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

Biocontrol agents are often considered to be harmless for the environment and therefore to have no side effects on any non target organism. Having in most cases much less killing potential than chemical pesticide we expect also more subtle side effects, which may go undetected in a short term unless appropriate research is done. To test the effect of a biocontrol agent on non target organism we examined the non-target effects of the introduction on apple trees of an epiphytic yeast, a potential biocontrol agent of apple powdery mildew, on scab (Venturai inaequalis) germination and infection, codling moth (Cydia pomonella) oviposition and damage and apple fruit quality. The treatment, that consisted of the yeast (Y16) supplemented with the detergent Tween 80, affected neither conidia germination of V. inaequalis nor their penetration of the leaf tissue but reduced the disease caused by this pathogen. The quantity of eggs laid by the codling moth during its second flight period on yeast treated trees was significantly lower than the quantity of eggs laid on the untreated trees, in one year and it was higher in the other year. Finally, the yeast treatment did not affect any of the examined fruit quality parameters. In the second part of this work, we examined some of the possible mechanism that could explain the yeast effect on codling moth egg laying. Laboratory experiments indicated, that the yeast had no direct effect on the codling moth egg laying; i.e. it did neither modify the contact between the insect and the oviposition surface, nor produce substances stimulating or repelling codling moth females. As indirect mechanism, we investigated the role of apple leaf surface primary metabolites in the interaction between the codling moth and the yeast treatment. Analysis of leaf surface washes revealed changes in quantities of sugars, sugar alcohols and amino acids, as well as ratio modifications between sugar and sugar alcohols, known to affect the codling moth egg laying behaviour. Laboratory experiments did not reveal any effect of blends representing the leaf surface chemical composition on the acceptance or on the stimulation of the codling moth for egg laying. Importance of the primary metabolites as a cue for the codling moth egg laying behaviour are not demonstrated in this work. The simulative effect of the yeast treatment on egg laying was attributed to the plant-yeast interaction, and might be enhanced by the Tween 80.