All species of sod webworms
spin threads of silk as they move, webbing leaves and soil particles
together, and often form horizontal silk tubes in the thatch.
The adults are buff-colored moths which are 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. When the moth is at rest, the wings wrap around the
body, giving it a tubular shape. Adults fly out of the grass when they
are disturbed by movement, and will move a short distance before darting
back into the grass.
The caterpillar stage is when they really start to damage your lawn.
They reach up to 1 inch in length when fully grown, but are only 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.
Some webworms complete one generation per year and some of which
complete two generations per year. Most species spend the winter as
large caterpillars 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. Damage appears as yellow or
brown patches of closely cut turf. Damage is often mistaken for
diseases. 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, further examination
of the area and brushing of the turf blades will uncover tunnels or
burrows in the thatch. Often these burrows will be lined with green,
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
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 won’t be located in 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.