Integrated management of diamondback moth using native parasitoids and commercial Bt productsύ

The diamondback moth, Plutella xylostella (L.) (Lepidoptera, Plutellidae), is the most important pest of crucifers worldwide. This insect has shown resistance to all insecticide groups, including some subspecies of Bacillus thuringiensis (Bt). Nowadays, world-scale integrated management of P. xylostella has been developed based on the more sustainable technologies, such as using hymenopteran parasitoids and pathogens. Due to recent outbreaks of P. xylostella at common cabbage and cauliflower fields in Isfahan province (central Iran), and the failure of synthetic insecticides for control of P. xylostella the combination of the native parasitoids and effective formulations of Bt can be useful as the components of integrated and sustainable management of P. xylostella. The present study aimed to identify the native parasitoids of P. xylostella, to determine natural percentage parasitism of P. xylostella in field, to examine the interactions between the parasitoids and Bt, and to perform integrating parasitoids and Bt against P. xylostella in field. Nine parasitoid species were determined as follows: (a) the larval parasitoids: (1) Cotesia vestalis (Haliday) (Hymenoptera, Braconidae), (2) Diadegma semiclausum (Hellen) (Hymenoptera, Ichneumonidae), (3) Oomyzus sokolowskii (Kurdjumov) (Hymenoptera, Eulophidae), (4) Bracon hebetor Say (Hymenoptera, Braconidae), and (5) Dolichogenidea appellator (Telenga) (Hymenoptera, Braconidae); (b) the prepupal and pupal parasitoids: (1) Diadromus subtilicornis (Gravenhorst) (Hymenoptera, Ichneumonidae), and (2) Diadromus collaris (Gravenhorst) (Hymenoptera, Ichneumonidae); and the hyperparasitoids: (1) Mokrzekia obscura Graham (Hymenoptera, Pteromalidae), and (2) Pteromalus sp. (Hymenoptera, Pteromalidae). The species D. appellator, D. collaris and M. obscura are new records for Iran. The predominant parasitoids were C. vestalis, D. semiclausum and O. sokolowskii with the relative abundance of 0.432, 0.421 and 0.118, respectively. The mean percentage parasitism of 34.7 and 38.3 were estimated for Falavarjan and Mobarakeh counties, respectively. The species C. vestalis, D. semiclausum and O. sokolowskii were found to be the most effective parasitoids of P. xylostella in Isfahan province, with a parasitism of 21.0, 12.9 and 3.5 per cent, respectively. The LC50 of BiolarvύΦ on P. xylostella larvae fed on Chinese cabbage was significantly greater than that on red common cabbage. The LC50 of BiolepύΦ, however, did not show any significant difference between the P. xylostella larvae fed on the different tested host plants. Both the formulations (BiolarvύΦ and BiolepύΦ) showed the highest LC50 on Chinese cabbage (2.310ύΓ-ύ4 gr/ml and 4.2010ύΓ-ύ4 ml/ml) and the lowest ones on red common cabbage (1.9710ύΓ-ύ7 gr/ml and 4.3510ύΓ-ύ6 ml/ml). When the susceptible population (Roth population) was tested, there was a significantly different percentage parasitism by C. vestalis between treatments. The proportion of Cry1Ac-treated hosts parasitized by C. vestalis was significantly greater than for Btk-treated hosts (0.862 vs. 0.667). Similarly, with the resistant population (SERD4 population) a significant difference in percentage parasitism between the treatments was found. The hosts treated with Btk pre-parasitism revealed significantly higher level of parasitism compared with the hosts treated with Btk post-parasitism (0.897 vs. 0.650). The present study established that besides the overuse of synthetic insecticides against P. xylostella, there was a high checking capability of larval parasitoids, which must be ecologically supported. The findings of combining Bt and C. vestalis are promising for the application of biological insecticides in field. Further studies on combination of the parasitoid and Bt for control of P. xylostella in field are in progress.