Alaphilippe, A.; Derridj, S.; Gessler, C.

With the development of pest resistance towards pesticides and the decrease of chemical input, the research on alternative methods for plant protection, such as the use of microorganisms as biocontrol agents (BCA), is developing fast. But the side effects of BCA on non-targeted organisms and on the host plant are poorly known. Mass introduction of an organism into a system can change it substantially. Similarly, the introduction of an epiphytic microorganism may change the phylloplane chemical composition. Furthermore, sugar quantities and ratios in the phylloplane can influence the egg laying behaviour of Cydia pomonella (L.) (Lepidoptera Tortricidae). In this work, we studied the effect on the egg quantity laid by C. pomonella of an epiphytic yeast, as BC-model organism, introduced to apple canopy surfaces. Two seasons of trial, during the second flight of C. pomonella, showed an effect of the yeast spray on the quantity of eggs laid on apple trees. In the preliminary greenhouse experiment, the number of eggs laid on the yeast treated trees was lower as compared with those laid on the untreated trees. But for the second year of experiment, performed in semi-artificial conditions, the yeast treatment increased significantly the quantity of eggs laid, especially on the sites close to the fruits. These conflicting results could be explained by the different environmental and experimental conditions, which may affect the yeast activity and as consequences the phylloplane chemical composition. We analysed the chemical composition of leaf water washings using gas chromatography. On the first year of analysis, we observed modifications of quantities and ratios within the sugar blend on the upper side surfaces compared to untreated ones. We are currently analysing the samples from the second season using liquid chromatography. Finally, the correlation between yeast treatment, sugar quantities and qualities and egg laying behaviour will be analysed.