Cranberry Girdler Rears its Ugly Head

 Posted by: Alison O'Connor, Larimer County Extension

Doesn’t it seem weird that we have a type of sod webworm called cranberry girdler that lives in lawns in Colorado? Because this insect is a pest of cranberry bogs, Douglas-fir, and true fir…along with cool season lawns. Weird.

Regardless of its preferred hosts, these guys can do damage. Quickly. And I’ve seen a number of lawns this fall with damage. For example, there was this lawn in Loveland, which was the worst damage I’ve seen. All of the brown areas are dead. This lawn was a mix of fine fescue, tall fescue, and Kentucky bluegrass.

As for the actual insect, it appears to be a western problem in states like Utah, Colorado, and Wyoming. There’s not a lot of information about it, nor much research. (Utah State University has the best publication.) It’s a sod webworm that does its feeding and damage in late summer into fall. The damage looks a bit like drought stress, until you tug on the grass. Then it peels up like a cheap toupee.

Cranberry girdler can kill lawns by chewing through the roots, resulting in browning and the resulting toupee. Identification of this pest can be hard because it looks like most other sod webworms—grayish, smallish, with an orange-brown head measuring about ¾” long at maturity. The adult is a small moth with three black dots at the base of the wings, a buff color, and measure about ½” wide.

Adult moths (which emerge from overwintered larvae) emerge in mid-June and actively mate for 6-8 weeks. Eggs are laid on turf blades and hatch 9-11 days later. The larvae burrow into thatch or grass crowns, where they feed for two months. And that’s when we start to see damage (August-September).

Larvae will burrow deeper into the soil when temperatures start to cool. I think that’s why we’re seeing damage so late this fall—we really haven’t had cooler daytime temps, so the larvae were still feeding near the surface and easy to spot in late September.  

Now, you will be hard pressed NOT to find some sod webworms in your lawn, but there is a threshold you should consider before treatment. First, how widespread is the damage? If it’s a significant percent of the overall lawn, then treatment could be considered. Do you have the ability to water in products? If so, then treatment can be considered. But just because you see little white moths flying about doesn’t justify treatment. Many lawns will do well with some sod webworms present—especially those that are properly maintained.

A couple of things about treatment. The first is that if you have damage this fall, you should consider a spring-applied treatment for control next year. A promising (and easily available) product for homeowners is acelepryn, which is found in Scotts GrubEx. Apply this in late May to early June, watering it in with at least ½” of water. This product will control sod webworms and all white grubs (here’s looking at you, Japanese beetle).

But this fall, acelepryn or other recommended insecticides (beta-cyfluthrin, carbaryl, and chlorpyrifos) will not work quickly enough to kill damaging larvae. One of the few options is trichlorfon (sold as Dylox) and can work as a “rescue” treatment. This should be applied by a lawn care professional. Prior to application, water the lawn well with at least ½” of water and after application, apply another ½” of water. While very effective, this insecticide only lasts a couple days, so timing is important. It will not work if the caterpillars have pupated deeper into the soil.

And what do you do with the damaged areas of the lawn? You’ll have to reseed or re-sod. Seeding now (in early October) will not likely be super successful, so waiting until spring would be a better option. You could sod into late fall, as long as you can water.

So chalk it up to another 2020 “kick-you-while-you’re-down” problem. Boo cranberry girdler. Boo.