Armyworms and Sod Webworms: Which Ate Your Lawn?

Two things separate the caterpillars from every other lawn pest, and both are visible without tools.

The grass has been chewed. Not wilted, not yellowed from below — bitten off. Blades cropped short, edges ragged, sometimes down to the stem.

It happened fast. UGA’s line on fall armyworms is that turfgrass can go from perfect green to complete brown within two to three days, because they arrive as a group and advance across a lawn as a front. Nothing else in this category moves at that speed.

Is it caterpillars at all?

This is a two-second check that sends you to the right shelf, because the three common lawn insects damage the plant in three different places.

Do the tug test first regardless. If the turf pulls back like a piece of carpet the roots are gone and this is not your problem — that is a grub, and nothing in this article will help.

The soap flush settles it

If the table leaves you unsure, there is a test that produces the animal itself, and NC State gives the recipe exactly:

“Mix 2 tablespoons of dish detergent in 1 gallon of water and apply to a 2-square-foot section of turf. Wait two to five minutes and examine any arthropods that come to the surface.”

The soap irritates anything living in the thatch and drives it up where you can see it. Two minutes, no cost, and it converts a guess into a count — which matters because the decision below is about how many, not whether.

Pick a spot on the edge of the damage rather than in the middle of it. The centre is where they have already finished; the advancing edge is where they still are.

And look at the right hour, or you will conclude you have nothing. NC State notes fall armyworm larvae “are most active in early morning or late evening”, while sod webworm larvae “feed after sunset.” Both hide down in the thatch through the middle of the day, which is exactly when people go out to inspect a lawn and find no insects on visibly chewed grass.

One pest sits outside that table entirely because it does not damage grass by eating it: fire ants build mounds rather than feeding on turf, so a lawn full of domed soil heaps is a different problem with a completely different fix.

Which caterpillar do you have

Both are moth larvae and both eat blades, but they behave differently enough that the diagnosis changes what you do.

Which of the two Florida actually gets

Both are on this state's own pest list — recorded as “Fall armyworms” and “Tropical sod webworms”. So the table above is not academic here: you need the field marks, because either one is plausible and they are treated on different triggers. They share that list with Southern chinch bugs and Mole crickets, none of which is a caterpillar — which is why the chewed-blade test comes before any product decision.

And note which sod webworm. Florida's record names the tropical sod webworm, which is a different species from the northern ones — it is the pest UF/IFAS publishes on separately, it works on warm-season turf, and it does not follow the northern seasonal pattern. If you are reading a northern guide for a Florida lawn, that is the mismatch.

Sod webworms feed at night and hide in the thatch during the day, which is why you can have a serious population and see no insects at midday. Armyworms are far less subtle — they are often visible on the grass, and a lawn suddenly full of feeding birds is a reasonable first clue.

One line in that table is a local variable rather than a constant: sod webworm patches expand in dry weather, because a lawn under moisture stress cannot outgrow the feeding. Rainfall in Florida runs — Pronounced summer wet season (June–September, 60%+ of annual rain); dry spring. Humidity: Very high (year-round 70%+ average) — and the practical meaning is that the leaf-wetness thresholds behind brown patch, large patch and gray leaf spot are cleared most nights for most of the year, not seasonally. Watering time is therefore a disease decision here rather than a convenience one, and a shaded or enclosed corner may never fully dry.. So the dry window here is the window in which a tolerable population turns into visible damage, and irrigation during it is doing pest-management work as well as agronomic work.

Where to look first, and it is not the middle of the lawn

Penn State adds the detail that turns “check the lawn” into “check there”. Sod webworms are not distributed evenly:

“Areas frequently infested include steep slopes, banks, and other locations that are difficult to water.”

That is a shortcut worth having before you mix a bucket of soapy water. The hardest part of the lawn to irrigate is the part where a webworm population turns into visible damage soonest, because the grass there has no spare vigour to outgrow the feeding. Start the soap flush on the slope, the strip by the driveway, the mound the sprinkler arcs over — not the flat middle where the lawn looks worst by area.

Two more Penn State details worth carrying to the lawn with you. The scale:

“Sod webworm larval damage often is observed as brown patches up to the size of a baseball in the lawn.”

A baseball, not a dinner plate. If the brown areas are large from the start, that is an argument for something else — this insect starts small and joins up.

And the birds, which the table above mentions but which leave a mark of their own:

“In some instances, the brown patches are punctured with pencil-sized holes” — the result of “birds searching for the webworm burrows.”

So a brown patch peppered with narrow holes is not a disease with an odd texture. It is a patch that the birds found before you did, and the holes are corroboration rather than damage.

Which part of a Florida lawn that is

Penn State's "difficult to water" is a description of a site, and the soil decides how much of your lawn qualifies. In Florida the ground is Sandy state-wide, but the series is the point: Myakka fine sand is the Florida state soil and a Spodosol, one of the flatwoods family (with Leon, Immokalee, Pomona and Smyrna) defined by an accumulation of organic matter, aluminium and iron in the subsoil. High organic muck near Lake Okeechobee, and shallow limestone and coral rock in Miami-Dade where the soil is measured in inches rather than feet. Drainage: Both, and getting this wrong is the classic Florida mistake. The top foot is sand and drains and leaches accordingly — nitrogen moves through it fast, which is the whole reason the state fertilizer rules read as they do. But Myakka, the state soil, is a Spodosol, and UF/IFAS records that most Florida Spodosols are poorly to very poorly drained: there is a weakly cemented organic hardpan 12 to 40 inches down, and the water table above it rises seasonally to within about 6 to 12 inches of the surface. So a Florida lawn can leach fertilizer in the morning and sit on a perched water table in the afternoon. Test the depth to that hardpan before blaming compaction, and expect the wet season to behave nothing like the dry one. South Florida is the exception again — shallow coral rock in Miami-Dade, and organic muck near Lake Okeechobee.

Read that as a warning about the whole property rather than about one bank. Where the soil sheds water quickly, most of the lawn behaves like Penn State's hard-to-water ground during a dry spell, so the "look at the slopes first" shortcut helps less here — the droughty zone is not a corner, it is the default. In Florida the useful version of the advice is the opposite way round: the parts holding moisture are the parts least likely to show early damage, so search everywhere else.

And the same soil sets how fast the lawn recovers once feeding stops, which is the other half of the decision. Slightly acidic to neutral (6.0–7.0), strongly alkaline near coastal shell soils and South FL coral That is the ground the regrowth has to come out of, and UF/IFAS Extension Soil Testing Laboratory (ESTL) is where to establish what it is actually short of rather than guessing — the recovery this article describes later assumes the grass has something to recover on.

The thresholds differ on purpose

For fall armyworms, UGA gives a real number: three to five caterpillars per square foot in lawns and pastures is the point to begin treating. That is a strikingly low bar compared with grubs, and the reason is the speed — by the time a population is obvious the damage is already days from done.

For sod webworms, a number does exist, and it comes with its own hedge attached. Penn State:

“The economic threshold for sod webworm larvae has been suggested to be 4 to 6 per square foot but can be variable.”

Note the wording — “has been suggested” and “can be variable.” That is a real published figure, and it is not offered with the confidence UGA’s armyworm number carries. The practical reason is the insect: sod webworms are nocturnal and concealed, so a daytime count of a square foot is measuring how well you flushed rather than how many are there. Use 4 to 6 as the yardstick once a soap flush has actually brought them up, and use the appearance of expanding closely cropped patches as the trigger when it has not.

An armyworm decision is arithmetic on a confident number. A sod webworm decision is observation with a hedged number available to check it against. Both beat the alternative, which is treating because the lawn looks bad.

Armyworms arrive; they do not live here

This is the fact that changes the calendar, and it is the one most lawn advice leaves out. Fall armyworm does not overwinter across most of the country. UGA:

“Fall armyworms cannot overwinter in North Georgia. They may survive a mild winter (like this past year) in North Florida and extreme South Georgia.”

What happens instead is an annual migration:

“Each year, storms bring the adult moths north on wind currents.”

“Succeeding generations march northward through the state each summer, traveling on weather fronts and storms.”

So outside a narrow Gulf band, this is not a resident pest with a treatment programme. It is an arrival, it comes late in the season, and UGA’s own timing is that “in late summer, almost every year, caterpillars invade pastures, hay fields, and home lawns throughout the state.”

For Florida that reads as follows. You are in or adjacent to the band UGA describes, so the arrival is early and there may be several generations in a season — the multiple-generations warning applies to you rather than to the North. Armyworms are on Florida's own pest list, so this is a real risk here rather than a theoretical one.

Why Bt fits, and where it stops

Bacillus thuringiensis is a bacterium that has to be eaten to work, and it only affects the larvae of moths and butterflies. On a lawn that means it hits armyworms and sod webworms and leaves bees, beetles, earthworms and everything else alone — a level of selectivity no broad-spectrum insecticide can match.

Now the limit, which is the whole reason people conclude Bt “does not work”: UGA notes Bt products are effective only on young worms, half an inch or smaller. On larger larvae it does very little.

So Bt is not a weaker version of an insecticide, it is a product for a narrow window — an infestation you caught in its first days. Measure a larva against your fingernail before you buy. Under half an inch, Bt is the best choice available. Over it, you are choosing the wrong tool for good reasons.

Two more practical notes: Bt degrades in sunlight, so late afternoon or evening application gets more out of it, and because it must be ingested it needs to be on the leaf surface the caterpillars are actually eating — this is a spray, not something to water in.

If the larvae are already large

Once the caterpillars are past that size, the options change. UGA lists the common treatments for fall armyworms as carbaryl, the pyrethroids (the actives ending in -thrin), spinosad, and insect growth regulators. Spinosad is also specifically available for sod webworm management and is very effective against caterpillars.

This is worth marking, because the same active is wrong one layer down. A pyrethroid binds to thatch and never reaches a larva feeding in the soil, which is why grub preventer vs grub killer rules the whole class out for white grubs. Surface chewers are the exception: they are up here where the pyrethroid is.

That is one of the few places on this site where reaching for a pyrethroid is actually correct — these are surface-feeding insects sitting on the leaf, not larvae buried in the root zone, so the binding problem that makes pyrethroids useless against grubs does not apply.

Before that, the local layer: Florida has its own rules sitting on top of the label — 10-ft fertilizer-free buffer required around water bodies in most jurisdictions — administered by Florida Department of Agriculture & Consumer Services, Division of Agricultural Environmental Services. And check the grass: the dominant lawn grass in Florida is St. Augustine, and a pyrethroid label that covers one species does not automatically cover another.

Whichever you use, the same discipline as everywhere else: treat the affected area and its margin rather than the whole property, and read the label for the grass species and the re-entry interval. And spray in the evening — these insects feed at night, so that is when the treated surface meets them.

Where this lands in the Florida summer

An outbreak does not arrive into an empty calendar. These are the state's own summer tasks from our research, and a caterpillar front lands on top of them:

• Scout weekly for chinch bugs in hot, dry spots near pavement
• Watch for gray leaf spot during the wet season
• Respect local summer fertilizer blackout periods (June 1 – Sept 30 in many counties)

Read that list against this page before you spray. If watering or disease is on it — and on most state lists it is — then a browning lawn in Florida has competing explanations that cost nothing to rule out, and the blade test rules them out in one look: chewed edges mean caterpillars, intact blades mean something else.

And for an outbreak specifically, a local alert beats a national article. This is the one pest on the site where timing is announced rather than predicted — UF/IFAS Turfgrass Science at University of Florida IFAS is the office that publishes when a front is moving through, and UGA's own account of that northward migration is why that is worth checking in late summer rather than reasoning from a calendar.

The lawn recovers from this

Here is the genuinely reassuring part, and it is the opposite of the grub story.

Armyworms and sod webworms eat leaf tissue. The crowns and roots survive, which means the plant has everything it needs to regrow once feeding stops. A lawn that looks devastated after an armyworm front usually greens back up with water and a modest feeding — the damage was to this month’s growth, not to the plant.

But regrowth needs growing season, and that is the part the calendar decides. A front that arrives with two months of growth left is a cosmetic event. The same front arriving three weeks before the lawn shuts down for winter leaves a thin, chewed stand going into the cold, and UGA’s timing — invasions “in late summer” after a northward migration — puts the risk deliberately close to that edge.

In Florida the season closes at first frost: Mid-December (Panhandle/Tallahassee), January (Orlando/Tampa); frost-free in South FL (Miami). Count backwards from that. A warm-season lawn does the opposite: growth slows toward first frost rather than accelerating, so a late outbreak on St. Augustine has less recovery time than the same date would give a northern lawn. Feeding a warm-season lawn late to force recovery is the wrong answer — it goes into dormancy carrying growth it cannot harden. How much of that recovery is automatic depends on the grass. St. Augustine, the dominant lawn grass in Florida, spreads by runners or rhizomes — so where feeding thinned the stand rather than just stripping leaves, the lawn closes its own gaps and overseeding is optional rather than the plan. The local window is undefined.

Where an outbreak has genuinely thinned the stand, the repair is ordinary overseeding — when to overseed your lawn covers the method, and the overseeding window by state table covers whether you are inside the window to do it now.

One of them you can prevent

The two pests split again here, and it is the most useful asymmetry on the page.

Armyworms cannot be prevented locally, because they are not local. They arrive on weather fronts, as above, so there is nothing on your property to manage between seasons — only watchfulness in late summer.

Sod webworms live here. UGA:

“Larvae overwinter in the thatch, then start feeding again when the weather warms in the spring.”

Which puts the lever in your hands, and it is a thatch lever rather than a chemical one:

“Do not over-fertilize the grass with nitrogen, which may enhance the growth of the grass and thatch accumulation near the soil.”

“Greater grass growth and thatch accumulation can ultimately increase the sod webworm attack.”

And the summary line worth remembering, because it is the whole cultural case:

“Well-maintained turfgrass is relatively resistant to webworm damage.”

Note what that does to a common instinct. Feeding a lawn heavily to push it through a webworm problem builds the thatch the larvae overwinter in — so the response that feels like helping is the one that stocks next spring’s population. UGA’s mowing instruction is deliberately species-specific rather than a number: “Mow the lawn regularly but make sure to set the mower at the recommended height for the grass species.”

For Florida that means the recommended height for St. Augustine, not a generic figure — our mowing height guide carries the species ranges. And because the larvae overwinter rather than migrate, the prevention work sits in the autumn list rather than in an outbreak week. These are Florida's own autumn tasks from our research:

• Apply fall pre-emergent by early October
• Final fertilization with potassium emphasis (where permitted)
• Treat large patch preventively as soil temps drop below 70°F

The nitrogen timing on that list is the one that matters here, and the local window is April–May and September–October (check local summer blackout ordinances). Autumn feeding on a cool-season lawn is root storage rather than top growth, which is the version UGA's warning does not apply to — the growth it warns about is the lush summer flush. Feed on the local schedule and the thatch does not build; feed off it and you are managing the pest in the wrong direction.

The honest summary

Caterpillars are the most alarming lawn pest and among the least damaging in the long run. They work fast, they are visible, and they eat the part of the plant that grows back.

Which means the mistakes are timing mistakes. Bt is the right product and it only works on small larvae. Pyrethroids are the wrong product for grubs and the right one here. And the fastest way to waste a season is to treat the yellow patch you noticed without first tugging the turf to check whether the roots are still attached.