Loxostege sticticalis Linnaeus Lepidoptera Pyralidae

Natural History

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.

  1. Beet webworm eggs are flattened, oval in shape and measure about 1 mm long and 0.7 mm wide. They occasionally are deposited singly, but more often in small clusters, usually in a single row, with individual eggs overlapping slightly. They are usually found on the underside of leaves, though females sometimes deposit eggs near succulent plants on dry twigs and clods of soil. Females each deposit 200-300 eggs, and oviposit over a broad temperature range of about 20-32°C. Duration of the egg stage under field conditions is normally 3-5 days.
  2. Larvae are mostly green or yellowish green, but sometimes darker. A pronounced dark stripe is located dorsally, and a broken dark stripe on each side; each dark stripe is bordered on each side by a white stripe. Despite the presence of the stripes, perhaps the most striking feature is the numerous white circular spots on the body segments. Each circular marking consists of a dark spot from which protrudes a hair, and is surrounded by a white ring. Larvae display five instars and grow about 4 mm long at hatching to 20 mm at maturity. Total larval development time is 12,18, and 29 days when reared at a constant temperature of 32°, 26°, and 22°C, respectively. Under normal field conditions, duration of the larval stage is about 17-20 days. Larval feeding occurs over a temperature range of 15-44°C, but the optimal temperature is about 30°C. Larvae often construct a silken tube leading from a protected area on the host plant or in the soil, to the feeding site, and sometimes web together leaves. This webbing behavior is the basis for the common name. (See color figure 68.)
  3. At larval maturity insects enter the soil and construct a silk-lined cell that varies from 2.5-5.0 cm long. The cell is oriented vertically, with the uppermost end within 1 cm of the surface. Pupation occurs within the cell, with pupae changing from yellow to brown as they mature. They measure about 12 mm long. Duration of the pupal period is related to temperature, with development periods of 35, 16, 9, and 6 days when reared at 18°, 22°, 27°, and 32°C. Duration of the pupal stage under normal field conditions is about 11 days. Not all larvae that enter the soil proceed to immediate pupation, as many enter diapause in the prepupal stage. The proportion of each generation entering diapause varies; in Montana the proportion of first generation larvae that enters diapause varies from 0.5-60%. Induction of diapause is related to day length; short day length (less than 13 h of photoperiod)
Loxostege Sticticalis
Beet webworm larvae.

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

Adult beet webworm.

keys by Munroe (1976) and Capinera and Schaefer (1983). The larva was included in the field key by Capinera (1986).

Damage

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.

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. Thus, adult population census is an important element of population monitoring. However, sterility is common in adults, and their presence does not necessarily indicate impending damage; rather, it should serve as a stimulus to initiate careful monitoring of eggs and larvae.
  2. Beet webworm populations are usually suppressed by application of insecticide to foliage. Bacillus thuringiensis provides some control. Populations are infrequently damaging, so insecticides should not be applied unless high population densities of 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 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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