Population dispersal of parasitoid Coccobius azumai (Hymenoptera: Aphelinidae) introduced from Japan in Fujian pine forest of China

Parasitoid Coccobius azumai is an important biological agent against Hemiberlesia pitysophila had pine armored scale imported from Japan in the late 1980s and released in Fujian pine forest of China. This paper analyzed the dispersal ability, post-release parasitic ratio and the driving factors of parasitoid population using a series of observations in selected pine forests in Fujian Province of China for the period 2007~2009. The parasitoid population dispersed much slowly in the field and spread at an approximate velocity of 200 m per year. Even though jerk dispersal of the population was occasionally observed in the field experiment, generalized linear model analysis showed that a displacement dispersal trend which increased with the time of release. The interaction of forest stand and dispersal time significantly influenced population dispersal. High parasitoid release initially, ♀∶♂=7∶1 mixture and release in the upper/middle forest slopes enhanced population dispersal. Linear regression analysis revealed that C. azumai parasitic ratio on H. pitysophila adult females significantly decreased with increasing distance from parasitoid population dispersal site. Parasitic ratios were 41.8%, 34.7%, 19.8%, 11.0% and 3.8% for initial release intervals of 0, 50 m, 100 m, 150 m and 200 m, respectively. No significant effects of the number of initial release and sex-mix pretreatment were observed on parasitic ratio at 100 m site. It suggested that interaction of high pest-controlling efficiency and low dispersal capability of parasitoid population was disadvantageous to parasitoid stability and sustaining in the field. The long-term efficient control was not sustainable for present releases in small geographic ranges. An effectively considerable measure was to release parasitoids at greater number of sites with reasonable spatial intervals in large geographic ranges, and periodically do so at logical time intervals. In order to fully utilize the parasitoids, useful and detailed methods of release were suggested, involving spatial layouts of release sites, complementary releases, parasitoid collection modes, initial released number and sex-mix pretreatment.