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).
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.
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.
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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