DAMAGE TO OILSEED RAPE SEEDLINGS BY THE FIELD SLUG DEROCERAS-RETICULATUM IN RELATION TO GLUCOSINOLATE CONCENTRATION

Seedlings of nine commercial cultivars of oilseed rape were exposed to the field slug Deroceras reticulatum immediately after sowing in compost in trays. There was a small reduction in seedling numbers in the presence of slugs which was not related to glucosinolate concentration in seeds or seedlings. However, the number and leaf‐area of seedlings with damage symptoms were strongly and inversely related to the total concentration of glucosinolates in seeds and one wk‐old seedlings. The presence of barley seedlings as alternative food did not significantly affect this relationship. The glucosinolate concentration of seeds was closely correlated with that of wk‐old seedlings. Analysis of individual glucosinolates in four cultivars spanning the range of concentrations found, showed that the concentration of most components declined as total glucosinolate concentration decreased. However, 2‐phenyl ethyl‐glucosinolate (gluconasturtiin) concentration tended to increase in seeds and 3‐indolyl methyl‐glucosinolate (glucobrassicin) increased in seedlings as total glucosinolate concentration decreased. Damage by slugs was inversely related to the concentration of those glucosinolates which decreased and was positively correlated with the two compounds which increased as total glucosinolate concentration decreased. The results support the hypothesis that glucosinolates in brassicas protect them from polyphagous herbivores, and, in particular, that an important function of glucosinolates in rape seeds is to protect seedlings from slugs. As glucosinolate concentrations of oilseed rape cultivars continue to decline, so the risk of slug damage to seedlings may well increase. Copyright © 1990, Wiley Blackwell. All rights reserved