitoids in a Satsuma mandarin orange (Citrus unshiu Marc. (Rutaceae)) orchard. From 2–4 years after the release, the parasitism rate by C. fulvus gradually increased, eventually reaching 70%, while that by A. yanonensis showed a weak increase, remaining under 12%. During this period, the host density decreased to about 1/100 of the initial density. After the drastic decrease, the host density remained under 1/60 of the level previous to the release for at least 2 years with the populations of both parasitoids persisting. To estimate the ability of the two parasitoids to regulate the populations of U. yanonensis, life tables of U. yanonensis under natural conditions and predator/parasitoid-exclusion (bagged) conditions were compared. It was demonstrated that C. fulvus and A. yanonensis impose about 70% mortality rate on the host at mature adult stages and about 30% on the host at immature adult stages. The results have strongly confirmed the high capability of the two parasitoids as biological control agents for U. yanonensis, which was suggested by earlier studies. However, contrary to those studies, the present study did not support the complementarity of the two parasitoids in regulating the host population, suggesting that the effectiveness of C. fulvus alone in regulating the host population at low levels.