Lepidoptera Pyralidae

Natural History

Distribution. There are several native pasture-dwelling webworms that occasionally damage crops.

Commonly they are called sod webworms, because they usually are associated with pasture and lawn grasses. They occur throughout the United States and southern Canada. Among the species known to cause damage are corn root webworm, Crambus caligi-nosellus Clemens; silverstriped sod webworm, Cram-bus praefectellus (Zincken); larger sod webworm, Pediasia trisectus (Walker); and striped sod webworm, Fissicrambus mutabilis (Clemens).

Food Plants. Sod and root webworms feed principally on pasture and sod grasses in the family Gramineae. However, if pasture or sod is tilled and the ground is planted to non-grass crops, they too may be injured by the residual webworm population. In addition to grasses such as bluegrass, corn, orchardgrass, rye, timothy, and wheat, some webworms have been known to attack alfalfa, cabbage, clover, mint and tobacco. Consumption of the latter hosts is unusual. Weed grasses such as crabgass, Digi-taria sanguinalis, and even some broad-leaf weeds such as sheep sorrel, Rumex acetosella, and aster, Aster eri-coides, are consumed by larvae. Corn root webworm displays a particular preference for plantain, Plantago lanceolata, and oxeye daisy, Chrysanthemum leucan-themum.

Natural Enemies. The natural enemies are not well known, but their impact is thought to be significant. Cockfield and Potter (1984) estimated 75% reduction in eggs within 48 h due to predation. Among those thought to be important are the mite egg predators Hypoaspis sp., Cosmolaelaps sp. (both Acari: Laela-pidae), and Parasitus sp. (Acari: Parasitidae); the ground beetles Anisodactylus rusticus Say, Amara cupreolata Putzeys, A. familiaris Duftschmidt, Calathus opaculus LeConte, and Stenolophus rotundata LeConte (all Coleoptera Carabidae); the rove beetles Meroneura venustula (Erichson), Neohypnus sp., Philonus sp., and Tachyporus jocosus Say (all Coleoptera: Staphylinidae) and ants, especially Phedole tysoni Forel. Birds also are common predators, and where webworms are abundant the sod or soil often is heavily disturbed by birds probing for larvae. Flies are not uncommon parasitioids, including Aplomya caesar (Aldrich), A. confusionis (Sellars), and Stomatomyia floridensis (Town-send) (all Diptera: Tachinidae). Wasp parasitoids known from larger sod webworm include Macro-centrus crambi (Ashmead), M. crambivorus Viereck, Apanteles crambi Weed, Orgilus detectiformis Viereck (all Hymenoptera: Braconidae), and Diadegma obscurum (Cresson) (Hymenoptera: Ichneumonidae). Macrocen-trus crambi and M. crambivorus have also been reared from corn root webworm. Species reared from striped sod webworm include Apanteles terminalis Gahan, A. ensiger (Say), and M. crambi (all Hymenoptera: Braco-

nidae) and Campoletis argentifrons (Cresson) (Hyme-noptera: Ichneumonidae).

Life Cycle and Description. Following is a description of larger sod webworm, but the other sod webworm species are similar except for the larger size of P. trisectus. Overwintering occurs in the larval stage. In Iowa and Tennessee, these larvae give rise to flights of moths in June, followed by an additional generation-producing flights of moths in August. Light trap catches from Ontario indicate two flights of moths for numerous sod webworm species (Arnott, 1934). Three generations are thought to occur for some sod webworms in the midwestern states, however.

  1. The eggs are dropped individually and randomly by females, while either at rest or while flying. The eggs are quite small, though large relative to other sod webworms. They measure 0.45-0.56 mm long and 0.31-0.36 mm wide. The eggs are white initially, and turn yellow with age. They are elongate-oval, with one end more broadly rounded than the other. Duration of the egg stage is 5-7 days. Dry air is lethal to eggs (Morrison et al., 1972), but they tolerate a wide range of temperatures (Matheny and Heinrichs, 1971).
  2. The larvae move downward upon hatching, usually hiding between the blades of grass. They produce silk readily, and if disturbed spin down on a silken thread. As the larvae increase in size they produce a silken tube or tubes beneath the surface of the soil, to which sand and soil particles adhere, as shelter. The larvae may leave the tube to feed, or if food is convenient it remain at least partially within the tube while feeding. The larvae may undergo 7-10 instars, but normally display eight. Duration is about 3.0, 3.1, 3.0, 4.0, 3.6, 4.1, 5.9, and 9.9 days respectively, for instars 1-8. Head capsule widths average 0.21, 0.31, 0.45, 0.67, 0.99, 1.32, 1.55, and 2.20 mm and body lengths 1-2, 2.5-3.5, 3.7-5.5, 6.0-8.5, 10-12, 12-18, 18-24, and 21-28 mm, respectively, for instars 1-8. The young larvae are reddish-brown with a blackish head capsule, but at instar four and thereafter the head becomes yellowish-brown. The larvae bear reddish raised plates from which setae arise. The thoracic and caudal plates tend to be brownish or black until the final instar when they become lighter. Larvae that overwinter can do so in nearly any instar, but this normally is accomplished by instars 2-5.

Larger sod webworm larva.

Sod Webworms Pupae
Web enclosing webworm larva attached to seedling beneath the soil surface.
  1. The larvae abandon their feeding tubes at maturity and pupate in the soil nearby. The pupal cell is oval and lined with silk. It measures about 14 mm long and 6 mm wide. The pupa measures about 11 mm long and 3 mm wide. The cell is placed very near the soil surface so the moth has no trouble in escaping. Pupation may require 5-15 days, but 7-8 days is normal.
  2. The moth's front wings and body are yellowish-gray or yellowish-brown, the hind wings lighter and silvery at the base. A line of dark scales usually extends along the mid-line of the wing almost the entire length, but then turns up to the anterior tip. This moth measures about 21-35 mm in wingspan. Moths are nocturnal, but begin activity at dusk. They hide during the day, usually on the underside of broad-leaf weeds. They apparently require water or dew, but have not been observed to feed on flowers. Longevity of adults is normally 7-10 days, though some individuals have survived for nearly a month. Females are believed to produce 200-250 eggs. (See color figure 213.)

The biology of corn root webworm was described by Runner (1914), striped sod webworm by Ainslie (1923a), silverstriped webworm by Ainslie (1923b),

Sod Webworm
Adult larger sod webworm.
Sod Webworm
Adult striped sod webworm.
Sod Webworm Life Cycle
Adult silverstriped sod webworm.

and larger sod webworm by Ainslie (1927). Forbes (1904) and Ainslie (1922) provided a brief description of several sod webworms. Artificial diets have been developed by Ward and Pass (1969) and Dupnik and Kamm (1970).

Damage

Damage often occurs when larvae feed on the leaves of grasses, but feeding can also occur at the soil line or even on the roots. Under high density conditions or if there is a shortage of leaf material, the plant stems and growing point of grasses may be eaten and the plants killed. Larvae commonly chew pits into the side of underground stems or leave the foliage ragged. Plant mortality is most common during periods of drought, and consumption of leaf material by webworms in the pasture environment often goes unnoticed during periods of adequate rainfall.

Management

  1. Moths are highly attracted to light and can be captured in blacklight traps. Larval populations are assessed by careful examination of the soil surface.
  2. Insecticides are needed only when crops immediately follow sod or pasture infested with webworms. Webworms normally redistribute themselves within a year, dispersing from crop plants to grass-dominated areas. Liquid and granular insecticides can be applied at, or shortly after, planting to protect the seedlings.

Cultural Practices. Rotation from sod or pasture to crops, especially corn, is risky if webworms have been abundant. Disking and tilling can destroy overwintering larvae, but intense soil disturbance is necessary. Both autumn and spring tillage are suggested for effective suppression.

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