Distribution. Beet webworm is found in the northern regions of Europe and Asia, and apparently is an immigrant from Europe. In the United States and Canada, beet webworm is present from coast to coast, but is a pest principally in western sugarbeet-growing areas from Alberta and Manitoba in the north to Utah and Kansas in the south (Pepper, 1938).
Host Plants. Known principally as a pest of sugar-beet, this insect also feeds readily on table beet and chard. During periods of abundance over 80 species of plants are damaged, but infestation is normally limited to Beta vulgaris and to certain weeds. Among the vegetables occasionally injured are cabbage, cantaloupe, carrot, cucumber, garlic, lettuce, mustard, onion, pea, potato, pumpkin, rhubarb, spinach, and turnip. Grasses, including corn, are rarely eaten. Weeds readily consumed include lambsquarters, Chenopodium album; redroot pigweed, Amaranthus retroflexus; and Russian thistle, Salsola kali. These weeds commonly serve as the preferred oviposition site of moths, with larvae dispersing to other, less preferred plants, when the weeds perish or are consumed. Larvae have been reared successfully on such diverse flora as alfalfa;
onion; lambsquarters; sagebrush, Artemesia sp.; sunflower, Helianthus annuus; and Canada thistle, Cirsium arvense (Pepper and Hastings, 1941).
Natural Enemies. Several parasitoids are known from beet webworm in North America. Numerous other species have been identified in Europe and Asia, but none have been imported. Wasps seem to be the most important mortality agent, though this has been little studied. Among the fly parasitoids are Aplomya caesar (Aldrich), Euphorcera omissa (Reinhard), Lespesia archippivora (Riley), L. ciliata (Macquart), and Stomato-myia parvipalpis (Wulp) (all Diptera: Tachinidae).
Predators also are detrimental to webworm survival. Among the insect predators are potter wasps (Hymenoptera: Vespidae), digger wasps (Hymenop-tera: Sphecidae), robber flies (Diptera: Asilidae), and damsel bugs (Hemiptera: Nabidae). Numerous species of birds, particularly blackbirds (Icteridae), have been cited as contributing to webworm mortality, but no assessments of impact are available for North America.
Weather. Weather has been implicated repeatedly in the development of outbreak populations of beet webworm, and subsequent extensive damage. Damaging populations are generally limited to the Great Plains and Rocky Mountain region by too much precipitation to the east and too little precipitation to the west. Populations survive well in areas with 2.56.5 cm of precipitation monthly during the growing season. Also, mean temperature greater than 13°C is deleterious, which limits the southern occurrence of webworms (Pepper, 1938). The condition of pre-pupae as they enter the winter is considered critical. High temperature during autumn hastens larval development, shortens the feeding period, and reduces larval weight. Thus, cool weather during autumn promotes development of large larvae that have high fecundity during the following spring (Bykova, 1984).
Life Cycle and Description. One generation requires 30-40 days, and 3-4 generations occur annually. Moths first appear in June; thereafter, all stages of development are present until cold weather. Mature larvae overwinter in the soil, and though few larvae from the first generation diapause, an increasing proportion of larvae from each generation enters diapause as the season progresses.
induces diapause (Khomyakova et al., 1986). Pupation of overwintering larvae (prepupae) occurs in the spring. The posterior, pointed end of the pupa bears eight small spines. This character serves to distinguish beet webworm from alfalfa webworm, Loxostege cereralis (Zeller), which bears eight small spoon-shaped appendages instead.
Adult. Emergence of moths from the overwintering population occurs in May-July in Montana. In some years, large synchronous emergences follow protracted periods of warm weather. Other years, brief periods of favorable weather interspersed with unfavorable weather result in protracted emergence. The moths are grayish brown in general color, with irregular dark and light markings crossing the forewings. Most prominent of the markings are a dark border distally on the front wing bordered by a cream-colored band. When at rest, the wings are folded back to give the triangular form typically found in the family Pyr-alidae. The wingspan of beet webworm moths is about 21-22 mm. The moths may disperse in great aggregations, and are attracted to lights. Heavy flights do not necessarily precede high larval populations because infertility is common among females. Adults are commonly seen collecting nectar. A female-produced sex pheromone was identified by Struble and Lilly (1977). (See color figure 206.)
Beet webworm moths are often confused with adults of alfalfa webworm. 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.
Beet webworm was described by Gillette (1905), Marsh (1912c), Paddock (1912), Pepper and Hastings (1941), and Maxson (1948). Adults were included in
keys by Munroe (1976) and Capinera and Schaefer (1983). The larva was included in the field key by Capinera (1986).
The first two instars feed on the underside of foliage, skeletonizing the leaves. Large larvae consume holes in foliage, eventually eating all except the principal veins and stems. At high densities, fourth instar or older larvae may disperse long distances in dense aggregations, a behavior typical of "armyworms." It is under these high density, and dispersing conditions that so many plants are destroyed. Damage to crop plants also results when preferred weed species are exhausted and larvae are forced to seek alternate food sources.
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 egg hatching, however, tends to drive larvae to nearby crop plants. Crops planted into land immediately after alfalfa are at greater risk because not only it is a suitable host but in the later stages of its growth cycle it is often interspersed with numerous weeds.
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