Agrotis orthogonia Morrison Lepidoptera Noctuidae

Natural History

Distribution. This native species is found in semiarid regions of the western United States and Canada. Its distribution is largely restricted to the western edge of the Great Plains and eastern portions of the Rocky Mountains, including southern Alberta and Saskatchewan, most of Montana, Wyoming, Colorado and Utah, and northern New Mexico. It also occurs in portions of adjoining states, and has been particularly troublesome in western Kansas.

Host Plants. Larvae generally feed on grasses and grain crops, and also on some weeds. However, they occasionally have been known to damage vegetables, including bean, beet, carrot, corn, onion, potato, and tomato. Most commonly damaged are the small grain crops such as wheat, barley, rye, oats, and millet, but other field crops such as alfalfa, flax, Sudan grass, sugarbeet, and sweet clover may be fed upon. Among weeds known to be consumed are sunflower, Helianthus annuus; tumble mustard, Sisymbrium altissi-mum; Russian thistle, Salsola kali; wild lettuce, Lactuca scariola; mallow, Malva sp.; and dandelion, Taraxacum spp.

The adults feed on nectar from flowers, preferring goldenrod, Solidago spp., sunflower, Helianthus spp., and rabbit brush, Chrysothamnus spp. In the absence of the preferred nectar sources other flowers are used, including snakeweed, Gutierresia sp.; Canada thistle, Cirsium arvense; fleabane, Erigeron spp.; and Russian thistle, Salsola kali.

Natural Enemies. Natural enemies are believed to play an important role in the occurrence of this species, but it is the interaction of weather and natural enemies that is critical. Larvae of this cutworm normally spend most of their time below-ground. However, wet weather in the spring months causes larvae to move to the soil surface, where they can be attacked by parasitoids and predators. The proportion of the population lost to natural enemies varies from about 20-70%. Among the wasps known to parasitize pale western cutworm are Meteorus leviventris Wesmael, Chelonus sp., Zele sp. (all Hymenoptera: Braconidae); Apanteles griffini Viereck, Paniscus sp. (both Hymenop-tera: Ichneumonidae); and Copidosoma bakeri (Howard) (Hymenoptera: Chalcididae). Other parasitoids include Bonnetia comta (Fallen), Mericia sp., Gonia aldri-chi Tothill, G. longiforceps Tothill, G. longipulvilli Tothill, Peleteria texensis Curran, and Periscepsia rohweri (Townsend) (all Diptera: Tachinidae); Anthrax molitor Lowen, Villa alternata (Say), V. willistoni (Coquillett), and Poecilanthrax sackenii (Coquillett) (all Diptera Bom-byliidae). Numerous avian and insect predators have been observed to feed on cutworm larvae; among the insects are leaffooted bugs (Hemiptera: Coreidae), assassin bugs (Hemiptera: Reduviidae), and ambush bugs (Hemiptera: Phymatidae), ground beetles (Coleoptera: Carabidae), and predatory wasps (Hyme-noptera: Sphecidae and others). The role of pathogens is uncertain; fungi and viruses seem to be unimportant, but bacterial diseases are sometimes suggested to be a significant mortality factor.

Weather. The abundance of pale western cutworm is directly related to precipitation patterns. Outbreaks of pale western cutworm rarely occur in areas with more than 12 cm of precipitation during the period of May-July, when pale western cutworm is in the larval stage. Within the semi-arid area of western North America inhabited by this cutworm, spring periods with fewer than 10 precipitation events exceeding 6.4 mm are followed by increases in cutworm number during the following year. Similarly, spring periods with more than 15 such precipitation events are followed by population decreases. The effect of the precipitation is to drive the larvae to the surface of the soil, where they are susceptible to attack by predators and parasitoids. Higher moisture levels may also favor spread of disease among the insects, but this is less certain.

Life Cycle and Description. There is only a single generation per year throughout the range of this insect. Eggs are laid in the autumn and hatch in the winter or early spring. Larvae feed until early June and then enter a quiescent prepupal period that may last for 40-50 days if the weather is warm. Pupation occurs in July or August, with moths common in late August-October. Not surprisingly, the active period of this species is shorter in the north, approximately April-September in Alberta, whereas in New Mexico the insects are active from February-October.

  1. Eggs are deposited in the soil at a depth of 612 mm, apparently singly or in small clusters. They are white, turn yellowish-gray and then slightly bluish as the embryo matures. In shape the egg is a slightly flattened sphere, measuring about 1 mm in diameter and 0.8 mm in height. The egg bears 27-32 ridges radiating from the apex. Incubation requires 30-50 days in the field, and embryos require a cold period before hatching. Under laboratory conditions, embryo development requires 11, 14, 21, and 33 days at 30°, 25°, 20°, and 15°C, respectively. They must have contact with moisture or high humidity in order to hatch. As noted above, hatching occurs early in the spring.
  2. The larvae feed below-ground for their entire life. They normally display 6-8 instars. Jacobson (1971) gave mean development times of 8, 6, 7, 7, 8, and 14 days (excluding the prepupal part of the terminal instar of another 13 days) for instars 1-6, respectively, when reared at 20°C. In contrast, Parker et al. (1921) gave mean development times of 11.2, 8.0, 9.4, 9.8, 11.3, 14.4, 22.6, and 29.6 days (excluding the pre-pupal period) for instars 1-8, respectively, under unspecified insectary conditions. Thus, larval development time in the latter study, which averaged 118 days, was more than twice in the former study, where development required only 50 days. The difference is even greater if development times at warmer tempera-

Pale western cutworm larva.

Pale western cutworm larva.

Head capsule of pale western cutworm.

ture are compared. Jacobson (1971) gave mean larval development times of only 29 and 24 days at 25° and 30°C, respectively. Head capsule widths are 0.27-0.31, 0.35-0.45, 0.48-0.67, 0.70-0.98, 1.00-1.50, 1.60-2.20, 2.30-2.80, 2.90-4.00 mm, respectively, for instars 1-8 (Sutter et al., 1972). Mean body lengths are reported to be 1.4, 4.0, 5.9, 9.3, 15.9, 24.6, 31.1, and 36.6 mm, respectively. The larva is gray or bluish gray, and is largely free from distinctive markings. The head capsule, which is yellowish-brown, bears two dark vertical bars. The larva attains a length of about 35-40 mm at maturity. At maturity the larva digs deeper into the soil, usually 5-15 cm, and forms an earthen cell with the aid of salivary secretions. While in this cell, the pre-pupa shrivels to a length of about 20-25 mm and its color changes to yellowish white. A period of inactivity follows, 30-75 days in duration, that is quite long compared to similar species, and likely is an adaptation allowing the insect to escape the heat and dryness of the summer in its hostile environment. (See color figure 55.)

  1. Pupation occurs within the cell formed by the larva. The pupa is yellowish initially, turning brown with time. The pupa measures about 13-19 mm long. Duration of the pupal period is 20-40 days.
  2. The adult is an attractive moth, gray with yellowish and brownish spots on the forewing, and white on some of the veins. The hind wings are whitish, but darker distally. The body is robust and clothed with long scales. The wingspan of the moth is 2540 mm. Although the moths are considered to be principally nocturnal, females fly and oviposit in late afternoon, and both sexes begin feeding at flowers about sunset. Females select loose soil for oviposition, avoiding hard or crusted surfaces. Fecundity is not well documented, but apparently females can produce 300-400 eggs. Adult activity usually ceases by mid-
Adult pale western cutworm.

night, principally because it becomes cool, but there are reports of mating occurring at this time. Length of adult life in the field is uncertain, but appears to be 7-14 days. (See color figure 246.)

The biology of pale western cutworm was described by Parker et al. (1921), Cook (1930), Sorenson and Thornley (1941), and Jacobson (1971). Rearing was described by Sutter et al. (1972). A sex pheromone was described by Struble and Swailes (1978). Keys to the larval stage were found in Whelan (1935), Walkden (1950), Capinera (1986), and in a key to armyworms and cutworms in Appendix A. Adults were included in a key by Capinera and Schaefer (1983).

Damage

The larvae feed below-ground just above the growing point of plants. Young larvae are not large enough to completely sever the plant, but this is accomplished routinely once the third instar is attained. This feeding usually results in death of the plant. In grain crops, entire fields may be killed. The larvae are reported to move underground from plant to plant, often feeding on just the small below-ground section of each seedling. They rarely move above the soil, and are not known to disperse long distances in an "armyworm" fashion in search of food.

Management

Sampling. The eggs and small larvae are very difficult to detect. Large larvae can be recovered from soil, but their densities are usually assessed indirectly from plant damage. Adult densities can be monitored easily with light traps.

Damaging populations are predicted on the basis of pattern of precipitation during the larval stage. The days of the larval stage when at least 6.4 mm of precipitation occur—termed "wet days"—are tabulated, and if they are less than 10, cutworm densities are expected to increase during the next year. In contrast, if the number of "wet days" exceeds 15, then cutworm populations are expected to decrease (Seamans, 1935). Two successive years of dry weather are required to cause high and damaging population densities.

Insecticides. Persistent insecticides applied to the foliage or soil in a liquid formulation provide good control of pale western cutworm. Larval mortality in fields treated with insecticide is not rapid, often requiring several days. The use of persistent insecticides is very important because larvae are inactive and remain below-ground for 3-5 days during molting cycles. Thus, if insecticides are to be effective, they must be persistent enough to remain until the larvae resume activity and come into contact with insecticide (Byers et al., 1992; Hill et al., 1992). Bait formulations generally provide only partial control because larvae generally feed below the soil surface and therefore have little contact with baits (DePew, 1980; McDonald, 1981a). Bacillus thuringiensis is not usually recommended for control of cutworms.

Cultural Practices. Cultural methods of management have been developed for grain production systems, but they may have applicability for other cropping systems. Moths infrequently deposit eggs in soil that has a crust, apparently because females cannot penetrate hard soil with the tip of their abdomen. Therefore, a widely practiced technique is to allow a crust to form on the soil in the autumn, before oviposition. Another common practice is to till the soil 10-14 days before planting. This destroys weeds and other alternate hosts on which larvae may be feeding, and causes them to starve before the new crop germinates. A practice that is often suitable for vegetable production is to increase irrigation frequency during periods of cutworm abundance; frequent irrigation in the spring is deleterious to larval survival.

Barriers are sometimes used to reduce access by cutworms to plants grown in the home garden. Metal or waxed-paper containers with both the top and bottom removed can be placed around the plant stem to deter consumption. The barrier should be extended below the soil surface because larvae may burrow below the soil line. For pale western cutworm, the requirement that the lower edge of the barrier be deeply recessed in the soil is especially important, because larvae burrow actively in the soil, rarely coming to the soil surface except during wet weather.

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