The Southern Cornstalk Borer (Diatraea Crambidoides (Grote), Lepidoptera: Crambidae) a New Pest of Eastern Gamagrass (Tripsacum Dactyloides (L.) L., Poaceae)
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JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY 84(3), 2011, pp. 209–216 The Southern Cornstalk Borer (Diatraea crambidoides (Grote), Lepidoptera: Crambidae) a New Pest of Eastern Gamagrass (Tripsacum dactyloides (L.) L., Poaceae) 1 2 1,3 1 T. L. SPRINGER, G. J. PUTERKA, D. L. MAAS, AND E. T. THACKER ABSTRACT: The southern corn stalk borer [Diatraea crambidoides (Grote)] has become a serious pest to eastern gamagrass [Tripsacum dactyloides (L.) L.]. Managing this insect will be important to the future of this forage crop in the United States. An experiment was conducted to understand the life cycle of the southern corn stalk borer infesting eastern gamagrass. For a two year period, four plant crowns which contained numerous shoots were dug randomly each week from a field plot located in Woodward, OK. All shoots from each crown were dissected and the number of larvae and pupae present was recorded for each shoot type, i.e., reproductive or vegetative shoot. The life stages of the southern corn stalk borer in eastern gamagrass can be described by three distinct populations in northwestern Oklahoma: a) over- wintering, b) first generation, and c) second generation. Over-wintering larvae feed within a cavity near the base of the shoot or within the proaxis. Pupation occurred within the feeding cavity. Larvae occurred in reproductive shoots 2.5 times more often than in the vegetative shoots, which suggested an oviposition preference by adult females for reproductive shoots. The life cycle of the southern corn stalk borer in eastern gamagrass was completed in about 911 cumulated growing degree days. Understanding the life cycle of this insect devastating to eastern gamagrass forage and seed production will help formulate methods of control. KEY WORDS: Southern corn stalk borer, life stages, populations, Oklahoma, degree days The southern corn stalk borer [Diatraea crambidoides (Grote)], as its name implies, is an economic pest of corn, Zea mays L., throughout the southern USA from Maryland and Kansas on the north and southward into the southern and southwestern states. This pest also occurs in Mexico and in South America (Heinrichs et al., 2000). Southern corn stalk borer larvae tunnel and feed in the leaf whorls of corn creating irregular shaped holes in the leaves as they unfurl, tunnel in leaf mid-ribs, and tunnel and create cavities in the upper and lower corn stalk (Howard, 1891; Ainslie, 1919; Phillips et al., 1921; Cartwright, 1934). Corn plants will often out grow damage caused by low infestations of this insect; however, reduced grain and ensilage yields occur with moderate infestations, and plant death is often reported with high infestations (Heinrichs et al., 2000). Howard (1891) reported the host range of the southern corn stalk borer to include eastern gamagrass [Tripsacum dactyloides (L.) L.], and stated that, ‘‘The borer in this food plant introduces a variation in habit, and it feeds mainly on the upper joints, some larvae even having been found feeding upon the seed heads.’’ He reported that eastern gamagrass plants adjacent to a corn field were highly infested with southern corn stalk borer larvae while only one stalk of corn was found with borer damage. He further suggested that burning eastern gamagrass in the vicinity of corn fields 1 USDA, Agricultural Research Service, Southern Plains Range Research Station, 2000 18th Street, Woodward, Oklahoma 73801, USA. 2 USDA, Agricultural Research Service, Wheat, Peanut, and Other Field Crops Research Unit, 1301 N. Western Street, Stillwater, Oklahoma 74075, USA. 3 Current address: Monsanto, 800 North Lindbergh Blvd, St. Louis, Missouri 63141, USA. Accepted 10 June 2011; Revised 23 August 2011 E 2011 Kansas Entomological Society 210 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY every winter should reduce the number of hibernating individuals which would otherwise infest corn fields in the spring. Ainslie (1919) reported the host range to include sorghum [Sorghum bicolor (L.) Moench], Johnsongrass [Sorghum halepense (L.) Pers.], sugar cane (Saccharum officinarum L.), and eastern gamagrass. He stated that, ‘‘The injury to plants other than corn is never severe but, in planning methods of control, these plants must be considered and an examination made to determine whether or not they are harboring the pest.’’ Contrary to Ainslie (1919), Krizek et al. (2003) reported severe dieback to eastern gamagrass plants in research plots at the USDA-ARS Beltsville Agricultural Research Center, Beltsville, MD, in 2001 and consequently attributed the dieback to the southern corn stalk borer. They observed several larvae emerging from eastern gamagrass crowns. In April 2002, in field plots at the USDA-ARS Southern Plains Range Research Station, Woodward, OK and in pastures at the USDA-ARS Southern Plains Experimental Range near Ft. Supply, OK, southern corn stalk borer larvae and pupae were collected in the base of the prior year’s reproductive shoots and in the base of current year vegetative shoots of eastern gamagrass plant crowns (Springer et al., 2003; Maas and Springer, 2005). Maas and Springer (2005) did not observe the severe dieback to eastern gamagrass plants in Oklahoma that was reported by Krizek et al. (2003). However, Springer et al. (2004) estimated forage yield losses caused by the southern corn stalk borer to eastern gamagrass of as much as 1000 kg ha21, or economic losses of $50.00 per hectare for severely infested fields. Eastern gamagrass is a highly productive and palatable perennial, warm-season grass used for pasture, hay, and conservation purposes. Its breeding and agronomic potential and insect pest problems have been reviewed by Springer and Dewald (2004). As the number of hectares of eastern gamagrass increases in the United States (Springer and Dewald, 2004), the incidences of disease and insect pests have become more evident. Plant diseases and insects commonly found to occur in corn are now becoming prevalent in eastern gamagrass. Cultural practices used to control many of these insects in corn are not applicable to eastern gamagrass because it is grown as a perennial crop. Additionally, few chemical control measures are labeled for use on eastern gamagrass. The USDA-ARS Southern Plains Rangeland Research Station maintains the largest collection of temperate eastern gamagrass germplasm in the United States with more than 500 accessions. We have assessed our collection and found no resistance to the southern corn stalk borer. Eastern gamagrass, a close relative of maize, has been used as a genetic source for developing disease and insect resistant maize lines (Bergquist, 1981; de Wet, 1979; Moellenbeck et al., 1995). Maize lines with limited resistance to the southwestern corn borer [Diatraea grandiosella Dyar] are available. It is possible to transfer genes from maize into eastern gamagrass, but the process could require 10 years or more. Development of Bt gamagrass is another possible alternative (Krizek et al., 2003), but is unlikely due to regulatory issues. The utilization of eastern gamagrass for pasture, hay, and soil stabilization is increasing every year (Springer and Dewald, 2004). Other uses, such as pharmaceuticals and grain for human consumption or livestock feed, also are being explored. As the hectares of eastern gamagrass increase, it will be important to develop strategies to minimize the economic impact imposed on the crop by the southern corn stock borer and other pests. We anticipate that these problems will be solved through an VOLUME 84, ISSUE 3 211 integrated approach of plant breeding and cultural practices. Because knowledge of the life cycle of the southern corn stalk borer in eastern gamagrass will be useful for developing cultural practices for control, research was conducted to understand the life stages of development of the southern corn stalk borer in this host. Materials and Methods This experiment was conducted at the USDA-ARS Southern Plains Range Research Station, Woodward, OK (36u259N99u249W, elevation 615 m). The field plot consisted of 1200 plants of eastern gamagrass germplasm ‘FGT-1’ (Dewald and Kindiger, 1996) which were transplanted from the greenhouse to the field in 1996. Plants were spaced in 1.1 m rows on 1.1 m centers within rows on Devol fine sandy loam (Coarse-loamy, mixed, superactive, thermic Typic Haplustalfs) soil. Each year after establishment, the plot was burned in mid-March and atrazine [2-chloro-4- ethylamino-6-isopropylamino-s-triazine] was applied 7 to 14 days later for weed control at 1.68 kg of AI per hectare. The field plot was fertilized each year in April with nitrogen in the form of urea (46-0-0) at 70 kg N ha21. Beginning in January 2003 and continuing through December 2004, four plant crowns were dug randomly each week for a two-year period. Sampling was conducted on the east side of the field plot the first year of the experiment; sampling was conducted the second year on the north side of the field plot. Each year, the first row of the plot was designated a border row and all samples were taken by digging plants from rows 2–9. Sampling was conducted in this fashion to protect the integrity of the plot because it was used to maintain seed and vegetative propagules for the FGT-1 germplasm release. Each year, replicate 1 was sampled randomly from rows 2 and 3, replicate 2 was sampled randomly from rows 4 and 5, replicate 3 was sampled randomly from rows 6 and 7, and replicate 4 was sampled randomly from rows 8 and 9. For each week of the experiment, four plants (one from each replicate) were dug from the field plot, transported to a barn where each crown was hand-split into single and compound shoots (Dewald and Louthan, 1979), and then visually classified into reproductive or vegetative shoots. Pruning shears were used to cut each shoot transversely, beginning at the tip and proceeding down to the base, to determine presence or absence of southern corn stalk borer larvae or pupae.