My Lawn Has Never Looked This Bad!
Tony Koski, Extension Turf Specialist

Winter desiccation (aka "freeze-drying); made worse because the thatch layer was a little
thick? Probably some rabbit activity, and maybe some cranberry girdler as well? That 
combination will kill grass in the best of lawns during a severe winter.

It seems like nearly every lawn up and down the Front Range has experienced some level of turf injury this spring – more than I can remember in my 30+ years at CSU. Many have been cared for by avowed lawn geeks (like Alison’s lawn, pictured here!). These folks are used to turf perfection, so are especially shocked, dismayed and embarrassed by the spots and patches that have appeared in their normally picture-perfect lawn. What has caused so many beautiful (in 2021, anyway) lawns to become so bad this spring that owners are reluctant to send me pictures of them? Pinning down the exact cause of the turf damage without visiting the lawn can be difficult, but if you are seeing dead patches in your lawn this spring it’s almost certainly one of the following problems. Lawns that were seeded or sodded in 2021 (especially later in the year) were more likely to be injured. If the lawn hasn’t greened up by this point in time – especially after the recent rain – then it’s almost certainly dead, with the solution being new sod or seeding (go here

for some videos on seeding and sodding lawns https://planttalk.colostate.edu/youtube-videos/lawn-videos/#reseed)

Winter Desiccation (freeze-drying)
This is probably the most common cause of lawn damage this spring, most likely to occur on areas of the lawn that were drought-stressed going into winter last fall.  Turf stressed by poor irrigation coverage in 2021 (broken/plugged/low/crooked heads, low pressure, bad spacing) was more susceptible to being damaged by the dry, cold, sunny, windy winter conditions we experienced this year. Lawns with thick layers of thatch also appear to be more likely to have been injured. Check spots that are dead now for irrigation coverage deficiencies – before re-seeding or sodding. You rarely can detect non-uniform water application by simply watching your sprinkler system (“It looks like water is going everywhere” you say…but it’s not going everywhere uniformly). Don’t blindly trust your “irrigation guy”/sprinkler company when they tell you that everything “looks fine”. To be certain about uniformity of water application, it has to be measured by a formal or informal irrigation audit (here’s a DIY method to help you identify dry spots in your lawn).

This turf died from winter desiccation. When you can see where the
irrigation heads are, you know there are irrigation coverage problems. The
dead areas in this lawn were drought-stressed LAST FALL - which resulted
in the turf succumbing to the severely dry and cold winter. The turf also 
survived around the young tree - because of winter watering of the tree.

Insect Feeding Damage

Feeding by cranberry girdler (https://csuhort.blogspot.com/2020/10/cranberry-

This lawn was weakened by cranberry girdler feeding
last fall (2021); the cold, dry, windy winter resulted in
the death of the stressed turf

girdler-rears-its-ugly-head.html) or white grubs (https://extension.colostate.edu/topic-areas/insects/billbugs-and-white-grubs-5-516/) last fall could have gone undetected and so weakened turf root systems that the turf more easily succumbed to winter desiccation. Dead turf that peels up easily from the soil surface was likely fed upon by these insects. If the lawn hasn’t greened up by now, it’s dead and the only alternatives are to seed or sod the affected areas.

Turfgrass mites damaged this lawn. Notice the
lack of mite injury in the neighboring lawn (top
of photo) which received winter watering.

Mite Feeding

Winter mite activity can kill large areas of turf, usually on lawns facing south or west. The damage caused by winter mites can look identical to winter desiccation injury: orange or brown grass (early death), changing to grey/silvery grass (dead for awhile). Mite-damaged turf will still be strongly rooted, distinguishing it from damage caused by root-feeding insects. Even when mites are actively feeding in lawns, they can be difficult to detect – so mite injury is often confused with other types of winter damage. The best control for mites is late winter/early spring irrigation, or a few good snowfalls or rainy spring weather. Since they aren’t insects, insecticides don’t provide great control of turfgrass mites.

Rabbit Injury

Look for rabbit pellets (aka "poop") in the brown spots
in your lawn as a sign of rabbit activity.

Rabbits feeding is often concentrated in small areas of the lawn. The persistent feeding and deposition of urine can kill small to large areas (depends on how many rabbits are in the yard) over the winter. Rabbits can be trapped and relocated (where legal; check with local animal control for the laws that apply where you live); applying the fertilizer Milorganite to your landscape (lawn and garden beds) can repel rabbits - if they have somewhere else to feed and live.

Rabbit activity can result in the death of large areas in the home lawn.

While it’s good to know what may have killed parts of your lawn – so that you can take measures to prevent the same thing from happening in the future – the “fix” is generally the same: repairing the dead areas by seeding or sodding (here and here, for how to do this).

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.

Cranberry Girdler Found in Jefferson County Lawns

Posted by: Mary Small and Curtis Utley, Jefferson County Plant Diagnostic Clinic/CSU Extension

Yes, you read that right! Cranberry girdler larvae are infesting lawns.  Customers look at you strangely when you tell them this.  “Well, where did it come from? We don’t have any cranberries around here!”  Admittedly, it sounded so strange that we had to do more investigation on the origin of the insect’s name.  (We’ll get to the pest part momentarily!) 

Turns out, the larvae of this insect feed on cranberry runners at or below the soil surface, removing bark and/or conductive tissue. So their feeding actually girdles the stems and results in patches of dead cranberry plants.

Cranberry girdler larvae

In addition to cranberries, the insects have an affinity for grasses, true firs and Douglas fir. They are a member of the sod webworm group of insects. In fact, another name for this pest is the subterranean webworm, because of where it feeds. (At least this is beginning to make more sense!)
Now for the pest part. In less than a couple weeks’ time, Jefferson County’s horticulture staff found several cases of cranberry girdler damaging lawns. Diagnosis was really confusing, since the damage is similar to drought stress and can be exacerbated by drought stress.

But then, to add to the confusion, we’ve had many inches of rain and cool temperatures recently. Lawns turned around pretty quickly from summer’s heat and drought stresses.   In fact, most area lawns look pretty darn good.

While examining the lawns, horticulture staff noticed that the sod was hardly attached to the soil underneath.  We began searching for white grubs since the symptoms sure fit!  Imagine our surprise to discover ¾”, grey caterpillars crawling around in the thatch and soil.  Their feeding on turfgrass crowns and roots created the brown patches of dead grass that led homeowners to us.

Management is more difficult than other sod webworms that are surface-feeding. And cranberry girdlers aren’t very cooperative when using irritants for sampling.(It’s not that they don’t irritate, they’re usually too far down for the irritant to reach them.)

Birds can provide some natural control, but not in heavy infestations. Insect parasitic nematodes are one means of control, but must be applied early or late in the day to avoid sunlight.  They also must be watered in thoroughly to move them into and below the thatch layer where the food is.

Cranberry girdler infestations can be treated with Spinosad and certain formulations of Bt, but they are most effective when  larvae are young.  Scouting for larvae in July and August, when eggs are laid and young larvae are developing, will help with application timing.

Dose makes the poison, for this reason the late instar caterpillars that are large and deep in the ground are better protected from an insecticide application; because of their size the caterpillars must come in contact with a larger dose of insecticide to be killed. So instead of trying to control the caterpillars now  consider scouting for small caterpillars next year and treat with an insecticide if you find them. Insecticide applications should be followed with a water application to help move products into the feeding zone.

Damaged turf areas can be over-seeded *now or next spring to get your lawn looking good again. Over seeding operations are most successful if you follow these simple steps.

1.       Core aerate the lawn 3-4 different directions – just make Swiss cheese out of it.

2.       Apply 3 lbs. of grass seed per 1000 ft.²

3.       Apply lawn fertilizer at the recommended label rate.

4.       Water once per day for one month. You don’t have to stop mowing.

*Over-seeding after October 1 can potentially fail if the new seedlings freeze before being mowed a few times.