Distribution. This species occurs widely in the western hemisphere, but not elsewhere. It is known from much of the southern United States, though in the eastern regions it occurs much farther north than in western states. The northernmost limits seem to be Massachusetts to southern Iowa in the east, and Oklahoma to southern California in the west. It is found throughout Central and South America. Despite its wide range, damage is limited principally to sandy soil, so it tends to cause injury in the coastal plain of the southeastern states from South Carolina to Texas.
Host Plants. Lesser cornstalk borer damages several crops grown in the southeast, though it is mostly a pest of peanut, sorghum, and soybean. Among vegetable crops injured are bean, beet, cabbage, cantaloupe, corn, cowpea, lima bean, pea, pepper, sweet potato, tomato, and turnip. Legume and grass crops are most often damaged. Field crops injured are corn, chufa, millet, oat, rice, rye, sorghum, peanut, soybean, sudan grass, sugarcane, and wheat. It also infests crabgrass, Digitaria sanguinalis, wiregrass, Elusine indica; and Johnsongrass, Sorghum halepense.
Natural Enemies. Several natural enemies of lesser cornstalk borer are known, though they are not thought to be major determinants of population trends. Smith and Johnson (1989) constructed life tables for populations in Texas, and identified survival of large larvae as the key element in generation survival, but the causative factor remained unidentified. The predominant parasitoids are Orgilus elasmopalpi Muesebeck and Chelonus elasmopalpi McComb (both Hymenoptera: Braconidae), Pristomerus spinator (Fab-ricius) (Hymenoptera: Ichneumonidae), and Stomato-myia floridensis Townsend (Diptera: Tachinidae) through most of the range of lesser cornstalk borer. Other parasitoids sometimes are present include Bra-con gelechiae Ashmead (Hymenoptera: Braconidae), Geron aridus Painter (Diptera: Bombyliidae), and Invreia spp. (Hymenoptera: Chalcididae) (Johnson and Smith, 1981; Funderburk et ah, 1984b; Smith and Johnson, 1989). Parasitoids rarely cause more than 10% mortality.
Among the predators thought to be important mortality factors are a ground beetle, PHlophuga viridicolis LeConte (Coleoptera: Carabidae); Geocoris spp. bugs (Hemiptera: Lygaeidae); and larval stiletto flies (Diptera: Therevidae).
Pathogens are commonly present in lesser cornstalk borer populations. The most important pathogen appears to be a granulosis virus, but a Beauveria sp. fungus, microsporidia, and mermithid nematodes also have been found (Funderburk et al., 1984b).
Weather. Lesser cornstalk borer seems to be adapted for hot, xeric conditions, and therefore tends to be more abundant and damaging following unusually warm, dry weather. Mack et al. (1993) used data from Alabama and Georgia to develop a predictive equation that forecasts the potential for crop injury and the need to monitor crops. It is based on the concept of "borer-days." Borer-days is calculated as the sum of days during the growing season in which the temperature equals or exceeds 35° C and the precipitation is less than 2.5 mm, less the number of days in which the temperature is less than 35°C and the precipitation equals or exceeds 2.5 mm. Thus, it is the sum of the number of hot, dry days less the number of cooler, wetter days. If the number of borer-days equals or exceeds 10, damage is likely. If borer-days equals 5-9, damage is possible and fields should be scouted. The relationship between borer-days and larval abundance is nonlinear, and small increases in borer-days beyond 10 results in large increases in larval abundance. Reduction in larval feeding at high soil moisture levels was documented by Viana and da Costa (1995).
Life Cycle and Distribution. There are three complete generations annually in Georgia, and some members go on to form a fourth generation. Other southeastern states also experience 3-4 generations, but in the southwest there are only three generations annually. Activity extends from June to November, with the generations overlapping considerably and little evidence of breaks found between generations. Overwintering apparently occurs in the larval and pupal stage; diapause is not present. A complete life cycle usually requires 30-60 days.
Egg. The egg is oval, measuring about 0.6 mm long and 0.4 mm wide. When first deposited, the egg is greenish, soon turns pinkish, and eventually reddish. The female deposits nearly all her eggs below the soil surface, adjacent to plants. A few, however, are placed on the surface or on leaves and stems
Lesser cornstalk borer larva.
and then almost black just before the adult emerges. It measures about 8 mm long and 2 mm wide. The tip of the abdomen is marked by a row of six hooked spines. Pupal development time averages about 9-10 days (range 7-13 days).
Adult. Moths are fairly small, measuring 1722 mm in wingspan. Sexual dimorphism is pronounced. The forewing of the male moth is yellowish centrally, bordered by a broad dark band bearing purplish scales. In females, however, the entire fore-wing is dark, sometimes almost black, but also bears reddish or purplish scales. The thorax is light in males, but dark in females. The hind wings of both sexes are transparent with a silvery tint. Adults are most active at night when the temperature exceeds 27°C, relative humidity is high, and there is little air movement. Such conditions are optimal for mating and oviposition. Indeed, these activities cease if temperature falls below 18-20°C. Mack and Backman (1984) studied adult longevity and fecundity in relation to temperature. Longevity varied from 20 days if held at 17°C, to eight days if held at 32-35°C. Mean fecundity varied from only about 20 eggs per female when reared at 17°C, to a maximum of 110 eggs at 30°C, then decreased at higher temperatures. Mean fecundity per day was estimated at about 12 eggs. However, these values included an unknown proportion of individuals that did not reproduce. Other reports indicate that fecundity is about 100-450 eggs among females that successfully reproduce, with an average of about 200 eggs per female. Adult longevity under field conditions is estimated at about 10 days.
The biology was described by Luginbill and Ainslie (1917) and a review was published by Tippins (1982). Developmental data were given by Dupree (1965) and Leuck (1966). Rearing was described by Chalfant
Damage is caused by the larval stage which feeds upon, and tunnels within, the stems of plants. Normally the tunneling is restricted to the basal region of stalks, including the below-ground portion, and girdling may occur. In affected plants wilting is one of the first signs of attack, but buds may wither, and stunting and plant deformities are common. Plant death is not uncommon, and infested areas of fields often have a very thin stand.
Cultural Practices. Lesser cornstalk borer damage is largely restricted to sites with sandy soil. Modified planting practices have long been used to minimize crop loss in such locations. Populations tend to increase over the course of a season, so some damage can be avoided by early planting. Tillage and destruction of weeds are recommended before planting because this helps to destroy larvae that may be present in the soil and damage seedlings, the stage most susceptible to destruction. However, crop culture that uses conservation tillage (i.e., retention of crop residue at the soil surface) experiences little injury from lesser cornstalk borer feeding because the larvae feed freely on crop residue and other organic matter, sparing the young crop plants (All and Gallaher, 1977; All et al., 1979).
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