Field evidence and grower perceptions on the roles of an omnivore, European earwig, in apple orchards

Abstract

In agroecosystems, omnivores can be beneficial predators or harmful herbivores. In apple orchards, the omnivorous European earwig (Forficula auricularia) is thought to be a key predator of woolly apple aphid (Eriosoma lanigerum), but has also been implicated in feeding on apple fruit. Assessing the effects of earwigs in orchards is difficult because they are nocturnal, and their damage to fruits can resemble other wounds. Apple orchardists thus may manage earwigs as either predators or pests based on subjective opinions. To understand current opinions on earwigs in apple orchards, we interviewed 15 apple pest management decision-makers in Washington State, USA. To compare opinions to objective measurements, we manipulated earwig abundance within plots at four orchards and collected data on fruit damage, woolly apple aphid abundance, and molecular gut contents of earwigs. Most interviewees thought earwigs were aphid predators, but some thought earwigs could be minor pests, and most were more uncertain about earwigs’ effects relative to other aphid natural enemies. In the field, earwig abundance was negatively correlated to woolly apple aphid abundance, and earwig guts regularly contained woolly apple aphid DNA, even when aphid densities were low. We found no evidence earwigs damaged fruits. Overall, our results suggest earwigs improved biological control and were not pests, so discontinuing the occasional use of insecticides against earwigs could benefit apple growers. More generally, omnivores and difficult-to-observe natural enemies could often have important underappreciated benefits in agriculture.