Estimating parasitoid impact on aphid populations in the field

We developed a quantitative method to assess the capacity of parasitoids to reduce aphid populations in open fields. The method, designated as the propagated mortality analysis (PMA), estimates the impact of early parasitoid-induced mortality on forthcoming aphid populations and was built upon an existing model describing the bell-shaped population dynamics observed in many aphid species. The PMA was next used to assess the impact of the naturalized and most abundant parasitoid Aphelinus sp. on populations of the soybean aphid Aphis glycines under field conditions in north-eastern North America. Results from the PMA showed that Aphelinus sp. reduced both peak soybean aphid densities and cumulative aphid-days at the end of the pest management threshold period by only 1-7%, probably because of low levels of parasitism early in the season. The method we propose is simple to use, could apply to most aphid-parasitoid systems and would gain to be extended to other natural enemies.