Effects of Temperature and Extraguild Prey Density on Intraguild Predation of Coccinella septempunctata and Harmonia axyridis

The ladybirds <i>Coccinella septempunctata</i> and <i>Harmonia axyridis</i> are important biocontrol agents for the small walnut aphid <i>Chromaphis juglandicola</i>, a key walnut pest. <i>C. juglandicola</i> outbreaks occur in walnut orchards, and walnut yields have declined. Intraguild predation (IGP) is prevalent among natural enemies that coexist in shared habitats and prey upon the same extraguild prey. We designed laboratory experiments to evaluate the potential for IGP between these two ladybirds at different temperatures and extraguild (EG) prey densities, and the ability of IGP to control EG prey under different conditions. We measured IGP rates in first instar larvae, female adults, and male adults (both starved for 24 h) in the vulnerable immature life stages of two ladybird eggs. Intraguild (IG) prey (<i>H. axyridis</i> eggs or <i>C. septempunctata</i> eggs) and EG prey (<i>C. juglandicola</i>) consumption were tallied after 24 h. Temperature and EG prey density influenced IGP rates, with temperature contributing the most to the variance. IGP increased with increasing temperature (15-35 °C), with both factors interactively influencing the EG prey consumption rate and exhibiting highly significant effects. EG prey consumption increased with temperature and density. This research provides theoretical support for the rational use of <i>H. axyridis</i> and <i>C. septempunctata</i> in the joint biological control of <i>C. juglandicola</i>.