Earwig Releases Provide Accumulative Biological Control of the Woolly Apple Aphid over the Years

SIMPLE SUMMARY: The woolly apple aphid (Eriosoma lanigerum) is a worldwide pest that causes damage to shoots, branches, trunks, and roots and reduces the capacity of apple trees to sprout and bloom. Even though this pest has a diverse guild of natural enemies, the parasitoid Aphelinus mali and the European earwig (Forficula auricularia) are the ones that have the most potential to regulate this pest. In our study we assessed the impact of earwig releases over time on woolly apple aphid populations. To do this, we released earwigs once per season in consecutive years and we compared woolly apple aphid abundance with respect to a control treatment (without earwig release). Overall, we found that earwig releases reduced the size of the colonies, but this effect was only evident from the second year onwards. These results highlighted the importance of considering time on augmentative biological control strategies. ABSTRACT: Nature-based solutions, such as biological control, can strongly contribute to reducing the use of plant protection products. In our study, we assessed the effect of augmentative releases of the European earwig (Forficula auricularia) to control the woolly apple aphid (Eriosoma lanigerum), a worldwide pest that causes serious damage to apple trees. The trials were carried out in two organic apple orchards located in Catalonia (NE Spain) from 2017 to 2020. Two treatments were compared: with vs. without earwig release. For the treatment, 30 earwigs per tree were released by means of a corrugated cardboard shelter. These releases were performed once per season and were repeated every year. We periodically assessed the length of the woolly apple aphid colonies, the number of colonies per tree, the percentage of aphids parasitized by Aphelinus mali, and the number of earwigs per shelter. Our results showed that earwig releases reduced the length of the colonies, but this effect was noticeable only for the second year onwards. Moreover, we found that those releases were compatible with A. mali. Overall, we demonstrated the positive impact of earwig releases on the woolly apple aphid control and the importance of considering time on augmentative biological control strategies.