SOD WEBWORMS Contents:
Introduction
Most of the key turfgrass pest
sod
webworm species are Pyralid moths
in the genera Crambus,
Pediasia, Parapediasia and
Fissicambus spp. All the species spin threads of silk as they
move, webbing leaves and soil particles together, and often form
horizontal silk tubes in in the thatch. The single most important
species Parapediasia
teterrella, the bluegrass webworm
is detailed as an example for the typical webworm.
Identification
Adult bluegrass webworms will
reach lengths of 1/2" and hold their wings over their bodies when
at rest. They have a prominent snout and the adults are off-white
to tan in color. The larvae will range in size from 5/64" as first instars,
to 1/2" as seventh instars or, fully grown larvae. Larval body
color will vary from straw yellow to brownish green but will
always have distinct reddish brown spots on its abdominal and
thoraxic segments.
Distribution
The bluegrass webworm, a native
species, is very common in areas where Kentucky bluegrass is
grown. Its range consists of the eastern half of the United
States.
Hosts
All species live in the thatch
layer in grasslands, lawns, golf courses, cultivated pastures and
sod farms. Although Kentucky bluegrass and fine fescue mixtures
appear to be the most seriously impacted, most cool season
cultivated turfgrass species host one or more species. Evidently
slight variations in microhabitat can play a critical part in
delineating damage. In Maryland we have observed that severe
outbreaks (10-30 larvae per ft2) of the larger sod webworm and the
bluegrass webworms are restricted to turf areas receiving full
sunlight. Areas adjacent to fences, trees, shrubs or buildings
that provide partial shade host usually non damaging populations.
These shaded areas may also be less stressed during the drought
periods that exacerbate webworm injury.
Life Cycle
Depending on the sod webworm
species, length of growing season and geographic location, the
number of generations per year can vary from one to four or more.
The bluegrass webworm, for example, has 2 generations per year and
overwinters as mature larva within a silk-lined tunnel in the soil
or thatch. The larvae feed during the evening and pupate within a
cocoon at the end of its silk tube. In Maryland, adults emerge in
late May and early June, mate, and lay eggs for several days. A
second flight occurs in July and August with the mature larvae
overwintering. Adults of all species are active at night. During
the daylight hours, they rest in the thatch or on broadleaf plants
near the turf. Eggs are nonadhesive and randomly drop into the
thatch. Generation times require 4-10 weeks depending on the
temperature.
Damage
Although the sod webworm complex
represents over 20 species in the United States, the leaf
feeding damage and the
construction of the silken
tunnels appear to be common
behaviors. Generally the high risk period for damage is from
mid-July to the end of September when many of the cool season
grasses go dormant. The presence of 1-2 large species
caterpillars, such as the larger sod webworm, or 3-4 small
species, such as the bluegrass webworm, in Kentucky bluegrass or
fine fescue turf is sufficient to cause defoliation during late
summer. The damage is described as a browning of the turf but in
reality the green foliage is consumed during the night and what
appears after heavy feeding is the dead thatch layer. The feeding
is equivalent to a severe scalping of the turf with the mower.
Most bluegrasses will partially recover when fertilized and
watered to break summer dormancy. Fine fescues may be killed if
completely defoliated.
Larval damage can be expected 10-14 days after observations of heavy adult flight activity. Since adults are highly attracted to blacklights, this method can help indicate areas at high risk to damage. However, because of the natural high levels of insect predation on the webworm eggs and larvae, high trap counts may not always result in severe damage.
Sample larval populations using a visual thatch inspection or an irritant flush. Because the small first and second instar larvae are very difficult to find in the dense thatch, a visual inspection tends to underestimate populations. The pyrethrin or soap flush, if too concentrated, may also underestimate by killing the larvae immediately and allowing only the mature larvae to exit the thatch.
Excessive bird feeding activity in the turf may also indicate the presence or overabundance of mature larvae.
Biological-Biorational
Control
Webworms support a wide range of
native predators and parasites. The major predator of the eggs and
young larvae are ants, predatory mites and big-eyed
bugs. Older larvae fall prey to
birds, ground
beetles, parasitic flies and
wasps. Several pupal parasites are well established in the
northeast and together all these agents usually keep webworm
populations below aesthetic thresholds. Frequent use of
insecticides kill these beneficial species and their populations
may require 1-2 years to recover.
Several naturally occurring Beauveria fungal and Nosema and Phelohania microsporida diseases have been recovered from field collected larvae. However, the impact of these and other pathogens is poorly understood and probably greatly underestimated.
To date several commercial Bacillus thuringiensis (B.t.) products (Steward) and various species of parasitic nematodes (Vector, Cruiser) provide good larval control. Both products are limited, however, because of the restricted labeled uses and the costs for multiple applications required to control multiple webworm generations throughout the summer.
A plant extract from the Neem tree and known as azadirachtin has been developed to control different groups of insects. Turplex Bioinsecticide is a commercial preparation of this material and is labeled for sod webworm control. Recently, metabolites of bacterial fermentation known as spinosyns, have been shown to have insecticidal properties. Conserve SC is the commercial preparation of spinosyns.
Cultural Control
The warm season turf species such
as zoysiagrass and bermudagrass appear to be resistant to webworm
feeding. Tall fescue varieties are intermediate with Kentucky
bluegrass, fine fescue, hard fescue and perennial ryegrass
species, varying from highly susceptible to moderately resistant.
Unfortunately the selection for turfgrass insect resistance
appears less important and more difficult to obtain than other
genetic characteristics. Major success have been accomplished in
selecting for drought and disease resistance, pH and fertility
tolerances, green color, turf density and persistence.
Fortunately a newly discovered nongenetic "Enhanced Resistance" offers the plant breeders a combination of a high level of insect resistance, good turf persistence under drought conditions and some limited disease tolerance. This resistance is caused by a fungal endophyte. Endophytes are generally fungi or bacteria that live inside a plant but don't cause disease symptoms. They actually enhance a grass plant's survivability. At this time, a number of varieties of tall fescue, perennial ryegrass and fine fescues possess high levels of protective endophyte.
These endophyte infected grasses control insects in two ways. First they repell the insects from feeding, however, if webworms are forced to feed, they become poisoned and die. Maryland research has shown that the first instar and fourth instar larvae are highly sensitive but older larvae are slow to respond to the endophyte toxins. The young larvae die rapidly, 1-6 days after feeding, whereas older larvae stop feeding and die after 5-6 days.
Unfortunately no endophytes have been discovered in Kentucky bluegrass, but susceptible varieties with strong rhizome systems are more likely to recover than susceptible clump types. Irrigation and fertilizer applications in the fall will break bluegrass dormancy and stimulate regrowth. However, complete recovery may require 2 years and the weakened turf may become more susceptible to summer diseases.
Chemical Control
All species of webworms are easily controlled with any of the presently registered insecticides. Control is usually accomplished within 12-24 hours after treatment. The following insecticides have labeling for sod webworm control, however, some products have restrictive labeling and may only be applied by certified applicators:Pyrethroids: Astro, Battle, Scimitar, Tempo, and Talstar
Carbamates: Sevin and Turcam+
Organophosphates: Dursban, Diazinon*, Proxol or Dylox, Crusade+, Mainstay, Oftanol+ and Orthene.
*Diazinon is not registered for use on golf courses and sod farms
+ Restrictive labeling
by Dr. Lee Hellman, Department of Entomology and Dr. J. Kevin Mathias, Institute of Applied Agriculture