Predictive thresholds for forecasting the compatibility of Forficula auricularia and Aphelinus mali as biological control agents against woolly apple aphid in apple orchards

The woolly apple aphid (WAA), <em>Eriosoma lanigerum</em> (Hausmann) (Hemiptera: Aphididae) is a well-known pest of apple orchards world-wide. Several studies have demonstrated variable control of WAA populations by the European earwig, <em>Forficula auricularia</em> (L.) (Dermaptera: Forficulidae) and the WAA parasitoid <em>Aphelinus mali</em> (Halderman) (Hymenoptera: Aphelinidae). We examine whether a beneficial interaction between <em>F. auricularia</em> and <em>A. mali</em> exists and calculate optimal numbers for each species to maintain WAA infestations below acceptable levels. We demonstrate that trees possessing >14 earwigs per trunk trap per week within the first seven weeks post-blossom contained WAA infestations well below acceptable levels. Where these earwig thresholds were not met, a first generation of <em>A. mali</em> greater than 0.5 wasps per tree was required. If these beneficial insect targets were not met, severe WAA infestations occurred. Our findings suggest that if <em>F. auricularia</em> and <em>A. mali</em> numbers exceed these thresholds chemical intervention may not be required.