BioControl 44: 391–402, 1999. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. Effects of host stages and temperature on population parameters of Oomyzus sokolowskii, a larval-pupal parasitoid of Plutella xylostella Xin-geng WANG 1,∗ , Shu-sheng LIU 1 , Shi-jian GUO 2 and Wen-cai LIN 2 1 Department of Plant Protection, Zhejiang University, Hangzhou 310029, China; 2 Institute of Plant Protection, Zhejiang Academy of Agricultural Science, Hangzhou 310021, China; ∗ author for correspondence; Dept. of Applied and Molecular Ecology, The University of Adelaide, Waite Campus, Glen Osmond, SA 5064 Australia (e-mail: xwang@waite.adelaide.edu.au) Received 10 November 1998; accepted in revised form 14 September 1999 Abstract. Oomyzus sokolowskii is a larval-pupal parasitoid of diamondback moth, Plutella xylostella. In a host stage preference test, the parasitoid parasitised all larval and pupal stages, but exhibited a strong preference for larvae over prepupae or pupae, and did not show a preference among the larval instars. At 25 ◦ C, the developmental time, number and sex ratio of offspring per host pupa, and successful parasitism did not differ significantly among para- sitoids reared from host larvae of different instars, indicating similar host suitability between larvae of different instars. Mean developmental times from egg to adult at 20, 22.5, 25, 30, 32.5, and 35 ◦ C were 26.5, 21.0, 16.0, 12.7, 11.9 and 13.4 days, respectively. The favourable temperature range for development, survival, and reproduction of the parasitoid was 20–30 ◦ C. However, wasps that developed and emerged at a favourable temperature could parasitise effectively at 32–35 ◦ C for 24 hours. Life-fertility table studies at 20, 25, and 30 ◦ C showed that each female wasp on average parasitised 3.1, 13.2, 6.8 larvae of diamondback moth and produced 20.5, 92.1, 50.4 offspring, respectively, during her lifetime. The highest intrinsic rate of natural increase (r m ) of 0.263 female/day was reached at 30 ◦ C as a result of the short mean generation time at this temperature compared to that at 20 and 25 ◦ C, suggesting that the parasitoid had the highest potential for population growth at relatively high temperatures. Key words: host stage preference, host suitability, temperature effects Introduction Diamondback moth (DBM), Plutella xylostella (L.), is the most destructive pest on crucifer crops worldwide; the annual cost for managing this pest was estimated to be US $ 1 billion in 1992 (Talekar and Shelton, 1993). High reliance on insecticides in the last two decades has led to resistance of DBM to almost all types of insecticides, including the bacterial insecticide Bacillus thuringiensis Berliner, particularly in tropical and subtropical areas in the