European Earwig Functional Roles in Peach Orchards
The European earwig (Forficula auricularia) is native to Europe, western Asia, and parts of Africa, and was first introduced to Utah in the 1930s. It is a common insect in agricultural crops and home gardens in the western U.S. Interestingly, it has omnivorous feeding habits and so can be both a plant-feeder and predator. It is a pest of many plants, including fruits, vegetables, herbs, grains, stored foods, pollen, and ornamentals. Similarly, it has been found to prey on many types of small arthropods including aphids, scales, caterpillars, maggots, and mites.
Its abundance and crop injury in stone fruits, especially peach, is a concern for commercial growers and home gardeners. A survey of Utah peach producers in 2010 revealed that 68% had observed fruit injury from earwigs with most categorizing the damage as 5-10% of the crop affected. The majority (61%) reported use of insecticides to control earwigs, while only 8% had tried cultural practices, and 31% had done nothing to reduce the damage.
Tree fruit researchers in Europe and Oregon developed a simple monitoring trap for the European earwig. Strips of corrugated cardboard (4 in wide by 10 in long; corrugation flute size “A”) are rolled with the ridges turned inward and tied onto a tree trunk or limb. Earwigs are nocturnal in their activity and seek refuge in the cardboard traps during daylight.
We tested the refuge trap in peach orchards and found that the number of earwigs collected in a trap can provide a relative measure of abundance as the number that seek shelter in a trap is influenced by crowding (density). The traps, however, provide easy-to-gather information on phenology (timing of life stages during the season) and location preference within orchards.
The European earwig overwinters as an adult, and femalesbegin to lay clutches of 30 to 50 eggs in the soil or other moist locations in the spring. Females provide maternal care, an uncommon behavior in the insect world. There are 4 nymphal instars (immature stages with a molt of the outer cuticle between each).
The 1st instars remain primarily in the nest, and few 1st instars have been caught in the cardboard traps. The 2nd through 4th instars depart the nest and forage for food in the surrounding environment.
In six peach orchards in 2010, abundance of 2nd instars peaked at 300 degree-days (DD) (lower threshold for development = 44°F) while 50% trap catch of 2nd to 3rd instars, 3rd to 4th instars, and 4th instars to adults occurred at 425, 500, and 775 DD. We plan to develop a degree day model that can be used to determine when the different life stages will be active, when crop injury may occur, and optimal timing for control.
Studies so far have shown that earwigs will feed on the leaves and fruit of peach trees. In the spring and summer, adults and nymphs chew along the edges and tips of leaves. Last year, in a research orchard in Kaysville where no earwig controls were applied and the crop load was small, from 38-58% of the peach fruits were fed upon by earwigs. Fruit injury occurred from mid August to early September, most likely by adults, which were found to be the primary life stage in the orchard. Peach fruit-feeding increased with decreasing firmness (i.e., maturity) of the fruits, and the size of feeding holes substantially increased when fruit firmness dropped below about 0.2 kg/mm using a fruit penetrometer.
The earwig population size measured in cardboard traps was not correlated to fruit injury, and suggests that refuge traps may not provide an accurate measure of actual population density.
In an insecticide control study, an average of 6% of the fruit in the untreated plots had earwig injury as compared to 1% in each the spinosad (Success)- and carbaryl (Sevin)-treated plots. The insecticides were applied on August 16 just after the first earwig fruit injury was detected.
The abundance of green peach aphids in orchards this spring allowed us to evaluate the feeding rate of earwig adults and nymphs on aphids. Further research on earwig ground habitat preference (orchard cover crops, tillage, herbicide-treatments, and mulches), dispersal distances, feeding preferences throughout the season, predation rates and preference for prey types, timing of crop injury, predation feeding behaviors, phenology models, and other life history characteristics is underway. The project goals are to gain a better understanding of European earwig functional ecology and roles in the peach orchard, and how to better manage this insect to take advantage of the benefits it can bring through biological control while minimizing crop damage.
This research is supported by the USDA Organic Agriculture Research and Extension Initiative, the Utah Department of Agriculture Specialty Crop Block Grant Program, and the Utah Agricultural Experiment Station. Look for research updates at upcoming tree fruit field days, grower meetings, and Master Gardener classes.
-Diane Alston, Entomologist, and Andrew Tebeau, PhD student in Dept. of Biology, USU