RELATIONSHIPS BETWEEN TURNIP ROOT FLY (DELIA-FLORALIS) LARVAL DEVELOPMENT AND THE SUGAR CONTENT OF SWEDE (BRASSICA-NAPUS SSP RAPIFERA) ROOTS

Eight genotypes of swede (Brassica napus L. ssp. rapifera [Metz.] Sins k.) at the 8-10 true leaf stage were inoculated with five, 10 or 20 eg gs of the turnip root fly Delia floralis (Fall). The roots were sample d, with control roots, after 6 weeks of larval development. D. florali s root damage, as measured by reduction in root weight, was found to b e linked to inoculation level. Neither D. floralis egg numbers nor swe de genotype had a significant effect on the percentage of larvae devel oping to pupation. Mean pupal weight varied by a factor of X1.4 and co nsistently decreased with increasing egg inoculation level. Changes in the root concentrations of glucose, sucrose and fructose were measure d. All swede genotypes showed a similar response in their sugar, conce ntrations after root damage. Glucose and fructose concentrations were reduced whilst sucrose concentration remained unaffected. The concentr ations of glucose and fructose were highly correlated. Pupal weight, u sed as a measure of larval development, was significantly correlated w ith the concentrations of individual and total sugars in the roots. Th e implications of sugar responses to damage in brassicas, and the corr elation between sugar concentrations in the roots and D. floralis pupa l weights are discussed.