Diatraea crambidoides Grote Lepidoptera Pyralidae

Natural History

Distribution. Southern cornstalk borer is, as its common name suggests, found predominantly in the southeastern states. It may be found as far north as Maryland and southern Ohio, and as far west as Kansas and New Mexico. However, these are the geographic extremes, and as a pest its range is mostly limited to the southeastern states from Alabama and Florida to Virginia. It frequently has been confused with related species, particularly sugarcane borer, Diatraea saccharalis (Fabricius), so some records of occurrence are suspect. Southern cornstalk borer apparently is native to the southeastern United States, but also occurs in Mexico and South America.

Host Plants. This insect is known principally from corn, but occasionally damages sorghum. It may also be found in some of the wild grasses with thick stems such as Johnsongrass, Sorghum halepense; Paspalum spp.; and panic grass, Panicum spp.

Natural Enemies. Mortality factors of southern cornstalk borer are not well documented. However, an undetermined fungus seems to be quite important in affecting survival of overwintering larvae. Tricho-gramma sp. (Hymenoptera: Trichogrammatidae) egg parasitoids as well as wasp parasitoids of the larval and pupal stages have been noted on occasion, particularly Syntomosphyrum clisiocampae (Ashmead) (Hymenoptera: Eulophidae) and Temelucha ferrunginea (Davis) (Hymenoptera: Ichneumonidae). The parasitic flies Lixophaga diatraeae (Townsend) and L. sphenophori (Villeneuve) (both Diptera: Tachinidae) also have been reared from this stalk borer. Probably several general predators consume larvae, and the goldenrod soldier beetle Chauliognathus pennsylvanicus De Geer (Coleoptera: Cantharidae) is among those insects known to attack larvae and pupae within the tunnels of the corn stalk.

Life Cycle and Description. There are two generations per year, with the larval stage overwintering in the base of the corn stalk. In Virginia and North Carolina, pupation of overwintering larvae commences in late April and May. Adults and eggs of the first generation are found in May and early June, larvae from May to July, and pupae in July. Second generation adults and eggs occur in July and early August, followed by the larval stage which persists until the following spring.

  1. The egg is flattened and oval. It measures about 1.6 mm long and 1.0 mm wide. They are deposited singly or in an overlapping, shingle-like fashion in small clusters. Initially, there may be as many as 20 eggs in a single cluster, but over time the female deposits smaller and smaller egg masses until she is depositing only single eggs. Deposition usually occurs on the upper surface of leaf blades. The eggs are whitish when first deposited, but gradually assume a yellow or orange-yellow color. Duration of the egg stage averages about nine days in the spring and eight days during the summer (range about 7-13 days).
  2. Larvae increase in size from about 1.5 mm long to about 25 mm, and display 5-6 instars during development. Newly hatched larvae are brownish, with a black head. Each body segment is darker toward the posterior, producing a transverse banded pattern. The appearance of the mature larvae differs slightly between seasons. Summer larvae bear a yellowish-brown to brown head capsule, with each body segment bearing several large dark spots dorsally and laterally on a whitish background. Overwintering larvae have a thicker, more robust appearance with the spots markedly lighter, barely darker than the whitish body color. Larvae of southern cornstalk borer are difficult to differentiate from other Diatraea spp. A key to the caterpillars boring in corn stalks is found in Appendix A. Mean development time of the first generation, or summer brood, is about 30 days (range 24-38 days). Second brood larvae, of course, persist for about nine months. Larvae tunnel freely within the stalk, with spring generation larvae usually feeding upward from the point of entrance. Overwintering larvae, however, eventually move down to the taproot to pass the winter in a more sheltered location. As they near the end of the larval stage the larvae eat to the
Corn Stalk Growth Stages

Southern cornstalk borer larva, summer form.

outer edge of the stalk, but leave a thin covering of the outermost layer of the stem in place or a few strands of silk across the exit hole. This presumably provides the mature larva and pupa protection from predators, but still allows the adult to escape. Second generation larvae apparently do not feed in the spring, but clear out the tunnel near the exit hole in preparation for pupation.

  1. Pupation takes place within the tunnel produced by the larva. The pupa is mahogany brown and measures about 13-17mm long and 3.5mm wide. Duration of the pupal stage averages about 20 days in the spring and 12 days during the summer.
  2. The moths are yellowish-brown except for the hind wings, which are white. Also, there is a small black spot near the center of the forewing, and the wing veins are darker brown. The tips of the front wings of the male moth are sharply angled rather than broadly rounded, approaching 90°, whereas the females are more rounded. The moths measure 25-35 mm in wingspan. As is characteristic of many pyralids, the palpi are enlarged and project forward, imparting the appearance of a pointed head. Oviposi-tion usually commences two days after emergence, and extends for 3-4 days. The moths are nocturnal, and are not strong fliers. The females, in particular, fly only short distances. Females usually deposit about 200 eggs during their life span of 4-6 days, with nearly all eggs produced during the first two nights of ovipo-sition.

A good summary of southern cornstalk borer biology was given by Leiby (1920). The reports by Phillips et al. (1921) and Cartwright (1934) are basically abbreviated versions of the same information. A key to the

Diatraea larvae can be found in Peterson (1948), and Stehr (1987), and to the adults in Dyar and Heinrich (1927). A key to stalk borers associated with corn in southern states is presented by Dekle (1976); this publication also includes pictures of the adults. A key to common stalk boring caterpillars also is found in Appendix A.

Damage

The corn plant may be attacked at various stages of growth by southern cornstalk borer larvae, resulting in different types of injury. Early damage occurs when larvae feed on the unfolded leaves at the tip of the plant. If fewer than four larvae are present the result may be only ragged foliage bearing irregular holes. If several larvae are present, however, the bud or growing point of the plant may be killed, stunting the plant and preventing production of a functional tassel. Larvae may also burrow through the veins of leaves and into the stalk. Tunneling larvae are found most frequently at the base of the stalk near the soil line and brace roots. Stalks may support up to 15 larvae, but usually no more than 2-3 attain the pupal stage. Plants that have been tunneled by larvae are susceptible to wind damage.

Management

Insecticides. Insecticides are not usually required for management of southern cornstalk borer. If necessary, insecticides can be applied in granular form to the whorl, where many larvae contact the insecticide. Insecticides can also be applied to the foliage, though because older larvae burrow within the plant, it is difficult to achieve high levels of suppression unless the insecticide is applied when the larvae are young. Systemic insecticides, particularly granular formulations applied to the soil, also are sometimes recommended.

Cultural Practices. Several cultural practices can alleviate damage by southern cornstalk borer. Destruction of stubble or lifting stubble from the soil can eliminate overwintering larvae. Burying stubble deeply can also prevent moths from escaping in the spring.

Modified planting dates are sometimes recommended because late-planted corn can escape infestation. In Virginia and North Carolina, corn planted in June generally escapes damage by first generation insects, but is susceptible to infestation by the second generation. In general, the earlier the planting the more heavily it will be infested. Because moths, particularly females, do not fly far after emergence, crop rotation has considerable benefit.

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