Bulletin of Entomological Research (2000) 90, 221–231 221 Seasonal abundance of the parasitoid complex associated with the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae) in Hangzhou, China Shu-sheng Liu 1* , Xin-geng Wang 1 , Shi-jian Guo 2 , Jun-hua He 1 and Zu-hua Shi 1 1 Department of Plant Protection, Zhejiang University, Hangzhou 310029: 2 Institute of Plant Protection, Zhejiang Academy of Agricultural Sciences, Hangzhou 310004, China Abstract An investigation of insect parasitoids of the diamondback moth, Plutella xylostella (Linnaeus), in brassica vegetable crops in the suburbs of Hangzhou was conducted during five periods from 1989 to 1997. Eight species of primary parasitoids were recorded: Trichogramma chilonis Ishii, Cotesia plutellae Kurdjumov, Microplitis sp., Oomyzus sokolowskii Kurdjumov, Diadromus collaris (Gravenhorst), Itoplectis naranyae (Ashmead), Exochus sp. and Brachymeria excarinata Gahan. Seven species of hyperparasitoids were also collected. Rates of parasitism of eggs of P. xylostella were usually very low. However, rates of parasitism of larvae and pupae were substantial and showed two peaks each year, around June–July and September–November respectively. Rates of parasitism during peaks were usually 10–60% and reached over 80% on a few occasions. Cotesia plutellae, O. sokolowskii and D. collaris were the major larval, larval-pupal and pupal parasitoids respectively. In the field, C. plutellae was active throughout the year. Oomyzus sokolowskii was active from May to October, entered a quiescent pupal stage in October–November to overwinter and did not emerge until next April–May. Diadromus collaris was recorded from April to July and October. Rates of parasitism of P. xylostella in radish and mustard fields were usually higher than those in cabbage and Chinese cabbage fields in the same locality. Negative correlations of parasitism rates between C. plutellae and O. sokolowskii indicate a competitive relationship for host larvae between these two larval parasitoids. Introduction The diamondback moth, Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae), was a minor pest of brassica vegetable crops in China before 1960. Its pest status has increased rapidly since the early 1960s when large scale application of chemical insecticides was started in vegetable crops. During the last 30 years, this insect has become a major pest of brassica vegetable crops, especially in the Changjiang River Valley and Southern China (Shi & Liu, 1995; Zhao et al., 1996; Liu & Yan, 1998). Similar to the situations observed in Southeast Asia and North America (Shelton & Wyman, 1992; Sun, 1992; Talekar & Shelton, 1993; Tabasknik, 1994; Iqbal et al., 1996), the populations of P. xylostella in China have developed resistance to all groups of insecticides, including insect growth regulators and Bacillus thuringiensis Berliner (Tang et al., 1992; Sun et al., 1995; Feng et al., 1996; Zhao et al., 1996). Development and implementation of integrated pest management (IPM) systems are now considered to be the only solution to combat this highly resistant insect pest (Liu et al., 1995; Zhao et al., 1996; Liu &Yan, 1998). During the past 20 years, development and implementation of biological control-based IPM has made * Fax: +86 571 6049815 E-mail: shshliu@zju.edu.cn