Effect of plant characteristics on host plant selection and larval performance of specialist insect pests on Brassicaceae

Substantial economic yield losses are regularly caused by insect pests to crops of oilseed rape (Brassica napus L.) in Europe. The control of these pests is usually achieved by application of synthetic insecticides. Emerging resistances of insects against specific active ingredients require integrated pest management strategies. The aim of this study was to identify cultivars of oilseed rape and other brassicaceous species which are less susceptibility or even resistant to herbivore feeding. Moreover, the suitability of turnip rape as a trap crop for insect pests in oilseed rape was investigated. Experiments were conducted under controlled climate conditions to assess the larval performance and adult feeding preference of the cabbage stem flea beetle (Psylliodes chrysocephala L.; CSFB) on different brassicaceous species. To identify potential resistance mechanisms, leaf laminas were analyzed for sugars, phenolics and glucosinolates and petioles were analyzed for glucosinolates. Additionally, trichome densities and dry weight of leaf laminas was assessed. Extensive field plot experiments were established to evaluate the suitability of different turnip rape (Brassica rapa L. silvestris) cultivars as trap crop. The effect of turnip rape, grown as perimeter strips or in mixtures with oilseed rape, on infestation by CSFB, rape stem weevil (Ceutorhynchus napi Gyll.), cabbage stem weevil (Ceutorhynchus pallidactylus Marsh.), pollen beetle (Meligethes aeneus Fab.) and cabbage seed weevil (Ceutorhynchus obstrictus Marsh.) on oilseed rape was investigated. Larval performance of CSFB significantly differed between brassicaceous species. While larvae reared in turnip rape gained the highest weight, larvae developing on white mustard (Sinapis alba L.) gained lowest. Larval performance was also significantly reduced when larvae fed on white cabbage (Brassica oleracea L. var. capitata f. alba) and oilseed radish (Raphanus sativa L. var. oleiformis) compared to oilseed rape. Performance of larvae was positively correlated with the content of the glucosinolates progoitrin and 4-hydroxybrassicin in the leave petioles. In dual choice experiments adult CSFB preferred turnip rape to oilseed rape for feeding, while white cabbage was less preferred. Feeding preference was not correlated with the content of glucosinolates, sugars and penolics in the leaf lamina while the feeding preference was positively correlated with the trichome density. In field experiments turnip rape was significantly higher infested by CSFB and pollen beetle than oilseed rape. Among four cultivars of turnip rape ‘Malwira' was preferred by pollen beetles. However, higher attractiveness of the turnip rape in perimeter strips and in mixtures with oilseed rape did not reduce the infestation levels of the main crop by pests. The antixenosis resistance of white mustard to CSFB larvae is regarded as having potential for breeding oilseed rape cultivars less susceptible to insect pests. Both trap cropping strategies did not reduce pest infestation of the main crop and are not suitable for use as a single pest management strategy. A combination of a turnip rape perimeter strips and targeted insecticide application to these strips may contribute to an effective trap crop strategy. Furthermore a less susceptible cultivar of oilseed rape as main crop in combination with a highly attractive trap crop could contribute to a better efficacy of the perimeter strips for insect pest management.