Distribution. Southwestern corn borer is a native to Mexico, and appears to have been first found in the United States in New Mexico in 1891. By 1931 it had spread to the nearby states of Arizona, Texas, and Oklahoma. By 1970 the northern limits of its distribution were Kansas, Missouri, and Kentucky and the eastern limits were Tennessee and Alabama. Thus, it is found widely in the southern United States east of California.
Host Plants. This species attacks only grasses, with corn as the principal host. Sorghum and sorghum hybrids, broomcorn, sugarcane, and pearl millet are other crops that sometimes serve as hosts. Among weeds, Johnsongrass, Sorghum halepense, may also serve as an alternate host, but development time is much longer and fecundity greatly reduced on this plant (Aslam and Whitworth, 1988).
Natural Enemies. Several natural enemies are known, though their value is open to question. In Texas, egg or early instar mortality is critical to survival of the spring population, whereas in the summer generation survival is most affected by mortality of large larvae and diapausing larvae (Knutson and Gil-strap, 1989a,b, 1990). Among the egg parasitoids are Trichogramma spp. (Hymenoptera: Trichogrammati-dae), and a larval parasitoid of Diatraea spp., Apanteles diatraeae Muesebeck (Hymenoptera: Braconidae) (Knutson and Gilstrap, 1989a,b). Parasitism by Tricho-gramma spp. usually occurs late in the oviposition period at low levels, but sometimes 20% or more of the eggs are parasitized (Moulton et al., 1992). General predators such as insidious flower bug, Orius insidiosus (Say) (Hemiptera: Anthocoridae), and lady beetles such as Hippodamia spp. (Coleoptera: Coccinellidae) are common in corn fields but have little impact because they often seem to be out of synchrony with susceptible stages of southwestern corn borer.
Pathogens such as the fungus Beauveria bassiana, and to a fewer extent Bacillus sp. bacteria, also affect larvae, particularly the overwintering population. Knutson and Gilstrap (1989a) reported that up to 45% of large larvae were infected with Beauveria. Rol-ston (1955) indicated that unspecified nematodes were sometimes observed, and Knutson and Gilstrap (1990) reported minor incidence of Heterorhabditis sp. (Nema-toda: Heterorhabditidae).
Mortality also occurs during the winter months when the larval overwintering cell is penetrated by stalk rot fungi and termites, apparently because such penetration allows seepage of water into the cell. Thus, heavy rainfall, and heavy and wet soils are detrimental to overwintering survival, and southwestern corn borer typically is more destructive in areas with sandy soil. Considerable overwintering mortality also results from bird predation, often 50-80% of the total mortality. The yellowshafted flicker, Colaptes auratus (L.), is the most important avian predator (Black et al., 1970). Rodents also sometimes consume overwintering larvae, but this occurs erratically.
Life Cycle and Description. In most locations, southwestern corn borer exhibits 2-3 generations per year. In Mississippi, moths are present in late April-early May, late June-early July, and in August. In northern Arkansas, corresponding moth flights appear in June, July, and August-September. In Texas, moths are found in late May-June, mid-July-early August, and sometimes in late August-early September. A generation normally requires 40-50 days. In most areas, larvae of the first or spring generation do not display diapause, but larvae from the subsequent generation(s) may enter diapause depending on photoperiod and temperature present when they are developing. Photoperiod of less than about 15 h often induces diapause. Larvae developing after early August usually enter diapause in Missouri.
Egg. The eggs of southwestern corn borer are oval, measuring about 0.8 mm wide and 1.3 mm long. They are flattened, and deposited in an overlapping manner, resembling scales of a fish. The eggs are creamy white, but bear three parallel, transverse, orange-red lines. They are deposited principally on the upper surface of foliage, but also on the lower surface, and occasionally on the stem (Poston et al., 1979). The number of eggs per mass varies from one to several, but averages 3-5 per cluster. Initially, egg masses tend to consist of several eggs, but the number diminishes as the females age, so by the fourth day of oviposition most females are depositing but single egg. The incubation period of the eggs is 4-7 days.
The biology of southwestern corn borer was reviewed by Davis et al. (1933), Rolston (1955), Hen derson and Davis (1969), and Chippendale (1979). Phenology was also presented by Walton and Bieberdorf (1948) and Knutson et al. (1982). Developmental biology was given by Whitworth and Poston (1979), Knutson et al. (1989), and Ng et al. (1993). An artificial diet for borer culture was presented by Whittle and Burton (1980). A bibliography was published by Morrison et al. (1977). A key to the Diatraea larvae can be found in Peterson (1948) and Stehr (1987), and to the moths in Dyar and Heinrich (1927). Southwestern corn borer was included in the larval key by Capinera (1986) and the moth key by Capinera and Schaefer (1983). A key to stalk borers associated with corn in southern states was presented by Dekle (1976); this publication also includes pictures of the adults. A guide to the common stalk boring caterpillars is found in Appendix A.
All stages of the plant may be injured by feeding of southwestern corn borer larvae. Early instars of the first generation feed on leaf tissue, especially new tissue within the whorl of young corn plants. This can result in destruction of the terminal bud (a condition called "dead heart''), loss of apical dominance, and development of lateral buds. Such plants are stunted and bushy. Early instars of the second generation feed mostly on leaf sheaths, the husk, shank, kernels, and cobs of ears. In sweet corn it is difficult to detect the presence of the larvae in the ear until the husk is removed. Late instars of all generations bore within the stalk. Stalk damage may result in stunting if it occurs early in the growth of the plant; in more mature plants tunneling may disrupt translocation of nutrients to the ears, causing decrease in kernel size. Larvae about to enter diapause also girdle the stalk internally just above soil level. Girdling increases the likelihood that plants break. Stalk breakage is not often a problem in sweet corn due to early harvesting, but is a severe threat to grain corn. Damage potential to grain corn was presented by Whitworth et al. (1984).
Cultural Practices. Several cultural practices are used to minimize the effects of southwestern corn borer on corn production. Early planting is often recommended, because damage to the growing point of the corn plant is minimized or prevented. Cultivation, especially if done early in the winter, prevents adults from emerging successfully (Archer et al., 1983). Lifting stubble from the soil exposes larvae to more severe overwintering conditions, and also reduces adult emergence in the spring. Corn planted at high densities is more likely to suffer girdling, than corn at low densities, despite a tendency for equal incidence of infestation; the basis for this disparity is unknown (Zepp and Keaster, 1977).
Host-Plant Resistance. Corn cultivars possessing considerable resistance to corn borer feeding have been located and incorporated into commercial varieties, with mixed results (Davis et al., 1991; Ng et al., 1990; Thome et al., 1992). Resistance is attributable to both limited damage by larvae and non-preference by adults (Ng et al., 1990).
Biological Control. The wasp Pediobius furvus (Gahan) (Hymenoptera: Eulophidae) was imported from Africa to attack nocutid and pyralid borers, and evaluated against southwestern corn borer in Texas. Low levels of borer parasitism were obtained under field conditions, and the wasp failed to overwinter, thus limiting its usefulness to augmentative releases
(Overholt and Smith, 1989). Southwestern corn borer is also susceptible to infection by the bacterium Bacillus thuringiensis and the alfalfa looper baculovirus (Davis and Sikorowski, 1978; Nolting and Poston, 1982), but these materials have not come into general use for southwestern corn borer.
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