Seasonal abundance and biology of hyperparasites and their hosts associated with Pieris rapae (L.) (Lepidoptera: Pieridae) in the Brassica crop system
Gaines, David N.
MetadataShow full item record
Hyperparasitism of beneficial parasites of Pieris rapae was studied on Brassica crops in Montgomery County, Virginia. The goal of the study was to determine whether the hyperparasites attacking the larval parasites of P. rapae were capable of eliminating an introduced population of the larval parasite Cotesia rubecula (Marshall) (Hymenoptera: Braconidae). Pieris rapae was found to be active from April to November with possibly six generations per year in this region. It was attacked throughout this period by five generations of the larval parasite Cotesia glomerata (L.) (Hymenoptera: Braconidae). Although P. rapae larvae were twice as numerous on broccoli (var. Premium Crop) than on cabbage (var. Rio Verde), a higher proportion of P. rapae larvae was parasitized by C. glomerata on cabbage, indicating that C. glomerata preferred to forage for P. rapae in cabbage. In spring plantings, up to 36.9% of C. glomerata cocoon masses had been affected by the hyperparasite T. galaetopus (Ratzeburg) (Hymenoptera: (Eulophidae) and 23.3% by the hyperparasite Spiloehaleis torvina Cresson (Hymenoptera: Chalcididae). In the fall, 93.2% and 4.2% of the cocoon masses were affected by T. galaetopus and S. torvina, respectively. Tetrastiehus galaetopus activity was observed from May to November, but it was most active after mid July. Tetrastiehus ga/aetopus had little impact on the gregarious C. glomerata, and even though it could affect >90% of the C. glomerata cocoon masses for extended periods, 500/0 of the cocoons in each mass were unaffected. However, it's high reproductive capacity could adversely affect a solitary host such as C. rubecula. Spilochalcis torvina was observed as early as late May but it was most active during the mid summer months. It's reproductive activity ceased by early October even though hosts were abundant in the month which followed. Four generations were seen between June and October. Spilochalcis torvina's hosts were Cotesia orobenae Forbes (Hymenoptera: Braconidae), C. glomerata, and Plutella xylostella (L.) (Lepidoptera: Plutellidae), but the principal host was C. glomerata. However, the proportion of female S. torvina progenies was <31% from the Cotesia spp. hosts and 70% from P. xylostella hosts. Spilochalcis torvina typically hyperparasitized <4 pupae per Cotesia spp. cocoon mass in the field. In laboratory experiments, S. torvina hyperparasitized an average of <7 C. rubecula pupae daily, and the maximum number hyperparasitized was 12. Due to its apparently low reproductive rate in the laboratory, and its low rate of hyperparasitization on hosts in the field, it is unlikely that S. torvina is a danger to populations of parasites like C. rubecula.
- Masters Theses