Stalk Borer (Lepidoptera: Noctuidae) Ecology and Integrated Pest Management in Corn

Stalk Borer (Lepidoptera: Noctuidae) Ecology and Integrated Pest Management in Corn Marlin E. Rice 1 and Paula Davis 1 Corresponding author, J. Integ. Pest Mngmt. 1(1):
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Stalk Borer (Lepidoptera: Noctuidae) Ecology and Integrated Pest Management in Corn Marlin E. Rice 1 and Paula Davis 1 Corresponding author, J. Integ. Pest Mngmt. 1(1): 2010; DOI: /IPM10006 ABSTRACT. The stalk borer, Papaipema nebris (Guenée), is a native North American insect with a broad host range. It frequently damages corn, especially in conservation-tillage systems, and occasionally soybean, in the Midwestern United States. A review is presented of crop injury, especially to corn, brief descriptions of the life stages, degree-day development, and larval host plants along with a discussion of control strategies including planting date, weed control, mowing oviposition sites, burning, insecticide and herbicide combinations, and transgenic corn. EILs and an IPM approach for managing stalk borers in corn are presented. Key Words: Papaipema nebris; Zea mays; maize; transgenic corn; IPM The stalk borer, Papaipema nebris (Guenée) (Lepidoptera: Noctuidae), is a native North American insect that occurs from the Atlantic Coast to the western Great Plains, and from Canada to the Gulf of Mexico (Decker 1931). Nineteen species of Papaipema occur in Iowa and P. nebris represented 95.3% of adult specimens collected, but it is the only species known to injure corn (Peterson et al. 1990). The larvae tunnel into the stems of annual and perennial plants and they are sporadic, yet serious pests of corn (Zea mays L.), in the Midwestern United States. Stalk borer injury to corn is especially prevalent where corn is grown adjacent to roadside ditches, fence lines, grass waterways, and conservation terraces (Meyer and Peterson 1998), or in minimum and no-tillage fields with grass or broadleaf weeds (Rubink and McCartney 1982, Stinner and House 1990, Levine 1993). On rare occasions, field-wide stand losses have been reported in corn where grass or giant ragweed, Ambrosia trifida L., problems occur. The stalk borer is occasionally found in soybean, Glycines max (L.), but yield losses have not been documented (Rice and Pedigo 1997). Pest management options to prevent yield loss in corn from stalk borers has been especially challenging in the upper Midwest. Here we present an overview of stalk borer ecology and discuss IPM for this insect in corn. Historical Perspective The stalk borer was first recorded as a crop pest of wheat in Since that time it has primarily been associated as a pest of corn and was first documented in 1877 destroying Illinois corn (Decker 1931). In 1927, the U.S. Bureau of Entomology s Insect Pest Survey listed the stalk borer as one of the 10 most destructive insects. In Iowa a major corn producing state it is a widespread pest and has been recorded in all 99 counties (Decker 1931). In the last quarter of the 20th century, increases in conservation tillage and perennial grass populations, plus the construction of more grass terraces and grass waterways in row-crop fields to combat soil erosion have favored the stalk borer, resulting in increased damage to corn. Additionally, increased acreages of no-till or minimum-till fields in the 1990s led to reported infestations in western Iowa of % of young corn plants in no-till fields where problems with giant foxtail, Setaria faberi Herrmann, or woolly cupgrass, Eriochloa villosa (Thunberg) occurred the previous year (Rice and Pedigo 1997). Large, field-wide infestations are the result of adults laying eggs on grass throughout the field during late summer, then when corn is planted again into the grassy-problem field the following spring, stalk borer damage to corn is highly probable. Herbicides that kill the grasses during May will force the larvae to move from the dead grassy weeds to the corn. In cultivated fields where grassy weeds are kept under control, most stalk borer damage is confined to the first several rows adjacent to grass ditches, terraces, fence lines, and waterways. Description of Life Stages Adult. There is one generation per year. Adult forewings are varying shades of brown sprinkled with gray and a thin, jagged, slightly curved, white line approximately one-quarter in from and parallel to the tip, with 2 3 groups of small clustered white spots. Hind wings are pale grayish brown. The wingspan is inch (25 40 mm; Fig. 1) (Decker 1931). Adult emergence from the pupal cell occurs during August 6 to October 5 in Iowa (Decker 1931). Peak occurrence is usually during the first two weeks of September with 50% trap capture of flying adults during September 8 14 (Bailey et al. 1985), but some adults fly as late as October 20 in Illinois (Levine 1983). Adults may live only 8 10 days if the weather is hot, but longevity is extended to days during cool weather conditions (Decker 1931). Egg. Stalk borer eggs are 0.02 inch (0.6 mm) in diameter, globular, and somewhat flattened, with 50 longitudinal ridges and numerous small cross ridges forming shallow pits on the egg surface (Decker 1931). Eggs are laid singly or in masses with egg production ranging from 75 to 2,199 and averaging 879 eggs. Nearly all eggs are laid on dead vegetation inside curled leaves or between the leaf sheath and stem, or in cracks of stems (Decker 1931, Levine 1985). Length of the egg stage ranges from months with eggs hatching the following spring from April 19 to June 5 in Iowa (Decker 1931). Larva. Early stage larvae have a combination of cream and dark purple or brown stripes on the thorax and abdomen (Fig. 2). A thin, Fig. 1. Adult stalk borer (J. Bradshaw). 2 JOURNAL OF INTEGRATED PEST MANAGEMENT VOL. 1, NO. 1 Fig. 2. Early stage stalk borer larva. Fig. 4. Smooth brome with dead heads caused by larvae tunneling into grass stems. Hosts Fig. 3. Late-stage stalk borer larva. cream-colored dorsal stripe extends from the behind the head to the anal plate, and a lateral cream-colored stripe extends from the fourth abdominal segment also to the anal plate. The third and fourth pairs of abdominal legs are cream-colored. The metathoracic segment (third segment behind the head) and first four abdominal segments are dark purple or brown, except for the light dorsal stripe. In late-stage larvae, the darker colors fade and the larva assumes a dirty-white color (Fig. 3). The dorsal pronotal shield (behind the head) is dark orange with a dark lateral stripe. The head is orange except for an oblique, lateral black stripe extending through the ocelli. This stripe remains through all stages and is a distinctive characteristic used to distinguish stalk borers from all other caterpillars that feed on corn in the Midwest. The number of instars is highly variable, ranging from 7 to 16, but most larvae complete their development in 7 9 larval stages if they are feeding on a high quality host (Decker 1931). The larval stages may last days because of this variability. Pupa. The pupa is reddish brown. Pupation occurs in an oval cell below the soil surface or in the plant stalk, and ranges from 16 to 40 days depending on temperature (Decker 1931). Levine (1983) calculated the total development threshold units (beginning January 1, base 41.2 F/5.1 C) with accumulated Fahrenheit (FDD) and centigrade degree-days (CDD), respectively, for 50% development of egg hatch (494 F/257 C), pupation (2,746 F/1,508 C), and adult emergence (3,537 F/1,947 C). Levine (1986) later used higher minimum thresholds, accumulated after January 1, and recalculated the average first hatch and 50% hatch to occur at 297.7FDD/147.6CDD (base 47.3 F/ 8.5 C) and 309.6FDD/154.2CDD (base 48.0 F/8.9 C), respectively. Davis and Pedigo (1990) compared the accuracy of the 41.2 or 48.0 F threshold for predicting 50% egg hatch and found that both gave reasonable estimates of hatch as measured by occurrence of first instars in the field. Therefore, Levine s (1983) calculations were used to validate pest management actions (Lasack and Pedigo 1986, Davis and Pedigo 1990) and are used in the IPM section (below). Females prefer to oviposit on narrow-leaved perennial grasses such as tall fescue, Festuca arundinacea Schreber, giant foxtail, orchardgrass, Dactylis glomerata L., quackgrass, Elymus repens (L.), and winter wheat, Triticum aestivum L., as opposed to wide-leaved annual grasses, like rye, Secale cereale L., or broad-leaved plants such as smooth pigweed, Amaranthus hybridus L., and giant ragweed (Levine 1985; Highland and Roberts 1987, 1989). Grass in terraces, ditches, fence lines, waterways, and weed patches are preferred egglaying sites. The larvae are rather indiscriminate feeders and have been collected from 176 plant species representing 44 families (Decker 1931). In addition to corn, other hosts associated with agriculture in the Midwest include green foxtail, Setaria viridis (L.); quackgrass; orchardgrass; smooth brome, Bromus inermis Leysser; bluegrass, Poa spp.; wirestem muhly, Muhlenbergia frondosa (Poiret); woolly cupgrass; smartweed, Polygonum spp.; lambsquarters, Chenopodium album L.; pigweed, Amaranthus spp.; alfalfa, Medicago sativa L.; sweet clover, Melilotus spp.; hemp dogbane, Apocynum cannabinum L.; giant ragweed; cocklebur, Xanthium spp.; goldenrod, Solidago spp.; and yarrow, Achillea spp. Larvae rarely may occur in other crops such as soybean, Glycines max (L.) (Rice and Pedigo 1997), oat, Avena sativa L.; wheat, Triticum sativum Lamarck; and sunflower, Helianthus annnus L. (Decker 1931). Newly eclosed larvae tunnel into the first suitable plant they find during the spring, which is frequently a grass species. Tunneling frequently kills the top of the plant, leaving a dead head, which is commonly seen in smooth brome growing in roadside ditches and waterways during late June and July (Fig. 4). Larvae often outgrow the stem diameter of their first host, especially if it is a grass, and must search for a larger diameter host (Decker 1931). Larvae are cannibalistic and small diameter hosts seldom contain more than one larva per plant, but large multi-branched hosts, such as giant ragweed, may contain up to 30 larvae (Decker 1931). Injury to Corn Corn is attacked when early stage larvae occur nearby or when partially grown larvae crawl from grassy areas to adjacent corn rows in search of larger hosts. In the later situation, the injury is usually limited to the first 4 8 rows of corn adjacent to the grassy area (Levine 1983, Davis and Pedigo 1990). Larvae may attack corn anytime after it emerges, and the most susceptible stages are V1 V5 plants (2 24 in/5 61 cm) (Decker 1931). Larvae mostly produce two kinds of injury: leaf feeding and stalk tunneling. OCTOBER 2010 RICE AND DAVIS: STALK BORER IPM 3 Fig. 5. Stalk borer injury to whorl-stage corn. Fig. 7. Dead heart in corn caused by stalk borer. Fig. 6. Stalk borer inside corn stem. Leaf feeding occurs when larvae crawl up the outside of the plant and then directly down into the whorl where they feed upon the young, unrolled leaves. Later, when these leaves emerge and unfold, large ragged holes will appear across the leaf blades (Fig. 5). Leaf feeding does not reduce grain yields (Bailey and Pedigo 1986). The other injury, stalk tunneling, is much more severe. It occurs when larvae tunnel through the plant (Fig. 6) and destroy the growing point, causing the innermost whorl leaves to die. This is known as flagging (Rubink and McCartney 1982) or dead heart (Fig. 7) (Decker 1931, Bailey and Pedigo 1986) even though the growing point is not always injured (Davis and Pedigo 1991b). Stalk tunneling results in upper leaves being cut off within the center of the plant, which then wilt and die. The outer leaves remain green and apparently healthy. Plants that survive often grow tillers, are delayed in development, and do not produce normal-sized ears. The younger a plant is injured, the more likely it is to not produce a harvestable ear (Levine et al. 1984). Direct yield losses may occur from stand reductions in young plants (Bailey and Pedigo 1986). Stalk tunneling may cause deformation of the upper plant and tassel (Decker 1931). Larvae also may tunnel into the corn cob (Decker 1931), but this injury is very rare. Plants with dead heart fail to produce ears 25 63% of the time, while plants that do produce ears have substantially smaller ears and grain reductions of 49 89% (Bailey and Pedigo 1986). Barren plants that survive are basically weeds competing with neighboring plants for moisture, nutrients, light, and space, but produce no grain. Because of this competition, the uninfested, adjacent plants do not compensate for the yield that is lost from the barren plant, unless the infested plant dies. Delayed silking is the primary physiological basis for yield reductions in plants with dead heart (Bailey and Pedigo 1986). The silks do not catch sufficient pollen to pollinate all the kernels. Younger plants are more susceptible to damage, therefore, yield losses caused by stalk borers decline if the plant is attacked later in development. Once the plant reaches the 6-leaf stage, the plant s ability to tolerate stalk borer Fig. 8. Corn border rows injured by stalk borers. injury greatly increases. Larvae also can tunnel into corn stalks below the growing point, but do not kill the plant (Davis and Pedigo 1991b). As many as four larvae per corn stalk have been found in silking-stage corn. In conventionally tilled fields without a grassy-weed problem, most damage occurs in the rows next to permanent grassy areas (Fig. 8). Densities of 0.2 and 0.5 larva per feet2 in grass are sufficient to injure 10 and 34%, respectively, of plants adjacent to grassy areas (Lasack and Pedigo 1987).When the first or second corn rows are shorter than the inside rows, stalk borers are usually the cause of this stunting. A common misconception is that rows are stunted because of competition for moisture from the neighboring grass or weeds. An examination of the stalks, however, should reveal stalk borers or their tunnels inside the plants. Injury to Soybean Yield losses or significant stand reductions in soybean have not been reported in Iowa. Larvae have been found tunneling in soybean 4 JOURNAL OF INTEGRATED PEST MANAGEMENT VOL. 1, NO. 1 Fig. 11. Stalk borer inside soybean stem. Fig. 9. Stalk borer injury to seedling soybean. Fig. 10. Stalk borer entry holes in soybean. shortly after plant emergence (Fig. 9) to the pod stages (Figs. 10 and 11) (Rice and Pedigo 1997). Infested plants usually have wilted terminals. Infested plants are easy to overlook in soybean because uninfested plants quickly outgrow and overshadow infested adjacent plants. Infested seedling plants may wilt and die, thereby remaining unnoticed. Managing stalk borers in soybean is not practical. Soybean are planted at rates of 100,000 seeds or more per acre and stalk borers are unlikely to reduce the plant stand to levels that show a yield reduction. In addition, healthy neighboring plants adjacent to infested plants will likely compensate for insect damage. Biological and Environment Influences Lasack et al. (1987) examined potential biological and environmental influences on stalk borer populations. They found that heavy rainfall during the egg-hatching period significantly reduced neonate stalk borer populations, but as the small larvae tunneled into grass stems, their survival increased as they were well protected from both adverse climatic conditions and predators. Later, when 4th through 6th-stage larvae migrated in search of larger-diameter hosts, predators such as ants, ground beetles, and spiders were significant mortality factors. Parasitoids accounted for 5% of stalk borer mortality. All of these factors contribute to larval mortality, but they cannot be relied upon to prevent yield losses. Management Options in Corn Cultural Management: Planting Date. Plants that are attacked at earlier developmental stages tend to produce fewer and smaller ears than do plants attacked at later developmental stages (Davis and Pedigo 1991). Early planted fields may escape some stalk borer damage, but this varies from year to year, depending on when the eggs begin to hatch. Fields that are planted late and have grass terraces or grass waterways and yearly grass problems within the field are at highest risk for stalk borer damage. Cultural Management: Weed Control. Large stalk borer infestations often occur in corn adjacent to areas with large stemmed weeds, especially giant ragweed. Decker (1931) maintains that the elimination of giant ragweed and similar hosts is the most important step in reducing stalk borer damage. However, herbicides applied to weeds, especially those scattered throughout a field, can force stalk borers from their weedy host and into adjacent corn, thereby aggravating stalk borer damage (Rubink and McCartney 1982, Levine 1983) and possibly destroying a significant number of plants. The most effective approach is a burndown herbicide-insecticide combination, either tank mixed or in a split application, rather than an insecticide used alone. The herbicide kills the grass, forcing the larvae out of the plant to search for another host and is then killed by the residual insecticide. The insecticide should be tank mixed with the herbicide if it is a fast burndown herbicide or the field should be sprayed with the insecticide 7 days after the herbicide if it is a slow burndown herbicide (Rice and Pope 2006). Long-term management of field-wide infestations requires grass control so that eggs will not be laid across the field during late summer. If the grass is not controlled, the problem can repeat itself the following year. Cultural Management: Mowing. Decker (1931) suggests that mowing grassy areas adjacent to cornfields during the second week of August and then removing the grass as hay will render the location undesirable for oviposition. Cultural Management: Burning. Using fire to destroy eggs in grass and weeds in egg-laying sites (Fig. 12) can reduce crop damage from stalk borers. Decker (1931) reported that stalk borer populations could be reduced by 82 97% by burning fence lines from November to early spring. Burning experiments in grass terraces during late March in Iowa resulted in average yield increases of 28% in five of six fields in corn rows adjacent to the terraces, presumably because of destruction OCTOBER 2010 RICE AND DAVIS: STALK BORER IPM 5 Fig. 12. Using fire in grass to kill stalk borer eggs. of stalk borer eggs (M.E.R., unpublished data). However, burning can result in no yield difference between burned and unburned areas because of low egg populations, eggs laid on grass out in the field, or eggs being blown into the field on grass leaves during the fall and winter. Successful reduction of stalk borer populations with fire is best attained by burning adjacent grassy areas and controlling grassy weeds in the cornfield. Grass is best burned in the spring just as it begins to break winter dormancy. Chemical Management: Insecticides. Chemical control can be difficult to achieve because larvae are exposed for very short periods of time and they cannot be killed after tunneling into corn. Therefore, insecticides with residual activity should be timed to coincide either with egg hatch or with movement from grass to corn. Therefore, predicting larval movement from weed hosts to corn hosts is crucial. Targeting hatching eggs works best in fields that have a history of field-wide infestations. An insecticide applied to grass terraces and timed at FDD, 41 F base ( CDD, 5.1 C base), reduced stalk borer populations by 54 85%, resulting in a 50 75% reduction in damaged corn plants (Davis and Pedigo 1990). Timing is critical for best performance. Unfortunately, this approach does not consider the potential size of the stalk borer population, so an insecticide may be sprayed on a population that would not be economically damaging. A second approach is to target the insecticide to coincide with migration of the larvae from the grass to the adjacent corn. First larval movement and 50% larval movement out of grass occurs at 1,110
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