Achyra rantalis Guenee Lepidoptera Pyralidae

Natural History

Distribution. This webworm is found throughout the eastern United States west to the Rocky Mountains, and also in California. Despite its wide distribution, it rarely is damaging except in the southern Great Plains region. Garden webworm also occurs in eastern Canada, in most of Mexico, and throughout the Caribbean. It is native to North America.

Host Plants. Garden webworm is quite similar to alfalfa webworm, Loxostege cereralis (Zeller), and beet webworm, L. sticticalis (Linnaeus), in most aspects of its biology, including host range. Though its biology is not well documented, it is known to attack such vegetables as bean, beet, cabbage, cantaloupe, cucumber, chard, corn, cowpea, eggplant, lettuce, onion, potato, pumpkin, squash, sweet potato, and tomato. Field crops including alfalfa, clover, pea, soybean, and sugarbeet are injured. If allowed to become abundant in alfalfa, larvae may disperse when the alfalfa is harvested and may damage nearby crops, including such crops as corn and cotton, which are not normally eaten. Grain crops generally are avoided. Weed hosts include dock, Rumex spp.; lambsquarters, Chenopodium album; Parthenium sp.; pigweed, Amaranthus spp.; ragweed, Ambrosia spp.; saltbush, Atriplex patula;

smartweed, Polygonum spp.; and sunflower, Helianthus spp. Pigweed and lambsquarters are regarded as favorite weed hosts.

Natural Enemies. Several parasitoids of garden webworm are known, but there is little information on their relative importance. Among the wasps are Cremnops vulgaris (Cresson), C. haematodes (Brulle), Apanteles conanchetorum Viereck, A. pyraloides Muese-beck, Cardiochiles explorator (Say) (all Hymenoptera: Braconidae), and Phytodietus rufipes (Cresson) and Diadegma pattoni (Ashmead) (both Hymenoptera: Ich-neumonidae). Among the fly parasitoids are Eusisyr-opa blanda (Osten Sacken), E. boarmiae (Coquillet), Hyphantrophaga hyphantriae (Townsend), Lespesia archi-ppivora (Riley), Lixophaga variablis (Coquillet), Nemor-illa pyste (Walker), Patelloa leucaniae (Coquillet), Pseudoperichaeta erecta (Coquillet), Stomatomyia parvi-palpis (Wulp) and Winthemia quadripustulata (Fabri-cius) (all Diptera: Tachinidae).

Life Cycle and Description. A complete life cycle requires about 40 days. The number of annual generations is thought to be 3-4 throughout its range. In Texas there are four flights of adults: they occur in May, late June-early July, early August, and mid-September. However, because of overlapping generations it is difficult to discern separate flights.

Egg. The oval eggs initially are nearly transparent, becoming cream or yellowish in color. They measure about 0.64 mm wide and 1.1 mm long. The flattened, overlapping eggs are deposited in clusters of 8-20, usually on the underside of foliage. Total fecundity is estimated at 300-400 eggs. Duration of the egg stage is 2-5 days, but averages 2.8 days.

Garden webworm eggs on foliage.
  1. The larva generally is pale green dorsally and yellowish-green ventrally. It measures 21-24 mm long at maturity. The head and thoracic plate are yellowish but marked with dark spots. The thoracic plate markings are in the form of two dark bars on each side. The body is well-marked with raised, dark spots enclosing a paler area from which emerges a stout black hair. This is especially evident in the lateral spots below the spiracles, which appear almost ring-like. There are six such spots on each abdominal segment. A light mid-dorsal stripe occurs dorsally along the center of the abdomen. Mean head capsule widths (range) are 0.25 (0.20-0.26), 0.37 (0.32-0.40), 0.58 (0.55-0.70), 0.92 (0.80-1.00), 1.10 (1.05-1.20), and 1.34 (1.251.50) mm, respectively for instars 1-6. Mean overall body lengths (range) are 2.1 (1.6-2.5), 3.7 (2.2-4.3), 6.1 (4.0-9.3), 9.5 (6.6-13), 11.0 (9-14), and 13.4 (9-20) mm, respectively. During the first instar, larvae normally are gregarious, but disperse afterwards. If disturbed, larvae wriggle violently, drop from the plant on a strand of silk, or rapidly retreat within a silken tube, if one is present. Duration of the larval stage requires 14-28 days, averaging 16 days, and is followed by a prepupal period of 1-3 days.
  2. The mature larva pupates in the soil within a silk-lined cell, usually under debris or near the soil surface. The pupal case measures about 12.5 mm long and 3.5 mm wide, and is encrusted with soil. It is closed at the bottom but open at the tip, allowing easy escape of the moth. The pupa is light to dark brown and measures about 8.0-9.5 mm long. The tip of the abdomen bears three stout spines. Duration of the pupal stage requires 4-13 days, averaging 8.7 days.
  3. This small moth has a wingspan of about 17-43 mm, males averaging smaller than females. It is yellowish-brown or reddish-brown, but bears lighter and darker markings. The forewing and hind wing usually are crossed by a light irregular band, and sometimes a dark band. The hind wing is yellow-
Garden webworm pupa.
Adult garden webworm.

ish. The moth is quite variable in coloration, which may account for the many times it has been described as a new species. Adults feed on nectar from various flowers and commence oviposition about 3-6 days after emergence.

Elements of garden webworm biology were given by Sanderson (1906), Sanborn (1916), Kelly and Wilson (1918), Poos (1951), and Smith and Franklin (1954). Capps (1967) provided a description of several stages. A key to some webworm larvae, including garden webworm, was published by Allyson (1976). A key to the adults was provided by Munroe (1976).

Damage

Larvae feed only on the leaf epidermis during the first two instars, skeletonizing the tissue. Thereafter they consume the entire leaf. Eventually, they defoliate plants, consuming all except the stems and major veins. Webworms usually wrap young leaves in a loose web and feed within the protection of the web. During periods of abundance, entire plants are shrouded in webbing. This webbing is not diagnostic, however, because beet webworm and alfalfa webworm also display this behavior.

Management

Infestations often result from the presence of weeds in crop ields. Destruction of weeds can deter oviposi-tion. The other major source of webworms is alfalfa ields, as adults disperse at maturity and larvae disperse when the alfalfa is cut. Early harvesting of alfalfa, especially if the larvae are young and incapable of long distance dispersal in the absence of food, can reduce webworm numbers. Alfalfa can also be treated with insecticide before harvest if it is heavily infested. Insec-ticidal suppression of webworms is normally accomplished by foliar applications, though this is seldom warranted. The bacterium Bacillus thuringiensis provides some control.

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