TURF SCIENCE
Sod Webworms
by John C. Fech
Photos by Tom Eickhoff, UNL
Turf issues that are
“out of sight, out of mind” are the most difficult to deal
with. Damage from sod webworms most certainly fits into this category.
Damage symptoms
When looking at damage from sod webworms, there
aren’t a lot of easily recognizable symptoms. In fact, the injury
mimics damage from summer patch disease, soil compaction, billbugs, and
sometimes leaf infecting diseases, such as bipolaris leaf spot. They feed
on most turfgrasses including tall fescue, fine fescue, zoysiagrass,
bentgrass and Kentucky bluegrass.
Webworms are an aboveground feeder, but do so in a
rather discreet manner. As with most caterpillars, adult sod webworms do
not damage turf directly. Damage results from larvae feeding at night on
grass leaves and stems near the soil surface, hidden from view. During the
day, they hide within small tunnels constructed with silken webbing, which
extend through the thatch layer and into the upper levels of soil.
In most cases, the first sign of infestation is small,
ragged, tan to brown spots in the turf. Upon closer inspection, these areas
have a scalped appearance, as if they have been mowed closely with a dull
mower or roughed with a paint scraper or sander. As feeding continues,
affected areas enlarge and may converge with nearby infestations, taking on
the appearance of one large mass of damaged sod.
When webworm numbers are high, and the turf is under
stress, turf status can change from damaged to dead. Because most stressors
occur in midsummer, the most serious turf injury occurs then, even though
webworms are active from early spring through fall.
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| Sod webworm larvae. |
An adult sod webworm. |
Inspection for webworms
Adult sod webworm moths often exhibit a zigzag-esque
pattern of flying close to the sod at dusk. Egg laying occurs during these
flights. If you observe .5-inch, buff-colored moths fluttering about just
before twilight, it’s wise to look closely for the presence of
larvae. They usually hatch within one to two weeks of egg laying.
Because many other damaging agents can cause similar
injury, it’s important to be sure that webworms are present before
applying an insecticide.
The two most effective ways to inspect for and confirm
an infestation of sod webworms are:
1. Using a lemon-scented soap flush—To verify
whether sod webworms are responsible, mix up a “detecting”
solution. Combine one tablespoon of 1 percent pyrethrins or .25-cup of
lemon-scented household detergent or dishwashing liquid with 2 gallons of
water. Choose the most fragrant product over
brand names or flashy looking labels. Mark off several 1-square-yard
sections of the turf that you suspect are infested with webworms and apply
1 gallon of the solution to each section. Wait about 10 minutes and look
closely for worms. The solution works by
irritating the caterpillars, causing them to move to the surface where they
can be seen and counted.
2. Probing with a pocketknife—Carefully cut
through the sod and examine the top inch of soil and thatch layer for the
presence of larvae, silken tubes and webbing.
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| A close-up view of sod webworm injury. |
Life cycle
Sod webworms overwinter as partially grown larvae in
silken tunnels in the upper soil layers and lower sections of thatch. These
larvae resume activity and begin feeding in midspring. After six to eight
weeks, they finish feeding and pupation (a brief resting stage) and emerge
as adults. Adults rest in shrubbery and on turf blades during the day and
lay eggs at dusk. Small, first-generation larvae hatch from the eggs after
a week or two, and then feed until midsummer. A second and third generation
takes place during the remainder of the season. Depending on the region of
the country, it’s quite common for the generations of webworms to
overlap, creating a scenario where all life stages are present by late
summer or early fall.
Threshold for treatment
In many turf areas, sound cultural practices, such as
efficient irrigation, proper nutrition, compaction relief and mowing
practices, will usually allow turf to outgrow light to moderate webworm
injury. However, if 15 or more webworm larvae are present per square yard,
an insecticide application may be justified.
Control steps
If an insecticide application is necessary, several
steps can be taken to enhance performance. First, the turf should be mowed
and the clippings removed, which will reduce the amount of vegetation that
might interfere with the insecticide-to-insect contact. Second, irrigate to
drive the worms upward in the soil profile. A thorough (.5 to .75-inch)
application will decrease oxygen levels and make the silken tunnels less
habitable, encouraging upward movement into a more vulnerable location.
Next, apply the insecticide in late afternoon or early evening when
webworms are generally more active. Finally, lightly irrigate (1/8 inch) to
wash insecticide residue off turf blades. This is especially important for
granular applications. After the light irrigation, avoid heavy watering for
24 hours to prevent dilution of the insecticide solution.
Many insecticide products are available for
controlling webworms, including bifenthrin, cyfluthrin, deltamethrin,
fipronil, permethrin, carbaryl and acephate. Ease of application, price,
need for additives, residual and overall effectiveness are some of the
features to consider. Local, state and regional turfgrass conferences are
key events for the comparison of various products, as well as a good
opportunity to receive research updates and briefings from university
researchers and extension professionals.
John Fech is an extension educator specializing in
turf and ornamentals at the University of Nebraska-Lincoln.
