Lawn Insects in Montana: Billbugs, Sod Webworm and White Grubs

How do I know if insects are killing my lawn?

Check before you assume. MSU's MontGuide on maintaining lawns says insect damage in turf is typically from root-feeding insects in the larval stage, that the damage often resembles drought stress, and that newly established turf is particularly vulnerable. Colorado State University Extension adds that most insects found in lawns are not harmful, and that even harmful ones are often not numerous enough to warrant treatment.

Three quick tests separate insects from dryness. First, the tug test: MSU says if a tug on the grass pulls up bits of sod, grubs may be feeding on the roots. Second, look at the base of the stems for sawdust-like material. Third, run a can test for water: if the brown area lines up with a weak sprinkler zone, it is probably coverage. Our guide to sprinkler repair covers that side.

Animals give clues too. CSU notes that birds feeding heavily in a lawn may indicate cutworms, webworms, billbugs or white grubs, and MSU says skunks or raccoons digging up turf can be an indicator of grubs.

What are white grubs, and which ones live in Montana?

White grubs are the larvae of several scarab beetles. MSU says May and June beetles (Phyllophaga and Polyphylla species) are the most common white grubs in Montana turf, and that the adults are often seen at porch lights in May and June. The adults do not damage lawns; the C-shaped larvae feed on roots under the soil most of the year, usually in the top few inches, and grow up to an inch long.

Damage shows as thinning or brown areas that peel up like loose carpet. CSU notes that most lawns have a few white grubs with no visible damage, and that large areas die only when populations are high.

Is the Japanese beetle in western Montana?

MSU's current lawn guide does not report it here. MSU says the Japanese beetle is established in several areas in Billings, that it prefers highly managed turf in good condition, and that grub damage shows as yellowing, stunting and death of turf in late summer. Adults also feed on the foliage of more than 300 plants. It is worth knowing what it looks like, but in Missoula and Ravalli counties a grub in the lawn is far more likely to be a May or June beetle.

What do billbugs look like, and how do I spot them?

Billbugs are weevils, not true grubs. MSU describes the larvae as smaller than white grubs (3/8 to 1/2 inch), white, C-shaped and legless, found just below the thatch. Young larvae feed inside the stem and crown; older ones feed on roots. Washington State University Extension describes the adults as dull gray or shiny black weevils 1/4 to 1/2 inch long with a long snout, and says Kentucky bluegrass is severely damaged by them.

The signature is in the stems. CSU says that if dead-looking stems break off at the soil line with sawdust-like material on the broken ends, billbugs caused it. MSU says that frass (insect excrement resembling sawdust) at the base of the turf can be diagnostic, and that brown, dying spots show in mid to late summer.

What are sod webworms?

The larvae of small moths. CSU calls the sod webworm one of the most common lawn insect pests in the Rocky Mountain region. The adults are small white and tan moths that flutter up from the lawn when you mow in late spring and summer; they do not feed on grass. Colorado State's IPM page names the cranberry girdler and the vagabond crambus as the two main turf species there, describes larvae living in silk-lined tunnels in the thatch by day and feeding on grass leaves at night, and says most species overwinter as larvae and have two to three generations a year, with overwintered larvae doing the most damage.

Damage starts as small brown patches of closely clipped grass that enlarge. CSU notes that adequately fertilized and watered lawns can usually withstand substantial webworm populations, and that birds often eat enough of them to prevent visible damage. MSU lists sod webworms among the other turf pests in Montana alongside ants, clover mites, leafhoppers and wireworms.

How do I check for sod webworms?

Use a soap flush. CSU's PlantTalk describes mixing a tablespoon of liquid dish soap in a gallon of water and pouring it evenly over a 3 foot by 3 foot area; webworms come to the surface within a minute or two. CSU's IPM page gives a similar method over one square yard and says larvae should surface within 10 minutes. The same page suggests treatment is worth considering only at 15 or more larvae per square yard in healthy turf, or 5 or more in stressed turf.

When is treatment actually warranted?

When you have confirmed the pest and the numbers. For white grubs and billbugs, MSU gives a common threshold of about one per square foot. Below that, a healthy lawn usually outgrows the damage. MSU also points to cultural causes: lawns that get heavy moisture while adult beetles are laying eggs are more susceptible to grubs, and turf that is overfertilized and overwatered is attractive to billbugs.

Grass choice helps over time. MSU says perennial ryegrasses and fescues with endophytic fungi can be highly resistant to billbugs, and WSU agrees that endophyte-containing grasses resist them. Overseeding a damaged bluegrass lawn with an endophyte-enhanced ryegrass or fescue is one long-term step. Our guide to Kentucky bluegrass versus fescue covers how those grasses fit together.

What control options does MSU describe?

Biological first. For white grubs, MSU names insect-parasitic nematodes in the genus Heterorhabditis and the bacterium Bacillus thuringiensis galleriae (Btg), most effective when applied in late July or early August, and describes both as non-toxic to beneficial insects, pollinators, birds and pets. For billbugs, MSU names nematodes in the genera Steinernema and Heterorhabditis. CSU notes nematodes and Bt products work against sod webworms only while the larvae are young.

Insecticides are available for each, and timing depends on the product. MSU says systemic treatments are most effective against billbug larvae in late spring to early summer, that aeration speeds penetration into heavily thatched turf, and that granular grub products must be irrigated in to reach the larvae. CSU's PlantTalk names chlorantraniliprole applied in June or early July as a preventive for lawns with a history of grub damage, and CSU's IPM page says to treat webworms two to three weeks after peak moth flight, in late afternoon or evening.

Protect pollinators: CSU says to mow before applying an insecticide to remove flowering weeds that bees are visiting, and MSU says to keep children and pets off the lawn for a day after application. With any herbicide or insecticide, the label directions are the law: the product has to be labeled for lawns, and the label sets the rate, timing and re-entry rules.

What else causes brown patches that look like insect damage?

Plenty. CSU's PlantTalk on brown spots says the most common cause is lack of water, and lists necrotic ring spot (a soil-borne fungus), dog urine and mites as others. MSU's lawn guide describes several fungal diseases in Montana lawns, from gray snow mold in spring to brown patch in hot weather. Our guides to dog spots on the lawn and what happens to a lawn under snow cover two of the most common lookalikes.

Getting a diagnosis

If you find larvae and are not sure what they are, your county MSU Extension office can help with identification; MSU's lawn guide directs readers to their local Extension agent for current recommendations. HJ Property Care is licensed for pesticide application, and our fertilizer and weed control program includes diagnosis before treatment: we confirm the pest and the numbers before anything goes down. This guide is part of our Montana Landscaping, Lawn & Irrigation Guide. Call (406) 215-7319 or request a free estimate.

More resources