PLANT STRESS AND LARVAL PERFORMANCE OF A DIPTEROUS GALL FORMER

According to the plant vigour hypothesis, galling insects should respo nd positively and perform better on vigorous plants or plant parts, th e opposite of the predictions of the plant stress hypothesis. I carrie d out field experiments to analyse the effects of sustained abiotic st ress on the interactions between the common reed (Phragmites australis ) and a gall-forming fly (Lipara lucens). The reed shoot diameter (a m easure of plant vigour) is strongly affected by environmental conditio ns, where dry and/or nutrient-poor habitats produce thinner (stressed) shoots. L. lucens gall density is negatively correlated with shoot di ameter. In a survival experiment with a wide range of shoot diameters, larval mortality was also highly correlated with shoot quality. Gall formation was higher on thinner, stressed shoots. An analysis of the g all tissues revealed that galls induced by L. lucens contain a high am ount of a nutrient-rich feeding tissue. The impact of L. lucens is hig her on thinner shoots. The results of this study showed that L. lucens performs better on stressed hosts, which contradicts the plant vigour hypothesis for galling insects. The low nutrient availability in the stressed shoots can be compensated by the production of galls with a n utrient-rich feeding tissue.