Papaipema nebris Guene Lepidoptera Noctuidae

Natural History

Distribution. This native insect occurs throughout the eastern United States and southern Canada east of the Rocky Mountains. It rarely is abundant in the southern states and along the western margin of its distribution, occurring as a pest principally in the midwestern states.

There are nearly 50 species of Papaipema found in northeast and northcentral United States and adjacent

Canada. Although, P. nebris is the dominant pest, on occasion related species such as P. cataphracta (Grote), the burdock borer, have been reported to damage crop plants. Accurate identification of these insects is difficult in the larval stage owing to their similarity in appearances and habits. Based on light trap collections in Iowa, Peterson et al. (1990) concluded that P. nebris was by far the most abundant species in this group.

Host Plants. Stalk borer has a very wide-host range, with almost 200 plant species recorded as hosts. In the spring the larvae burrow into grass stems, but as they grow the larvae move to nearby plants with thicker stems. Among the vegetable crops injured are asparagus, bean, cantaloupe, cauliflower, celery, corn, eggplant, parsnip, pea, pepper, potato, rhubarb, spinach, and tomato. Other crops sometimes injured include alfalfa, barley, cotton, oat, red clover, rye, sugarbeet, sweet clover, timothy, and wheat. Fruit damaged by stalk borer include apple, blackberry, currant, gooseberry, strawberry, peach, and plum. Shade trees can also be injured, including catalpa, elm, maple, poplar, and willow (Solomon, 1988). Among the numerous flower crops that have been reported to be damaged are anemone, canna, carnation, cosmos, daisy, gladiolus, hollyhock, iris, larkspur, lily, peony, phlox, purple coneflower, rose, and rose mallow. Some of the common weeds supporting stalk borer larvae, are cattail, Typha spp.; dock, Rumex spp.; Kentucky bluegrass, Poa pratensis; dogbane, Apocynum androsae-mifolium; groundcherry, Physalis spp.; goldenrod, Solidago spp.; lambsquarters, Chenopodium album; quackgrass, Agropyron repens; ragweed, Ambrosia spp.; smartweed, Polygonum spp.; sunflower, Helianthus spp.; thistle, Cirsium spp.; wildrye, Elymus canadensis, and many others. Ragweed is often suggested as the favored host. Some common weeds such as milkweed, Asclepias syriaca; and velvetleaf, Abutilon theophrasti; are not suitable. Small-stemmed grasses often induce larval wandering (Alvarado et al., 1989).

Natural Enemies. Stalk borer is attacked by many of the general predators that are found attacking other caterpillars. Because stalk borers often move among host plants, they likely are more susceptible to predation than some borers. Among the known predators are ground beetles (Coleoptera: Carabidae), lady beetles (Coleoptera: Coccinellidae), minute pirate bugs (Hemiptera: Anthocoridae), stink bugs (Hemi-ptera: Pentatomidae), and damsel bugs (Hemiptera: Nabidae).

Parasitoids may be more important natural enemies than predators, but the significance of individual para-sitoids varies among localities and habitats. Of the more than 20 parasitoids known to attack stalk borer, Lydella radicis (Townsend) (Diptera Tachinidae) was reported to be the most important in Iowa (Decker, 1931), parasitizing up to 70% of larvae. The most important wasp parasitoid in Iowa, and probably the second most important parasitoid, was Apanteles papai-pemae Muesebeck (Hymenoptera: Braconidae); this parasitoid attained levels of parasitism up to 38%, but averaged about 10%. In contrast, the important parasitoids in Ohio were Lixophaga thoracica (Curran) (Diptera: Tachinidae) in corn, Sympiesis viridula (Thompson) (Hymenoptera: Eulophidae) in potato and ragweed, and Lissonata brunnea (Cresson) (Hyme-noptera: Ichneumonidae) and Gymnochaeta ruficornis Williston (Diptera: Tachinidae) in ragweed (Felland, 1990). Lasack et al. (1987) reported only low levels of parasitism in Iowa.

Weather. Weather also influences abundance of stalk borer. Both excessive rainfall and hot, dry weather during the spring when larvae are hatching and moving from host to host are reported to reduce larval survival markedly (Decker, 1931; Lasack et al., 1987).

Life Cycle and Description. There is a single generation annually. The egg is the overwintering stage. Hatching occurs in April-June, followed by a larval development period of 60-90 days. Pupation occurs in late summer and autumn, with moths present and oviposition occurring during August-October. Population monitoring in Iowa demonstrated that nearly all moths were found during September (Bailey et al., 1985).

  1. The eggs are deposited singly or in clusters of up to 100 on the stems and leaves of dead grasses and weeds. The preferred site of oviposition is within rolled leaves, leaf sheaths, or cracks and crevices, particularly of narrow-leaved perennial grasses (Levine, 1985; Highland and Roberts, 1989). In shape, the egg is a slightly flattened sphere. It measures about 0.6 mm in diameter and 0.45 mm in height. It is white when first deposited, but turns grayish or amber with age. Narrow ridges, about 50 in number, radiate outward from the center of the egg. They normally hatch after a period of 7-9 months, usually in April or May. Under controlled conditions, post-diapause eggs require a mean of 12.4, 14.6, 18.8, 27.5, and 41.7 days for development when held at 24°, 21°, 18°, 16°, and 13°C, respectively (Levine, 1983).
  2. Larvae initially mine leaves, or if feeding on grass then the stem may be attacked. However, larvae relocate to plants with large-diameter stems such as ragweed as these hosts become available. The shift among host plants occurs principally during instars 4-6. Depending on food availability, larvae may be forced to move repeatedly before completing their development. The larva is cylindrical in form, but tapering toward both the anterior and posterior ends. The head and thoracic shield are dark brown or black during the first two instars, and yellowish thereafter, though marked with a dark narrow band laterally. The larva bears pairs of prolegs on the third-sixth abdominal segments in addition to anal prolegs. The body color is brown with a broad white stripe dorsally and on each side. The lateral stripes are interrupted by a large brown spot in the region behind the thoracic legs. The stripes fade at maturity, the larva assuming a whitish or purplish color. At maturity, the larva attains a length of about 27 mm. Larval development entails 617 instars, with larvae usually displaying 7-9 instars. Head capsule widths for larvae with seven instars are 0.25, 0.38, 0.57, 0.87,1.3, 2.0, and 2.9 mm, respectively, for instars 1-7. In contrast, head capsule widths for larvae with nine instars are 0.25, 0.34, 0.46, 0.63, 0.87,1.2, 1.6, 2.1, and 2.9 mm, respectively, for instars 1-9. Development time for larvae with seven instars when reared at 27°C is 4, 3.6, 3.7, 4.5,10,15, and 28 days for instars 1-7, respectively, producing a mean larval development time of 68 days. Development time for larvae with nine instars when reared at 27°C is 4, 3.4, 3.9, 4.2, 5.3, 6.2, 9.0, 14.1, and 26 days for instars 1-9, respectively, producing a mean larval development time of 76 days. Lasack and Pedigo (1986) indicated a developmental threshold of about 5.1°C. Cannibalism is common among stalk borer larvae, the larger individuals usually killing and consuming the smaller.
  3. When larval development has been completed, larvae usually move to the soil and prepare a small cell just beneath the surface, though in some hosts such as corn pupation often occurs within the stem of the host. After a prepupal period of 1-6 days, pupation occurs. The pupa is typical in form—elongate, broadly rounded anteriorly, tapering posteriorly, and terminating in a pair of small curved spines. It is reddish brown or brown and measures 16-22 mm long and 5-7 mm wide. Duration of the pupal stage is normally 22-29 days.
  4. The adult is medium in size, with a wing-span of 25-40 mm. The general color is grayish-brown, but close examination shows that the wings are dark-brown and covered with scales bearing white tips. The basal two-thirds of the front wings tend to be darker, and separated from the distal portion by a thin white transverse line. The hind wings are paler, similar to the distal portion of the forewings. Some moths also bear spots on the forewings—three small whitish spots about one-third the distance to the wing tip and a bean-shaped yellowish or whitish spot just beyond

Stalk borer larva.

Stalk borer larva.

the mid-point of the wing. Moths are nocturnal, and begin copulation and oviposition within three nights of emergence. The period of oviposition averages about 10 days (range 4-23 days), and is followed by death within a few days (mean 2.4 days, maximum 9 days). Females may produce 200-500 eggs daily, with average fecundity reported to be about 900 and maximum egg production just over 2000. Adults seem to be weak fliers, making only short flights.

An excellent treatment of stalk borer biology was given by Decker (1931). Temperature relations were given by Levine (1983). Egg diapause was described by Levine (1986b). A key for identification of larvae was provided by Crumb (1956) and Capinera (1986). A guide to common stalk boring caterpillars also is included in Appendix A.

Damage

Historically, damage has been sporadic and limited to border rows of crops. In recent years, however, as reduced tillage practices have become more widespread in corn production areas, population densities have increased and damage has become more frequent. Young larvae enter a variety of hosts in the spring, but often choose grass plants, because these tend to predominate early in the season. Larvae usually enter the plant by burrowing into the stem, but they may also mine leaves. Leaf mining in young corn plants causes no significant loss, but when larvae burrow into the whorl, causing its death (called "dead heart"), significant damage occurs (Bailey and Pedigo, 1986). The youngest corn seedlings are most susceptible to injury, and little damage is observed once corn attains the six-leaf stage (Levine et al., 1984; Davis and Pedigo, 1990b, 1991). If a food source of a single stem is exhausted, larvae move to other stems or plants. The entrance to the plant may be anywhere along the stem, and is usually made obvious by the large entrance hole. The stem is often completely hollowed out, causing the distal portions of the plant to perish. Stalk borer larvae sometimes feed on tissue of woody plants such as trees, but only the soft terminal tissue is damaged.

Management

  1. Moths can be attracted with blacklight traps, though mostly males are captured (Bailey et al.,
  2. The distribution of eggs and young larvae is highly aggregated, but larval distribution is altered and it becomes more uniform as larvae disperse. Many samples are required to estimate density, especially in wild grasses. However, it is not always necessary to dissect grass stems to locate larvae, because infested stems wilt and discolor (Davis and Pedigo, 1989).
  3. Persistent insecticides, including systemic materials, can be applied to rows of crop plants at the margin of crop fields to reduce damage by invading larvae. However, if crop fields support grass, particularly in the autumn when eggs are deposited, the entire field may require treatment. Davis and Pedigo (1990a) demonstrated that treatment of weedy areas adjacent to crop fields, especially if timed to coincide with larval hatch, could provide good crop protection.

Cultural Practices. The abundance of stalk borer is directly related to the availability of preferred weedy host plants in or near crop fields. Thus, field edges and small fields are more likely to experience damage. Stinner et al. (1984) noted the preference by ovipositing moths for grasses within crop fields, especially fields that were grown under reduced tillage practices. However, the presence of broad-leaf weeds also has been shown to be correlated with increased abundance of stalk borer (Pavuk and Stinner, 1991). Reduced tillage practices often result in higher weed densities within crop fields, and greater damage by stalk borer (Willson and Eisley, 1992; Levine, 1993). Thus, destruction of weeds and grasses at field margins is recommended to reduce the invasion potential by larvae dispersing from weeds, but weeds within fields must also be suppressed.

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