Effect of Brassica genotype on the infestation by cabbage stem weevil Ceutorhynchus pallidactylus (Mrsh.) (Col.: Curculionidae) and the parasitism of stem weevil larvae

Abstract

The search for plant resistance to insect pests in oilseed rape (Brassica napus L. var. oleifera Metzg.) is still in an early stage of development. In this study, a wide spectrum of Brassicaceae representing a broad genetic variability (B. napus and B. rapa cultivars, breeding lines, resynthesized rapeseed lines, other Brassica spp.) was evaluated for resistance characteristics to cabbage stem weevil (Ceutorhynchus pallidactylus) for the first time. Two new methods were developed for the screening of host plant quality of a large assortment under controlled conditions in climate chambers to allow a rapid and reliable selection of genotypes with partial or full resistance. For the quantification of leaf feeding by adult beetles, 106 genotypes were tested in no-choice screening tests for their susceptibility. In comparison to the standard cultivar Express , the average leaf area consumed by C. pallidactylus was reduced significantly by more than 40% on ten oilseed rape cultivars, four resynthesized rapeseed lines and five other accessions. In dual-choice screening tests for the evaluation of oviposition preferences, female C. pallidactylus laid significantly fewer eggs into plants of two oilseed rape cultivars, five resynthesized rapeseed lines and eight accessions of B. oleracea and B. fruticulosa, respectively, than into plants of the standard cultivar Express . Within three years, field experiments were carried out to evaluate the susceptibility of 42 Brassica genotypes to infestation by C. pallidactylus under field conditions. A significant correlation was found between the results obtained by laboratory screening tests and by field experiments, thereby confirming the reliability of the new laboratory tests for predicting the susceptibility of Brassica genotypes to C. pallidactylus under field conditions. In a semi-field trial, no significant relationship was found between the total leaf glucosinolate content and the intensity of attack by C. pallidactylus in 12 genotypes tested. However, the stem in jury coefficient was significantly correlated with single glucosinolate compounds, particularly aromatic glucosinolate gluconasturtiin and the indolic glucosinolates glucobrassicin, 4-methoxybrassicin and 4-hydroxybrassicin. The larval parasitism of C. pallidactylus and C. napi by Tersilochus obscurator and T. fulvipes (Hym.: Ichneumonidae), respectively, was found to be significantly different between host plant genotypes. In three cultivars of turnip rape, B. rapa, the level of parasitism of C. napi by T. fulvipes was significantly reduced compared to cultivars of winter oilseed rape, B. napus. The level of parasitism was also significantly affected by host plant architecture.