Earwigs in grain systems: When are they a pest and when are they beneficial?

by Matt Binns

GRDC project code CSP1805-016RTX.

There are at least 80 species of earwigs present in Australia, of which 10 are found in grain crops. These species are comprised of introduced and native earwigs, for which there is not a lot known. Reports of earwig outbreaks and damage to canola, cereals and pulse crops, have been increasingly recorded. However, the factors that influence the risk of earwig outbreaks and crop damage are not well documented. This study aimed to understand under what conditions earwigs are likely to cause damage, and what other roles they might have in grain crops.

Key findings:

• European earwigs (Forficula auricularia) can cause crop damage to canola early in the season. However, a very high density and widespread spatial distribution is required to cause significant biomass loss in canola.
• Timing crop germination to coincide with the period from egg lay (underground) to hatching (3-4 weeks) may be a useful way to help the avoid damage to crops at their most vulnerable stage.
• European earwigs can play an important role in the immediate control of aphids throughout the season, even in low densities. Further, European earwigs will not damage the canola plant until all the aphids nearby are consumed.
• European earwigs can cause pod damage to canola very late in the season when no aphids remain. However, this requires very high densities of earwigs present to cause more than just surface damage.
• The native earwig species, Labidura truncata an Nala lividipes will not cause any damage to canola or wheat, but will eat aphids.
• The use of a fipronil seed coating in canola is likely to reduce earwig populations, but it is not registered for this purpose and could harm beneficial populations of earwigs.

This study suggests that commercially significant levels of crop damage from earwigs requires very high numbers of European earwigs that are widely distributed across the paddock. Species identification is also important as the native species are unlikely to damage canola. Damage to young canola plants from European earwigs can look bad in isolation, but due to the low mobility of earwigs during the growing season, and the fact that they only have one generation per year, the situation can often look worse than it is. We have uncovered several traits that may be exploited for earwig management in canola. Field and laboratory observations have revealed that the 3-4 week period of egg care by the female European earwigs is associated with minimal crop feeding activity. There is potential to time crop germination with this period, allowing crop growth through its vulnerable stage unhindered by earwigs.

European earwigs are omnivorous and can be aggressive predators attacking invertebrates several times their size. In this study, we observed their clear preference for feeding on aphids over the canola plant. Each European earwig can consume about 100 aphids a day, giving them high beneficial value for controlling aphids in canola. Additionally, the fact that they are present in peak numbers as soon as the aphids colonise the crops means there is no lag such as with beneficials like ladybird beetles that need time to reproduce in the crop and build up numbers. The challenge is that the European earwigs will switch back to crop feeding when no aphids are present, and they will feed on pods. This adds complexity when managing European earwigs because, when present in low numbers, they may reduce aphid numbers to below spray thresholds and increase farm profits.

Links to extra information:

‘Earwigs – latest research on these damaging pests’ GRDC Update 2019

‘Earwigs – an appetite for destruction or are they beneficial?’ GRDC Update 2020

Binns M., Hoffmann A. A., Helden M. van et al. (2020) ‘Lifecycle of the invasive omnivore, Forficula auricularia, in Australian grain growing environments.’ Pest Management Science. https://onlinelibrary.wiley.com/doi/10.1002/ps.6206