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
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 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.
adult (R.A. Casagrande)
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.
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
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
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.
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.
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).
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
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
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.
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.
from the University of Massachusetts Extension, 1999