Antixenotic resistance of cabbage to onion thrips (Thysanoptera: Thripidae). I. Light reflectance.

Onion thrips (Thrips tabaci Lindeman) has become a significant pest of cabbage (Brasssica oleracea L.) in regions with a dry continental climate. Thrips-resistant cabbage varieties have been developed in breeding programs, but the mechanisms ofresistance remain largely unknown. Antixenosis, one of the three resistance mechanisms, may play a role but no plant trait has been identified as a source of antixenosis. A series of studies were conducted to identify resistance mechanisms in this insect- crop interaction and to seek plant traits that were correlated to resistance. In this first article of the series, the result of studying antixenosis and overall resistance of cabbage and the correlation between antixenosis and light reflectance characteristics are reported. There were distinct differences in the overall resistance to thrips between the six cabbage varieties studied. There were more pronounced differences between varieties based on the number of damaged head leaves compared with the use of damage ratings as a measure of overall resistance. Varieties also differed in their level of antixenosis; proportional abundance of thrips adults on head-forming leaves was more closely correlated to overall resistance of cabbage than actual thrips numbers. Some of the variables computed from the recorded reflectance spectra of cabbage were correlated to thrips abundance on head-forming leaves only in the first but not in the second year of this study, suggesting that either spectral characteristics do not affect antixenosis or other variables may affect thrips' responses to spectral cues. Furthermore, multiple spray applications of a kaolin particle-based product significantly changed the light reflectance characteristics of cabbage, but it did not reduce the actual thrips abundance on head-forming leaves.