Farmers are responding to the need for more sustainable agricultural practices by reducing their use of broad-spectrum pesticides. This often leads to an increase in on-farm natural enemy diversity. While there is evidence from lower trophic levels that greater diversity leads to improved ecosystem function, negative interactions among natural enemy species, such as intraguild predation, make the relationship between natural enemy diversity and pest suppression less clear. We are using the green peach aphid and eight natural enemy taxa that co-occur in Washington State potatoes as a model system to examine this relationship. In a large-scale field experiment, we are manipulating natural enemy diversity in a replacement series design to measure its effect on the strength of green peach aphid suppression. The results will have important implications for conservation, pest management and sustainable agriculture.