By Robert Orpet, WSU Entomology, robert.orpet@wsu.edu, June 2019
Earwigs are omnivores capable of damaging fruits, but they can also suppress pests such as woolly apple aphid and pear psylla. So, are earwigs pests or beneficials? Ongoing research in Washington and around the world suggests that earwigs are beneficial in pome fruits but pests in stone fruits. Earwigs rarely attack apples or pears except where there is existing damage like splits or mechanical injuries. However, earwigs clearly initiate damage to soft fruits like peaches. To manage earwigs, a variety of options are available for conservation or suppression.
Herbivore and predator behavior
Earwigs can make shallow or tunnel-like holes in fruit flesh. A distinctive feature of earwig feeding is its rounded edges, whereas wounds with jagged or straight edges may be from mechanical or bird damage. This type of damage is common on stone fruits like peaches. This following linked Utah State University fact sheet has useful pictures of earwig damage: https://utahpests.usu.edu/uppdl/files-ou/factsheet/European%20Earwig.pdf. However, do not use pesticide recommendations from the Utah fact sheet as pesticides listed there may not all be currently registered in Washington State.
When earwigs are seen on damaged apples, they are usually feeding or merely sheltering in existing wounds, which are attractive day-time hiding spots for these nocturnal insects. I released thousands of earwigs into research plots and occasionally found earwigs in stem-bowl splits (Figure 1), but the incidence of stem-bowl splitting was similar between earwig release plots and control plots with lower earwig populations. This is evidence that earwigs didn’t cause any new damage, but merely exploited existing damage.
As a predator, earwigs will eat almost anything small or soft. Several experiments from around the world have shown earwigs suppress woolly apple aphid and pear psylla, and video footage is available of earwigs consuming woolly apple aphids in the field (https://www.youtube.com/watch?v=sSFakIgkfMI).
How to suppress or conserve earwigs
Whether you want to suppress them or conserve them, it is important to time management tactics based on whether earwigs are in their underground nesting phase (winter and early spring), ground phase (spring), or canopy phase (summer to fall) (Figure 2B). Females overwinter in underground nests and produce one to two broods in Spring. There is only one generation of earwigs per year. Young immature earwigs eventually abandon nests and forage on the ground. Older immatures and adults spend most of their time in plant canopies instead of the ground.
Nesting phase (Tillage)
Tillage can destroy earwig nests. However, the nests usually occur >10 cm underground, so heavy disturbance of the soil is required to harm populations. To harm earwig populations, tillage would need to occur after October, when earwigs first beginning nesting, and before May, when the new generation starts to abandon nests.
Ground phase (insecticidal baits)
Spinosad is highly toxic to earwigs and is available as a soil-applied insecticidal bait formulation (Seduce) labeled for earwig suppression. To be effective against earwigs, this should be applied before May. Otherwise, canopy-foraging earwigs will not encounter these baits on the orchard floor. Note: the label of Seduce restricts application to 0.45 lb active ingredient spinosad/A/year, including foliar uses.
Canopy phase (insecticide sprays)
From late May to early October, earwigs are vulnerable to canopy sprays. However, earwigs sheltered in their daytime hiding spots can avoid direct contact with insecticides, lowering their efficacy. The best time of the day to spray to suppress earwigs would be during the first half of the night after sunset. The following insecticides have been implicated in suppressing earwigs in the field: carbaryl, diazinon, beta-cyfluthrin, imidacloprid, thiacloprid, spinosad, novaluron, diflubenzuron (note: not labeled for use in cherry), indoxacarb, and flonicamid. Kaolin clay can also suppress earwigs, but is not known whether residues from early-season applications can harm earwigs later in season, and this is a topic of investigation this year at WSU.
How big is my earwig population?
If you are thinking about trying to increase your earwig population, it is worth checking how large your current population is. Earwigs only have one generation per year, and disperse very slowly (they have wings but rarely use them), so conservation will have little immediate effect if there is no existing population. Also, if there is already a large earwig population, there is little benefit in increasing it further, and changes to management would be unnecessary.
To monitor, place rolled strips of corrugated cardboard in tree canopies in contact with trunks near any major limb. Place them during late June and a week later, around July 1, shake earwigs out of the tubes and count them. Studies conducted around the world with ca. 10 × 35 cm cardboard strips (Figure 3) suggest that the pest control benefits of earwigs level off at around 5 to 15 per shelter. If you find no earwigs, the population is either very sparse or there isn’t one.
Conclusion
Earwigs are often seen sheltering in previously damaged fruit during the day, but are underappreciated predators that attack pests at night when you aren’t watching them. In particular, earwigs can increase the consistency of biological control against woolly apple aphids and pear psylla. Whether you are aiming to conserve them or suppress them, it’s key to know where earwigs are at what time of year to use appropriate tactics. To develop more recommendations to improve biocontrol, research is ongoing at WSU on how to avoid non-target effects of pesticides on earwigs, and how to efficiently inoculate apple and pear orchards lacking earwigs with a new population.
Contact
Robert Orpet
WSU Tree Fruit Research and Extension Center
robert.orpet@wsu.edu
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