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University
of Rhode Island GreenShare Factsheets
Sod
Webworm
Papapediasia teterrella
Lepidoptera: Pyralidae
There are several species of webworms which attack turfgrass
in New England. All of the species spin threads of silk as
they move, webbing leaves and soil particles together, and
often form horizontal silk tubes in the thatch. The single
most important species, Parapediasia teterrella, the
bluegrass webworm, is detailed as an example for the typical
webworm.
The adults are buff-colored moths
which are 12 to 19 mm (1/2 to 3/4 inch) long. They sometimes
have a small, dark line
on the top of each wing cover. Two small fingerlike projections
are visible at the front of the head and look like a snout
(hence, the name "snoutmoth"). When the moth is at rest, the
wings wrap around the body, giving it a tubular shape. Adults
will fly upward when disturbed by movement (a person walking
by, a mower approaching), and will move a short distance before
darting back into the grass.
The caterpillars (the stage that actually damages turfgrass)
reach up to 25 mm (1 inch) in length when fully grown, but
are only 3 mm (1/8 inch) long at the beginning of their development.
The color pattern varies with the species and plant source,
but most webworms are greenish, graying or brownish, and usually
have dark spots scattered along the body. In most species,
the head capsule of the larger stages of caterpillars will
be relatively light brown with some dark markings.
There are several species of webworms
present in New England, some of which complete one generation
per year and some of
which complete two generations per year. Most species spend
the winter as large caterpillars in "hibernacula," which are
tent-like structures which larvae weave to provide extra protection
against temperature extremes, a few inches below the surface
of the lawn. When the caterpillars become active again in the
spring, they feed for a short period before pupating and emerging
as moths. Female moths flit around just above the surface,
popping out individual eggs as they fly. The eggs which land
in turf hatch about a week later into small caterpillars. These
begin feeding almost immediately, on the leaf tissue above
the thatch. Caterpillars go through six to ten molts as they
feed and grow. Because there are several webworm species present
and because weather conditions vary from one year to another,
there appears to be considerable variation in development of
webworms. Eggs, small caterpillars, large caterpillars, pupae
and adults can be found at any given time in turf in July and
August.
Sod webworms feed on a variety of grasses. While they
often thrive on lush, healthy grass, this is also the kind
of grass which is most able to withstand insect stress. Webworms
commonly attack Kentucky bluegrass, perennial ryegrass, fine
fescue and bentgrass maintained at a variety of mowing heights.
Webworm damage starts as small
yellow or brown patches in the leaf blades in lawn areas.
As the caterpillars feed
and grow, the patches gradually increase in size. Caterpillars
are nocturnal, so they will not be noticed on the surface during
the day. However, if a turf manager checks the area and teases
the turf blades apart gently, he or she will notice tunnels
or burrows in the thatch. Often these burrows will be lined
with green, pellet-like matter. This is "frass," or caterpillar
excrement.
By midsummer, large sections of the lawn may be destroyed
by the sod webworm caterpillars, which prefer sunny areas.
Webworm damage is particularly severe in droughty conditions,
in part because the turf is less able to recover from caterpillar
feeding. Damage is usually most apparent in July and August,
when temperatures are highest and cool season grasses are not
growing vigorously. Affected areas recover slowly from webworm
feeding and often are overrun by weeds.
The easiest way to determine whether sod webworm caterpillars
are present in a turf setting is to conduct an irritating drench
or soapy flush. This is done by putting one or two tablespoons
of lemon-scented dish detergent in one or two gallons of water
and pouring the soapy solution over an area which is about
two feet on each side. The caterpillars will be irritated by
the solution and will wriggle out of the thatch and up to the
surface, where they can be counted. Most caterpillars will
respond to the soapy solution within two to five minutes. This
technique is particularly helpful for determining what stages
(sizes) of caterpillars are present.
The soapy flush and the visual inspection should be conducted
on the edges of the affected patches, because this is where
the caterpillars will be concentrated (they will have already
moved out of the centers of the dead patches).
Webworms are relatively susceptible
to several turf insecticides, as well as some biological
control agents. The key to successful
control is to apply a control agent when most of the caterpillars
are still very small. Because webworms are nocturnal, traditional
insecticide applications should be made as late in the day
as possible. This will ensure that the insecticide is as "fresh" as
possible when the caterpillars leave their burrows in the evening
and begin foraging on the surface. The material should be watered
in lightly, just enough to move the insecticide off the blades
and into the upper thatch. If possible, the area should not
be mowed for two days after application.
One guideline for timing of application which seems to
work pretty well is to make an application two to three weeks
after a peak in moth flight activity. This allows time for
the moths to lay eggs and for the eggs to hatch into small
caterpillars, which are the most vulnerable stage.
There are several biological control options which appear
to work reasonably well for controlling sod webworms, as long
as the biological control agent is handled properly and applied
with adequate water and irrigation. Bacillus thuringiensis, a
bacterium which produces a toxin and paralyzes the gut of the
caterpillar, is available commercially for use on turf. The
bacterium will not kill the caterpillars quickly, but it does
paralyze the gut shortly after exposure, so feeding activity
slows soon after application.
An entomopathogenic nematode, Steinernema carsocapsae is
able to seek out caterpillars and penetrate the host, where
it introduces a different bacterium. This bacterium causes
a massive infection in the host caterpillar. The nematode then
reproduces inside the insect cadaver and moves on in search
of new victims. Field trials indicate that this nematode, available
commercially in a variety of formulations, can reduce caterpillar
populations substantially. However, the nematode is very vulnerable
to desiccation, so applications should not be made at mid day
and should be watered in (at least 1/2 inch, if possible) immediately
after application.
If a turf area is scheduled for renovation, endophytic
tufgrasses (certain cultivars of perennial ryegrass and
fescues) should be considered. The endophytes provide the
turfgrass with a level of resistance to caterpillar activity;
and so the turf seems to experience considerably less damage
than non-endophytic counterparts.
Adapted from the
University of Massachusetts Extension, 1999
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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