Masked Chafers

Masked chafers are underground bandits. They devour turfgrass roots all summer long, but the damage isn’t usually seen until late summer or early fall.

I couldn’t figure out why I had so many odd-shaped dead patches in my lawn. I watered and fed it all the same. In spite of my heavy clay soil, there were areas that felt spongy and didn’t look very healthy. When I grabbed a handful of lawn and gave a gentle tug, entire patches came up like a bad toupee. Also, one of my dogs kept digging shallow holes, even though she knew it was strictly forbidden. She could hear what I could not. It was masked chafers.

Masked chafer (Robert Webster) CC BY-SA 4.0

​Masked chafer description

Masked chafer beetles (Cyclocephala borealis) are small (3/4”) brown beetles with a black head. The larvae are fat grubs that live underground. They are about an inch long, white with a brown head and a brown stripe down the back. Masked chafer grubs tend to curl into a C-shape when exposed or disturbed. There are two other species grub that you may discover in your lawn: June beetles and black turfgrass ataenius grubs. The June beetle grub has two rows of bristles on the underside of its rear end (raster), while the masked chafer has scattered bristles, and the black ataenius has something of a wasp-waist. Whichever species is feeding on your lawn, you’ll want to get rid of it.

A case of mistaken identity

Another common grub looks very much like masked chafer larvae, but green fruit beetle larvae are actually beneficial. Green fruit beetle larvae feed on decomposing plant material and poop out some very nice plant food. They also help reduce soil compaction.

​Before you treat for pests, make sure you know what you’re dealing with!

Bucket of green fruit beetle larvae

Masked chafer host plants

Masked chafers prefer bluegrass, ryegrass, and other warm season grass species, but all turfgrass species are vulnerable. So are your blueberries. Masked chafers burrow underground, feeding on roots and cutting off the supply of water to your lawn. The first symptom of masked chafer infestation is the lawn looks drought-stressed. Well, here in California, all lawns look drought-stressed by the end of summer, so that doesn’t help. The symptoms also look similar to Southern chinch bug damage. The only way to really be sure about the presence of masked chafers is to watch for them whenever digging in the garden or landscape. If you have areas that look like masked chafers may be a problem, it may be worthwhile to grab your shovel and make a shallow cut, under the grass roots, to see what’s really going on down there. 

Masked chafer controls

Keeping your lawn healthy is the best treatment. That means planting the right variety for your microclimate, watering it deeply and infrequently, and feeding it appropriately. Avoiding the use of broad-spectrum pesticides will also help maintain a population of beneficial insects that may parasitize the grubs. You can learn more about the best turfgrass variety for your area in UC Davis’ Turfgrass Selection for the Home Landscape. Experts say that one grub per square foot of lawn is okay, but six is not. You decide what’s right for your lawn. 

You can buy a variety of parasitic nematodes that are advertised to kill off these pests, but not all of them work. Research at UC Davis has shown that Heterorhabditis bacteriophora is effective against masked chafers, while Steinernema bacteriophora are not. When I applied parasitic nematodes to my heavily infested lawn, I was rewarded the next morning with grubs on my patio. They may not move quickly, but they sure didn’t want to stay in my lawn! The hens were very happy about that and so was I!