Florence Grovida Gardening

Glassy Cutworm Apamea devastator Brace Lepidoptera Noctuidae

Natural History

Distribution. This native species occurs throughout the United States and southern Canada except for the southeastern states. Its range also includes South America.

Host Plants. Glassy cutworm is principally a grass-feeding species, and crop damage is most likely done when crops follow sod or are planted into fields heavily infested with grassy weeds. In Oregon, Kamm (1990) reported that bentgrass, Agrostis tenus; ryegrass, Lolium perenne; and wild oats, Avena fatua; were attractive to ovipositing females. Vegetables reported injured by glassy cutworm include beet, bean, cabbage, corn, lettuce, and radish. Among other crops injured are alfalfa, barley, bluegrass, fescue, oat, strawberry, timothy, tobacco, and wheat.

Natural Enemies. Several parasitoids of glassy cutworm larvae are known, though in general there is little information about natural population regulation of this species. Kamm (1990) reported that Lissonota montana (Cresson) (Hymenoptera: Ichneu-monidae) and Nowickia latianulum (Tothill) (Diptera: Tachinidae) collectively caused 30-48% parasitism in Oregon. Other wasp parasitoids include Macrocentrus crassipes Muesebeck (Hymenoptera: Braconidae), Pter-

ocormus ambulatorius (Fabricius), and Spilichneumon inconstans (Cresson) (both Hymenoptera: Ichneumoni-dae). Among other fly parasitoids known from glassy cutworm are Gonia aldrichi Tothill and G. frontosa Say (both Diptera: Tachinidae).

Life Cycle and Description. There is one generation annually. The winter is passed in the larval stage, with pupation beginning in May. Moth emergence begins in June, but peak abundance usually is during late July or August. Moths may be present until October, and produce eggs that hatch into the overwintering larval stage.

1. The egg stage of this poorly known species seems to be undescribed. In Minnesota, Knutson (1944) reported that egg laying was completed by the end of August, but Kamm (1990) inferred from adult and larval data that oviposition occurred over several months in Oregon. Duration of the egg stage is about 12-21 days.
2. The larva feeds entirely below-ground, or at least below the plant litter on the soil surface. The body of this cutworm is largely unpigmented, and many authors noted that this grayish larva resembles a white grub (Coleoptera: Scarabaeidae) in general appearance. The mature larva measures about 3540 mm long. The head is reddish brown, and measures 4.5 mm wide. A large prothoracic plate is also present, and is reddish-brown but with a darker margin. Duration of the larval stage is several months, depending on weather. (See color figure 52.)
3. The larva prepares a pupal cell several centimeter below the surface of the soil. The reddish-brown pupa is about 18-20 mm long and 5 mm wide. Duration of pupation is not well-documented but there are reports of 15-60 days in the literature, with the lower value more typical.
4. The adult is light gray to brownish gray in general, with extensive amounts of dark brown mottling. A narrow white transverse line is usually present distally on the forewing, with a series of dark triangles located along the inner margin of the transverse line. The hind wing are brownish, and darker distally. The wingspan is 35-45 mm. A sex pheromone produced by females has been identified (Steck et al., 1977,1980b). (See color figure 241.)

Glassy cutworm larva.

Glassy cutworm larva.

The most complete description of this insect is found in Crumb (1929). Knutson (1944) and Kamm (1990) also provided useful observations. A bibliography was published by Rings and Arnold (1974). Keys including the larva of this species were given by Crumb (1929, 1956), Whelan (1935), Rings (1977b), and Capinera (1986). It is also included in a key to armyworms and cutworms in Appendix A. The moth was included in pictorial keys by Rings (1977a) and Capinera and Schaefer (1983).

Damage

The larva lives below-ground, feeding on roots and the base of plant stems. Plants are readily killed by this type of injury, and the first sign of injury usually is wilting plants.

Management

Glassy cutworm is not a common pest unless crops are planted into fields that previously had been pasture or grass sod. The problem normally dissipates within 2-3 years after destruction of the grass. Population monitoring is most easily accomplished with pheromone traps because the other stages are associated with the soil and difficult to detect.

Chemical insecticides are useful for prevention of injury, but are most effective when placed in the furrow at planting. Baits are not very effective because larvae remain below-ground and have little contact with bait. Mechanical barriers such as metal cans with the top and bottom removed are often recommended for prevention of cutworm damage in home gardens. For glassy cutworm, a burrowing species, the lower edge of the barrier must be sunk well below the soil surface to become an effective deterrent to feeding.