Factors influencing the abundance of Trioxys pallidus, a successful introduced biological control agent of walnut aphid in California

This field study reports on factors influencing the abundance of Trioxys pallidus 35 years after its successful introduction in California, and in the presence of continued hyperparasitism and intraguild predation. The seasonal activity of Chromaphis juglandicola and T. pallidus were monitored in 2005–06 in relation to hyperparasitism and mummy predation in three orchards. Walnut aphids remained under the economic threshold in all three orchards with insecticide treatments for other pests in some cases reducing aphid populations as well. No significant density-independent or density-dependent aggregation in the risk of hyperparasitism was found at the individual tree scale, suggesting the absence of a refuge from hyperparasitism. In contrast, significant negative density-dependent aggregation in the risk of intraguild predation was found, but no density-independent aggregation, suggesting a refuge at higher mummy densities. Using a linear mixed effects model, the following factors were found to influence mummy density: aphid density from the previous sample date and intraguild predation, with the latter linked to hyperparasitism from the previous sample date through a significant interaction that appears to be mediated by current aphid density.