Abstract:

Objective: Sclerodermus alternatusi was first observed to parasitize the larva of pine sawyer beetle (PSB, Monochamus alternatus) in Yunnan Province in 2010 and has become a potential agent in current biocontrol of PSB. This study investigated their fitness gains associated with different numbers of foundress population in order to enhance the efficiency of mass-rearing of this parasitoid. Method: The parasitism behavior of foundress,the developmental process and the number of the offspring,brood size,and sex ratio were recorded under six different foundress densities (ranged from 1 to 6) using Thyestilla gebleri as host. Result: The results showed that both the first attack time on the host and the pre-oviposition time of female were declined with the increase of foundress densities. The duration of egg stage was not significantly different among different foundress densities,while the duration of both larval and pupal stages was gradually decreased as the foundress densities increased. With one foundress,the larval duration was the longest,with an average of 9.6 d,while with 6 foundresses,the larval duration was the shortest,with an average of 8.5 d. The pupal duration of males ranged from 18.7 to 19.7 d in different foundress densities,and that of females ranged from 20.0 d to 21.5 d,1-2 d longer than that of males. Brood size of the parasitoids increased significantly when the number of foundress increased from one to six. The net increments of brood size were approximate 60% as the number of foundress ranged from one to four; however,the further increases in foundress density did not affect the net brood size. The sex ratios of this bethylid wasp were always female biased. The proportions of males in the progeny colonies were less than 10% throughout all experimental treatments and showed no significant differences among treatments,but a slightly increase of sex ratio exhibited in the treatments of higher foundress densities. Conclusion: These findings suggest that moderate increase of the foundress densities could obtain more offspring in artificially mass-rearing of S. alternatusi. Meanwhile,the breeding durations of each generation can be shortened when more founderesses are inoculated to a host. Our findings provide a new approach to enhance the efficiency of mass-rearing of this bethylid wasp.

Key words: Sclerodermus alternatusi, parasitoid colonies, duration of immature stage, brood size