Abstract: Host size models assumed that the host quality for parasitoid growth and development is a linear function of host size. To test this assumption, a single-coice experiment was conducted to study the parasitism and offspring fitness performances of Lysiphlebus ambiguus developed on different nymphal instars of black bean aphis Aphis fabae. The results showed that all host instars and adults were susceptible to parasitism, but L₂ was most favored (35.25%) while adults were least attacked (14.75%). The eclosion rate of offspring wasps was significantly higher in young nymphs than in older hosts. The development duration of offspring was the shortest in L₂ (8.4 d) and the longest in adults (9.3 d), while the body size of offspring was greater in L₂-L₄ than in L₁ and adult. The female proportion in offspring wasps was significantly greater in L₄ (75.74%) than in L₁ (62.89%) and adult (65.19%), but not statistically different from that in L₂ and L₃ hosts. Based on the fitness returns to the females of parasitoid, the quality of the host aphid varied in the order of L₂>L₃=L₄>adult>L₁, indicating that host quality was not a linear function of host size for the parasitization in L. ambiguus-A. fabae system.

Key words: Alexandrium tamarense, Prorocentrum donghaiense Lu, Brachionus plicatilis