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Causes Of Brown Spots In Lawn

One of the most common questions that I’m sure every lawn company hears is, “Why are there brown spots in my lawn?” No one wants the grass to be greener on the neighbor’s side of the fence. There are a few different reasons why you can get brown spots in your lawn. Today we’re going to identify the most common reasons for brown spots and help determine what might be causing yours.

The three most common causes we have seen for brown spots in your lawn are ineffective/inefficient watering or drought, billbug grubs, and sod webworm. Each of these three causes can cause the dreaded brown spots. However, if you look closely, you can determine which of these three is the culprit.

Let’s start with the most common: watering issues.

How To Identify Watering Issues

One of the ways to identify if drought is causing the brown spots is to tug on the grass in a brown area. If the brownness is caused by lack of water, the grass will be firmly rooted and it won’t pull up easily. Another way to test is take a screwdriver and push it down into both a green area and a brown area of your lawn. If the screwdriver easily penetrates the ground in the green area, but won’t easily penetrate the brown area it is likely that lack of water is causing the brown areas.

Another simple way to identify drought is to look at your lawn to see if the areas that are green are around sprinkler heads, or in the shade. We often get calls from people thinking that they have some kind of insect problem in their lawn, but upon inspection, there is a bright green patch of grass around each sprinkler head and then it is brown in between the heads. If it is green around the heads, but not elsewhere, lack of water or ineffective sprinklers is your problem.

Click here to learn more about how much water your lawn should be receiving.

Recognizing Grub Damage

Another common cause of brown spots in lawns is grubs. Both White Grubs and Billbug larvae can cause extensive damage to lawns. White grubs and billbug larvae are usually found within the top inch of the soil. They feed on the organic matter in the soil, including grass roots. Since they eat the grass roots, the sod in the damaged area will roll back easily like newly laid sod. The grass stems themselves usually stay intact since the grubs eat mainly the root. Another sign that you may have grubs/billbugs in your lawn is that you may see skunks, raccoons or birds digging for the grubs.

White grubs are the larvae stage of Scarab beetles, Japanese beetles, May/June beetles or masked chafers. They usually curl into a “C” shape and have a reddish-brown head and three sets of legs. The billbug larvae look very similar. They are a white color as well with a brown head but are legless.

An easy way to determine if you have grubs in your lawn is to take a shovel and in one of the brown areas use the shovel to peel back a section of sod. If you have a heavy grub infestation you will see grubs below the surface of the grass. If you find 6 or more grubs per square foot then you should consider taking action to control them.

It is important to keep in mind that healthy lawns can tolerate more grubs per square foot, and the damage won’t be seen as easily. Billbugs and other types of insects thrive in unhealthy lawns. That isn’t to say that healthy lawns don’t get them, because they do as well. But, by ensuring your lawn has sufficient water, and keeping it on a regular fertilizing schedule you can increase its ability to resist damage caused by insects and disease.

Grub Control Treatments

Professionals vary in their opinions of the best time to control for grubs. However, most agree that the young larvae are the easiest to kill. One common chemical used to treat grubs is imidacloprid. It is used as a preventative treatment that is applied to your lawn (often in June). By using this systemic, the chemical gets in the grass, so when the larvae begin actively feeding they will eat the chemical and die. It is important not to do the treatment too early, as the residual can wear off before the grubs begin feeding.

Sod Webworm Symptoms

The final culprit that may be causing brown spots in your lawn is sod webworm. Although not as common in our area, we have seen cases of sod webworm and they can cause extensive damage to your lawn.

The adults are light-colored lawn moths. They are ½ to ¾ inches long and have 2 fingerlike horns and a snout. The adults don’t damage your lawn; the damage is caused by the larvae. The larvae (caterpillars) are gray/tan in color with dark spots and brown heads. They usually range from 3/4 in. to 1 inch long.

The larvae become active in April/early May. They begin feeding and then emerge as adults in mid-May to mid-June. They scatter eggs into the grass in the late afternoon/early evening. These eggs hatch in a week to ten days and then the larvae feed until mid-summer. There are two generations. The 2nd generation hatches mid-late August and causes the most damage to the grass.

Larvae feed at night on the grass leaves and stems near the soil surface. During the day they hide in silk web-lined burrows. One of the early signs of a sod webworm infestation is moths flying a zigzagging pattern over your lawn at dusk. This is how they spread their eggs. If you look closely at the grass once the larvae have started eating, you will notice that the grass is missing/eaten, not just dead. The ends will have ragged chewing. This is one way you can distinguish between sod webworms and drought. If there are sod webworms in your grass, upon closer examination, you will often see green fecal pellets near the grass stems and within the top inch of the soil, you can find their silken tubes and webbing.

Treating Sod Webworms

If you believe you may have a sod webworm infestation you can mix ¼ cup detergent with 2 gallons of water and dump it in a 1 sq ft area of your lawn. In 1-10 minutes you should see the worms coming to the surface. A few webworms in your lawn is not cause for concern, but if you see more than 4-6 in a square foot area you should consider treating to avoid significant damage.

Common chemicals used to control sod webworms are Cyhalothrin, bifenthrin, cyfluthrin, and permethrin. When treating sod webworms with chemicals you want to water a couple of days before the treatment. This will help bring the grubs to the surface. It is also a good idea to mow before the treatment, to remove extra turf growth that can keep the chemical from penetrating.

Everyone wants a beautiful, green lawn in the summer where they can enjoy spending time outdoors with their friends and family. If you find that your lawn has unsightly brown spots, it is very likely that drought, grubs or sod webworms are causing the problem. Thankfully, all of these can be treated/prevented if they are diagnosed correctly.