INFLUENCE OF THE LEAF CURLING PLUM APHID (BRACHYCAUDUS-HELICHRYSI) ONSTEM DIAMETER, SEED YIELD, AND THEIR RELATIONSHIP, IN SUNFLOWER

The leaf-curling plum aphid (Brachycaudus helichrysi Kalt.) is one of the main pests of sunflower (Helianthus annuus L.) in Europe. Yield lo sses were assessed in open field conditions and natural infestations o ver a 3-year period in Central West France. Plant variability and the choice of the more vigorous plants by aphids in open field conditions were two factors interfering with the experiments. To improve their se nsitivity and to correct for biases without expanding the experimental design, an analysis of covariance was the best compromise. Field and cage experiments were conducted between 1985 and 1989 to assess the in fluence of aphids on the relationship between stem diameter at harvest and seed yield. As an indicator of plant vigour (size and yield), ste m diameter at harvest was the best choice, as neither its relationship with seed yield nor its mean value was affected by aphid infestation. It was then used as a covariate to study yield losses. Aphid populati ons were quantified weekly from the beginning to the end of the infest ations according to the leaf-curling symptom and to the number of aphi ds per plant. Multivariate analyses followed by cluster analysis ident ified groups of plants displaying similar infestation dynamics. It was found that when aphid populations reached > 100 per plant at the budd ing stage, yield losses occurred. When populations were < 100 aphids p er plant at the budding stage and then decreased, no yield loss was ob served. This gives growers ample time to monitor populations and treat crops.