What Insects Eat Grass? Identify and Treat Lawn Pests

What Insects Eat Grass? The Three Feeding Categories

When homeowners notice thinning turf, yellowing patches, or severed grass blades, the immediate question is often: what insects eat grass? The answer depends entirely on where the damage is occurring. Lawn-destroying insects are not a monolith; they are highly specialized feeders that target specific anatomical zones of the turfgrass plant. Understanding these feeding zones is the first step in accurate identification and effective eradication.

Turf entomologists categorize grass-eating insects into three distinct groups based on their feeding behavior: blade and stem chewers (above-ground feeders), root devourers (below-ground feeders), and sap suckers (vascular feeders). Each category requires a radically different diagnostic approach and chemical intervention. Applying a soil drench for a surface-feeding armyworm will waste your money, just as spraying a contact foliar insecticide will do nothing to stop a subterranean grub infestation.

The Blade and Stem Destroyers (Above-Ground Feeders)

These insects physically sever the grass blade or chew through the crown and stolons. Damage from above-ground feeders often mimics drought stress or fungal diseases, leading to widespread misdiagnosis.

Chinch Bugs (Blissus spp.)

Chinch bugs are notorious for devastating St. Augustine, Zoysia, and Bermuda grasses, particularly in the Sun Belt. Adults are tiny (about 1/6 inch long), black with white folded wings, while nymphs are bright red or orange. They pierce the grass stem and inject a toxic saliva that blocks the plant's vascular system, causing the grass to yellow, then brown, and die.

Armyworms and Cutworms (Spodoptera and Agrotis spp.)

These are the larvae of nocturnal moths. Armyworms are surface feeders that march in massive numbers, devouring grass blades overnight. A severe armyworm outbreak can strip a 5,000-square-foot lawn to bare dirt in 48 to 72 hours. Cutworms, conversely, hide in the thatch during the day and emerge at night to chew through the grass stem right at the soil line, leaving behind distinct, circular dead patches the size of a golf ball.

The Root Devourers (Below-Ground Feeders)

Subterranean pests are often the most destructive because they sever the plant's anchor and nutrient supply. According to University of Minnesota Extension, root-feeding insects are responsible for millions of dollars in turf replacement annually across North America.

White Grubs (Scarab Beetle Larvae)

White grubs are the larval stage of scarab beetles, including the Japanese beetle (Popillia japonica), European chafer, and June bug. They are C-shaped, milky white, with brown heads and six legs near the head. Grubs feed aggressively on grassroots from late summer through early fall, and again briefly in the spring. The primary visual symptom is turf that rolls back like a loose carpet because the root system has been entirely consumed.

Billbugs (Sphenophorus spp.)

Billbugs are weevils with a distinct, elongated snout. The adults chew into the grass stem to lay eggs, and the resulting legless, white grubs with brown heads tunnel down to feed on the roots. As noted by Purdue University Turf Entomology, the hallmark of a billbug infestation is the presence of 'frass'—a fine, sawdust-like excrement that accumulates at the base of the grass crown. If you pull up a dead grass stem and the inside is hollowed out and filled with sawdust-like material, billbugs are the culprit.

Grubs vs. Billbugs: A Diagnostic Comparison

Diagnostic Framework: How to Identify the Culprit

Do not guess which insect is eating your grass. Use these two field-tested diagnostic protocols to force the pests out of hiding and confirm their identity before purchasing any treatments.

1. The 'Tug Test' for Root Feeders

Walk to the edge of a brown patch where dead grass meets healthy green grass. Grab a handful of the green grass near the margin and pull straight up with moderate force. If the grass pulls up easily with no roots attached, or if the entire section of sod lifts like a rug, you have a root-feeding problem (grubs). If the grass breaks off cleanly at the soil surface, leaving the roots firmly anchored, suspect billbugs or crown-feeding insects.

2. The Soap Flush Test for Surface Feeders

This is the gold standard for identifying chinch bugs, armyworms, and cutworms. Mix exactly 2 tablespoons of lemon-scented liquid dish soap (such as Dawn) into 1 gallon of water. Pour this solution evenly over a 1-square-yard section of turf at the margin of the damaged area. The soap irritates the insects' cuticles, forcing them to the surface within 3 to 5 minutes. Count the insects that emerge. For chinch bugs, a threshold of 20 to 25 insects per square yard warrants immediate chemical intervention.

Targeted Treatment Protocols (2026 Standards)

Once you have identified what insects eat grass in your specific yard, select a treatment based on the pest's life cycle. The NC State TurfFiles database emphasizes that timing the application of active ingredients to the insect's most vulnerable instar (developmental stage) is more critical than the sheer volume of product applied.

Preventative vs. Curative Grub Control

For white grubs, you must decide between prevention and cure. Preventative treatments utilize Chlorantraniliprole (found in Scotts GrubEx and professional Acelepryn). This active ingredient is highly targeted, safe for pollinators once dry, and must be applied in late May or early June before the eggs hatch. It costs approximately $14 to $18 per 1,000 square feet. If you missed the preventative window and are facing an active fall infestation, you need a curative contact insecticide like Trichlorfon (Dylox). Trichlorfon degrades rapidly in the soil (within 7 to 10 days) but kills active grubs within 24 hours. Apply at a rate of 1.5 to 2 lbs of active ingredient per acre, followed immediately by 0.5 inches of irrigation to move the chemical into the root zone.

Knocking Out Surface Feeders

For chinch bugs and armyworms, soil drenches are ineffective because the pests live in the thatch and on the blades. Use a liquid contact insecticide containing Bifenthrin (Talstar P) or Lambda-cyhalothrin (Scimitar). Apply using a hose-end sprayer in the late afternoon when the insects are most active and UV degradation is minimal. Do not water the lawn for at least 24 hours after application to allow the chemical to remain on the grass blades where the pests feed. For an organic alternative, products containing Spinosad or Beauveria bassiana (a beneficial fungus) provide excellent control of chinch bugs and armyworms without harming beneficial nematodes in the soil.