Role of Diadegma semiclausum (Hymenoptera: Ichneumonidae) in Controlling Plutella xylostella (Lepidoptera: Plutellidae): Cage Exclusion Experiments and Direct Observation XIN-GENG WANG 1,2 , JOHN DUFF 3 , MICHAEL A. KELLER 1 , MYRON P. ZALUCKI 4 , SHU-SHENG LIU 2 AND PETER BAILEY 1 1 School of Agriculture and Wine, University of Adelaide, SA 5064, Australia; 2 Institute of Applied Entomology, Zhejiang University, Hangzhou 310029, China; 3 Gatton Research Centre, Queensland Department of Primary Industries, QLD 4343, Australia; 4 School of Life Sciences, University of Queensland, QLD 4072, Australia (Received 27 May 2003; returned 17 October 2003; accepted 3 December 2003) We evaluated the role of the larval parasitoid, Diadegma semiclausum Helle ´n (Hymenoptera: Ichneumonidae), in controlling Plutella xylostella (L.) (Lepidoptera: Plutellidae) by cage exclusion experiments and direct field observation during the winter season in southern Queensland, Australia. The cage exclusion experiment involved uncaged, open cage and closed cage treatments. A higher percentage (54 /83%) of P. xylostella larvae on sentinel plants were lost in the uncaged treatment than the closed (4 /9%) or open cage treatments (11 /29%). Of the larvae that remained in the uncaged treatment, 72 /94% were parasitized by D. semiclausum, much higher than that in the open cage treatment (8  /37% in first trial, and 38  /63% in second trial). Direct observations showed a significant aggregation response of the field D. semiclausum populations to high host density plants in an experimental plot and to high host density plots that were artificially set-up nearto the parasitoid source fields. The degree of aggregation varied in response to habitat quality of the parasitoid source field and scales of the manipulated host patches. As a result, density-dependence in the pattern of parasitism may depend on the relative degree of aggregation of the parasitoid population at a particular scale. A high degree of aggregation seems to be necessary to generate density-dependent parasitism by D. semiclausum. Integration of the cage exclusion experiment and direct observation demonstrated the active and dominant role of this parasitoid in controlling P. xylostella in the winter season. A biologically based IPM strategy, which incorporates the use of D. semiclausum with Bt, is suggested for the management of P. xylostella in seasons or regions with a mild temperature. Correspondence to: Xin-Geng Wang, Department of Plant and Environmental Protection Sciences, University of Hawaii, 7370 Kuamoo Road, Kapaa, HI 96746, USA. Tel.:  /1-808-822-4984; Fax:  /1-808-822-2190; E-mail: xingeng@hawaii.edu Biocontrol Science and Technology (September 2004), Vol. 14, No. 6, 571  /586 ISSN 0958-3157 (print)/ISSN 1360-0478 (online)/04/060571-16 # 2004 Taylor & Francis Ltd DOI: 10.1080/09583150410001682304