The European earwig (Forficula auricularia) has been identified as a potential pest in citrus groves. Researchers have developed a cost-effective sampling method to study the earwigs’ impact and movement, though thus far the economic impact of their damage to citrus crops has been “hard to pin down.” (Photo by Joseph Berger, Bugwood.org)

By Andrew Porterfield

Andrew Porterfield

The European earwig (Forficula auricularia) has been in the United States for more than a century, thriving in all but the far southeastern states. An omnivorous arthropod, it has been noted as both a pest and as beneficial insect, feeding on crops as well as on aphids.

Recently, researchers have found evidence that suggests it also damages citrus groves, both in Europe and in the U.S. They have fed on very young, emerging citrus fruits in California, causing fruit scarring, but it’s not known how extensive this damage is. To get an idea of what kind of citrus damage comes from the earwig, researchers at the University of California, Davis, looked at the behavior of earwigs in citrus orchards in California’s Central Valley. Their results, which include a new, farmer-friendly method of collecting earwig samples, were published in February in the Journal of Economic Entomology.

The researchers, led by now-retired entomology professor Jay Rosenheim, Ph.D., found that earwigs begin feeding on budding citrus fruits much earlier in the growing season than expected but that the density of earwigs seemed to bear no relationship to fruit damage. They determined that the earwigs did travel up and down the trunks of citrus trees during early fruiting season, well before flower petals began falling.

But first, the researchers had to develop an easy and relatively cheap sampling method. After considering four methods, they found that small wooden boards (costing less than $2 each) were effective as traps for the nocturnal earwigs. Testing 27 such boards, which were laid flat on the ground at the edge of a tree canopy, they found the board traps had many more earwigs per trap than the other methods. They also either applied a sticky barrier around the tree trunk, or no barrier (as a control). They then used the boards to sample insect movements in 93 commercial citrus blocks over two years.

The European earwig (Forficula auricularia) has been identified as a potential pest in citrus groves. To better understand its potential impact, researchers at the University of California, Davis, looked for a cost-effective sampling method to study the earwigs’ behavior and movement. They found that small wooden boards laid flat on the ground at the edge of a tree canopy were most effective as traps for the nocturnal earwigs (and the boards cost less than $2 each). They then used the boards to sample insect movements in 93 commercial citrus blocks over two years. Shown here are Kelley Morrow, a pest control advisor at Fowler Packing Company and a co-author on the study, along with colleagues Edgar Cuevas, Jose Constantino,  and Jesus Garcia. (Photo courtesy of Kelley Morrow)

The sticky barriers were effective at halting earwig movement into citrus trees, but only if applied early enough in the season. Other researchers had found that the barriers resulted in a 50 percent reduction in fruit damage. Density of earwigs did not seem to have any impact on fruit damage.

The researchers also were trying to develop an estimate of economic damage caused by earwigs. “The economic impact is hard to pin down, because when we see damage, we don’t know if it’s generated by earwigs or by other early-season pests that also chew on tiny fruits—in particular, the fork-tailed bush katydids,” Rosenheim says.

“This is true despite successful experimental demonstrations of the damage potential of F. auricularia,” the authors write.

The European earwig (Forficula auricularia), shown here with a clutch of eggs, has been identified as a potential pest in citrus groves. Researchers have developed a cost-effective sampling method to study the earwigs’ impact and movement, though thus far the economic impact of their damage to citrus crops has been “hard to pin down.” (Photo courtesy of Jay Rosenheim, Ph.D.)

The reasons behind earwig movements in citrus trees, where the insects climb up and down during feeding, are still a bit of a puzzle. “We still don’t fully understand what all that movement was about,” Rosenheim says. The fact that the team could not find any relationship between earwig population densities and fruit damage underscores the need for more research on the biology of this insect.

As for next steps, Rosenheim, who retired from UC Davis in June 2024, says, “New workers will follow me, and I hope that my work will be a stepping stone towards bigger things to come.”

Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.