Predator to prey ratios: A way to judge biological control potential in the field?

Agricultural intensification has led to increasing use of pesticides, with significant negative human and environmental health risks. As the demand for food is growing, it is imperative to explore and develop production techniques that minimize these negative impacts. One way to reduce pesticide use while maintaining yields is to improve pest suppression through biological control. Conservation biological control, using resident natural enemies, is a particularly promising form of biological control that has been underutilized in the past. An important barrier to the more widespread adoption of conservation biological control is our current inability to accurately predict when and where it will be effective. Predator to prey ratio and predator density are relatively easy to calculate metrics that could provide farmers with an indication of the biological control potential in their fields. It’s not clear, however, if these ratios are reliable indicators in walnuts. Over two seasons of field data in walnuts, late season aphid density did not show a relationship with predator to prey ratios in the early season while predator density did. However, when sentinel aphids were placed in orchards, the proportion predated increased with predator to prey ratio but showed no relationship with predator density. Differentiating between metrics that are useful to predict concurrent biological control and those that can provide insight into future biological control could improve integrated pest management and support reduced pesticide use.