The impact of natural enemy evenness on biological control in organic and conventional walnut orchards in California.

Agriculture is a vital industry in the United States; however the intensification of farming practices, mainly the increased use of fertilizers and pesticides, has led to significant water and air pollution, human health risks, and environmental costs. Many environmental and health risks stem from the use of and run off from fertilizer and pesticide; therefore, reducing pesticide and fertilizer use while maintaining yields is a priority for agriculture. In some situations biological control, the use of natural enemy populations to decrease pest populations is a good alternative to pesticide use and can reduce pesticide pollution. The practice of biological control has a long history in agriculture; however, the mechanisms that underpin its successes are not fully understood, particularly in systems with multiple natural enemies. Fertilizer and pesticide are generally managed separately despite the fact that it is their combined influence that impact pest population abundance. Understanding how fertilizer management and plant nutrient status impact pests and their natural enemies is an important step in integrating fertilizer and pesticide use and management. A natural contrast in management practices can be found in a comparison of organic and conventional farms. While natural enemy diversity is generally higher on organic farms the functional significance for pest control is variable. In this project, the impact of orchard management, natural enemy diversity, and natural enemy evenness on aphid density was investigated in California walnut orchards. Walnuts, a $738 million specialty crop in California, are an excellent study system in which to address these questions. Walnut aphids, Chromaphis juglandicola, are mainly controlled by the specialist parasitoid wasp Trioxys pallidus, introduced from Iran. Green lacewings and lady beetles are generalist predators that also contribute to aphid mortality. Aphid populations can grow rapidly in late summer if control is disrupted. Although the natural enemy richness is not different between organic and conventional orchard and isn’t correlated with aphid density, organic orchards exhibit more even natural enemy communities. In this study, evenness is correlated with lower aphid density across orchard types. By creating and testing management strategies that encourage evenness, farmers may be able to improve biological control of pest insects.