Loxostege cereralis Zeller Lepidoptera Pyralidae

Natural History

Distribution. This native insect is primarily western in distribution, occurring from British Columbia to Quebec in the north and from California to western Texas in the south. It is highly dispersive, however, and sometimes attains the northeastern states. As a pest, its range is limited to the western Great Plains and Rocky Mountain region.

Host Plants. Alfalfa webworm has a broad host range, very similar to that of the better-known beet webworm, Loxostege sticticalis (Linnaeus), with which it is sometimes confused. Its common name is mislead ing, because though females oviposit in alfalfa and larvae consume alfalfa foliage, it is not routinely associated with this crop. In general, plants in the family Chenopodiaceae are preferred. However, when this insect is abundant and the favored plants are no longer available, a wide range of plants can be injured. Among vegetables susceptible to injury are bean, beet, cabbage, cantaloupe, carrot, eggplant, lettuce, parsnip, pea, pepper, spinach and likely many others. Grasses normally are avoided, but corn can be fed upon if nothing else is available. Alfalfa and sugarbeet are the field crops most often damaged during periods of great abundance. Among the weeds fed upon by alfalfa webworm, at least during periods of abundance, are dock, Rumex sp.; lambsquarters, Chenopo-dium album; common mallow, Malva neglecta; ragweed, Ambrosia sp.; redroot pigweed, Amaranthus retroflexus; saltbush, Atriplex sp.; Russian thistle, Salsola kali; sunflower, Helianthus annuus; and sweetclover, Melilotus sp. (Hoerner, 1933). In moth oviposition and larval food preference tests, Capinera et al. (1981b) reported that kochia, Kochia scoparia, and Russian thistle were favored for oviposition, though some eggs were deposited on all weeds tested, whereas only lambsquarters, redroot pigweed, and shepherdspurse, Capsella bursa-pastoris, were fed upon. Maxson (1948) reported finding eggs on dandelion, Taraxacum officinale; field bindweed, Convolvulus arvensis; kochia; lambsquarters; plantain, Plantago major; purslane, Por-tulaca oleracea; nightshade, Solanum sp.; ragweed, Ambrosia sp.; redroot pigweed; and saltbush. Adults feed on nectar from dandelion and field bindweed. (See color figure 9.)

Natural Enemies. Several natural enemies are described, though none are known to be particularly effective. In Colorado, for example, Hoerner (1933) reported that only 3% of the larvae were parasitized. Among the parasitoids are Cremnops vulgaris (Cresson), Meteorus campestris Viereck (both Hymenoptera: Braconidae), Aplomya caesar (Aldrich), A. trisetosa (Coquillett), Lespesia archippivora (Riley), and Phryxe vulgaris (Fallen) (all Diptera: Tachinidae).

Life Cycle and Description. In Canada, 2-3 generations are reported annually, but in the United States three generations seem to be normal. A generation can be completed in about 40 days under optimal conditions. As with beet webworm, mature larvae overwinter and the proportion of each generation that diapauses increases as the summer progresses. In Colorado, the first flight of moths occurs in May, followed by additional flights in June-July and September. However, Capinera et al. (1981a) suggested that the first two flights might both result from protracted emergence of overwintering larvae.

  1. Moths begin oviposition within a few days of emergence, and continue to oviposit for about two weeks. The flattened, oval eggs are about 1 mm long and 0.7 mm wide. They are white initially, but eventually become yellow. They usually are deposited in clusters of 2-20 overlapping eggs on the underside of foliage. Although individual eggs greatly resemble those of beet webworm, the clustering arrangement of alfalfa webworm serves to distinguish them from the single-row oviposition pattern of beet webworm. Eggs of alfalfa webworm hatch in 4-6 days during warm weather. (See color figure 258.)
  2. The larval stage has six instars. Larvae increase in length from about 3 mm at hatching to about 25 mm at maturity. Mean head capsule widths are 0.24, 0.41, 0.64, 1.01, 1.36, and 1.75 mm during instars 1-6, respectively. Development time of larvae fed sugarbeet foliage and reared at 27°C was 2.7, 1.8, 1.9, 2.3, 2.4, and 5.5 days, respectively, for instars 1-6. The optimal temperature for larval development is about 30°C, and larvae fail to develop at 15°C. Young larvae are pale yellowish-green, but they become darker as they mature. The head and thoracic plate are dark during instars 1-3, becoming irregularly colored with light and dark blotches thereafter. During the final instar the larva bears a broad whitish dorsal line along its body, bordered by broad black dorsolateral stripes, a distinct contrast with the dark dorsal line of beet web-worm. Most abdominal segments bear six dark spots with light centers dorsally, with a single dark hair arising from each. Larvae sometimes spin a mat of silk on foliage, where they rest during the daylight hours. They also produce strands of silk which are used to web together foliage and to construct a long thin tube into which they may retreat if disturbed. The silk tube often connects to clods of soil or other protected retreats. Duration of the larval stage is about 17 days at 30°C, increasing to about 25 days at 25° and 35°C. Duration of the larval stage is extended to several months for larvae that enter diapause. During diapause they remain below-ground within the silken pupation cell and then pupate in the spring. (See color figure 67.)
  3. Larvae enter the soil to a depth of up to 2.5 cm when they are ready to pupate, where they construct a silk-lined cell. The pupa is yellowish initially, turns dark-brown with age, and measures about 25-30 mm long. The posterior, pointed end of the pupa bears eight small spoon-shaped appendages. This character serves to distinguish alfalfa webworm from beet webworm, which bears eight spines instead.

Duration of the pupal stage is often 14-21 days under field conditions, but requires only 8-9 days when reared at constant temperature of 25-35°C.

Adult. The moth is fairly small, measuring only about 2.5-3.0 cm in wingspan. It is grayish brown in general color, though containing some irregular black marks and a transverse cream-colored band on the distal margin of the forewings. Adults are attracted to light, and during periods of abundance tremendous numbers aggregate around light sources. Many observers have described the appearance of the moth flights as being equivalent to a blinding snowstorm.

Alfalfa webworm moths are often confused with adults of beet webworm, Loxostege sticticalis (Linnaeus). However, they are easily differentiated by viewing the underside of the wings. Both species have a narrow dark line along the distal edge of the wings, but whereas the line is complete in beet webworm is it broken in alfalfa webworm. (See color figure 205.)

The biology of alfalfa webworm was given by Hoer-ner (1933), Maxson (1948), and Capinera etal. (1981a,b). Keys to the moths were included by Munroe (1976) and Capinera and Schaefer (1983). The larva was included in the key to the genus, Losostege, by Allyson (1976), and in the field key by Capinera (1986); Capinera et al. (1981a) pictured the instars.

Damage

Larvae injure plants by consuming foliage. Young larvae skeletonize leaves, but older larvae consume leaf tissue completely, often leaving only the stems and large veins. They often web together leaves and feed within the clustered foliage. Damage potential varies greatly among crops. For example, larvae consume over 50 sq cm of sugarbeet foliage during their development, but only about 13 sq cm of alfalfa foliage (Capinera et al., 1981b).

Adult alfalfa webworm.

Management

  1. The eggs and larvae can be found by visual examination of plants, but both of these life stages are difficult to detect. Moths are attracted to light and can be captured in light traps. Adults also can be flushed during the day, especially toward evening.
  2. Alfalfa webworm populations are usually suppressed by application of insecticide to foliage. Bacillus thuringiensis affords some control. Populations are infrequently damaging, so insecticides should not be applied unless high densities of adults or larvae are observed.

Cultural Practices. Several practices can alleviate webworm damage. Tillage can disrupt and destroy overwintering larvae within their silken tubes in the soil. Destruction of preferred weeds before adult ovi-position flights can minimize the deposition of eggs within crops. Destruction of weeds after eggs hatching, however, tends to drive larvae to nearby crop plants. Construction of deep furrows with steep or slippery sides has sometimes been recommended to stop the advance of dispersing larvae.

0 0

Post a comment