Crows digging up lawns in Washington and Oregon may be after the invasive European chafer grub.
The beetles are native to Europe, but by 2016 they had taken hold in Western Washington.
As grubs, they feed on grass roots in the spring and fall.
The fat, C-shaped larvae are a high-value treat for crows in the winter, when many food sources are scarce, said John Marzluff, an emeritus professor at the University of Washington who has written several books on crow behavior.
“It's a rich food source, so it's worth the energy they expend to tear up your lawn to get to them,” Marzluff said. “They are responding to this basically exploding population of this non-native pest. They're helping to reduce the spread of them by eating them.”
As the European chafer grubs spread, there have been more reports of animals such as crows and raccoons tearing up lawns to feast on them, said Jim Kropf, who directs the Washington State University Pierce County Extension.
European chafer beetles have been on the East Coast since the 1940s. They were first discovered in the Pacific Northwest near Vancouver, BC, in 2001.
According to Kropf, the grubs had been sighted in locations including SeaTac, Burien, Renton, Federal Way, northeast Tacoma, Lakewood, the Spanaway area, and the Tri-Cities.
"There's been quite a migration of them across parts of Washington," Kropf said.
Large infestations can lead to dead patches in lawns, as the grubs eat grass roots.
“This time of year, there really isn't anything to treat for,” Kropf said. “We recommend people go out and scout in their lawn, pull back loose sod and see if they have the grubs in the lawn.”
The more inch-long grubs in a lawn, the greater the feast for backyard critters. WSU experts say lawns with five to 10 European chafer grubs per square foot should be treated.
While animals digging through lawns can provide some beneficial aeration, Kropf said, people "can completely lose their lawn to the damage caused by both the insects and the predators going after the grubs.”
He said watering during the dry summer months makes lawns a less desirable place for the adult chafer beetles to lay their eggs. This can help prevent lawn damage in the fall.
People can also prevent large grub infestations by planting beetle-resistant grasses. And in some cases, they may consider insecticides or parasitic nematodes, though that comes with other risks.
“We caution people about starting to spray a bunch of insecticide at any time onto their lawn without absolutely knowing what the problem is, when it should be applied, what products to use, how to use them safely,” Kropf said.
People can report European chafer sightings to the Washington Invasive Species Council and get support from WSU Master Gardeners in the Pacific Northwest.
