Role of Diadegma semiclausum (Hymenoptera : Ichneumonidae) in controlling Plutella xylostella (Lepidoptera : Plutellidae): Cage exclusion experiments and direct observation

We evaluated the role of the larval parasitoid, Diadegma semiclausum Hellén (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 near to 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.Xin-geng Wang; John Duff; Michael A. Keller; Myron P. Zalucki; Shu-sheng Liu; Peter Baile

Egg parasitoids of the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), from south-east Queensland

Field surveys of egg parasitoids of the diamondback moth, Plutella xylostella, were conducted at Redlands and Gatton, south-east Queensland. Eggs of P. xylostella were present all year round in both localities, and parasitized eggs were consistently found between late spring and early winter. Percent parasitism in the range 30-75% was recorded on many occasions, although rates less than 10% were more common. The major parasitoids included Trichogrammatoidea bactrae Nagaraja and Trichogramma pretiosum Riley. Laboratory evaluation showed that the T. pretiosum from Gatton has a high capacity to parasitize P. xylostella eggs under suitable conditions. This study represents the first record of egg parasitoids of P. xylostella from Australia

Parasitoid complex of diamondback moth in south-east Queensland: first records of Oomyzus sokolowskii (Hymenoptera : Eulophidae) in Australia

Surveys for parasitoids attacking larvae and pupae of Plutella xylostella L. (Lepidoptera: Plutellidae) were conducted in the Brassica agro-ecosystem of the Lockyer valley, south-east Queensland between 1999 and 2002. Parasitised hosts were collected from infested plants placed in Brassica crops during the growing season (February-October) and from the natural P. xylostella population on the common weed Rapistrum rugosum L. (Brassicaceae) during the summer production break (November-January). Five species of primary parasitoids were recorded and they ranked in order of abundance: Diadegma semiclausum Hellen (Hymenoptera: Ichneumonidae) > Diadromus collaris Gravenhorst (Hymenoptera: Ichneumonidae) = Apanteles ippeus Nixon (Hymenoptera: Braconidae) > Brachymeria phya Walker (Hymenoptera: Chalcididae) = Oomyzus sokolowskii Kurdjumov (Hymenoptera: Eulophidae). In crops, parasitoids were reared from 23 of the 30 P. xylostella cohorts studied; D. semiclausum was collected at 21 sites, O. sokolowskii at eight sites, D. collaris at six sites, A. ippeus at three sites and B. phya at a single site. The incidence and abundance of hosts parasitised by D. semiclausum increased markedly in winter and early spring months and parasitism rates as high as 95% were recorded. Diadromus collaris and O. sokolowskii were recovered at low levels throughout 2000 and 2001 but parasitism rates never exceeded 11% or 8%, respectively. Parasitism rates are compared with other mortality factors and the contribution that each species of parasitoid makes to the biological control of the pest in the region is considered. Oomyzus sokolowskii has also been reared from P. xylostella collected in Tasmania and Western Australia. This study is the first report of the incidence O. sokolowskii in Australia; however, its wide distribution indicates that its establishment is not a recent event