Distribution. Western corn rootworm is a widely spread native species, consisting of two recognized strains. The principal strain is Diabrotica virgifera virgifera LeConte, which occurs from Montana, Utah, and Arizona east to New York, Connecticut and Virginia. In Canada, it is known only from southern Ontario. First noticed attacking corn in Colorado in 1909, it has gradually expanded its range through the midwestern corn belt and eastern states, where it often displaces northern corn rootworm, Diabrotica barberi Smith and Lawrence. This species is absent from the southeastern states, where banded cucumber beetle, Diabrotica balteata LeConte, is present, but successfully overlaps the northern range of the related spotted-cucumber beetle, Diabrotica undecimpunctata Mannerheim.
In southern Oklahoma, most of Texas, and south through most of Mexico the strain Diabrotica virgifera zeae Krysan and Smith predominates. It is known as "Mexican corn rootworm.''
Host Plants. The larval stage of this species feeds successfully only on plants in the family Gramineae. Although corn is the principal host, larvae can complete their development successfully on lovegrass, Eragrostis spp.; wheatgrass, Agropyron spp.; foxtail, Setaria spp.; rice; spelt (a primitive type of wheat); wheat, and perhaps barley (Branson and Ortman, 1967b; 1970). The adults feed readily on blossoms and foliage of Cucurbitaceae, and on the inflorescence of goldenrod, Solidago canadensis, and sunflower, Helianthus annuus, but these are not as satisfactory for adult longevity as is corn (Siegfried and Mullin, 1990). Although all cultivated squashes are attacked, beetles greatly prefer the cucurbitacin-rich wild species, especially the xerophytic species (Howe et al., 1976). Adults also feed on pollen of weeds such as pigweed, Amaranthus sp., and ragweed, Ambrosia sp., and spores of fungi (Ludwig and Hill, 1975). In the midwestern states, adults have shown an increased tendency to consume soybean leaves, but this is a nutritionally inadequate diet. (See color figures 6, 34, and 35.)
Natural Enemies. Western corn rootworm is remarkably free of natural enemies (Chiang, 1973). Some general predators, such as ambush bugs (Hemiptera: Phymatidae), and assassin bugs (Hemiptera: Reduvii-dae) will feed on adults, and mites (Acari: Laelaptidae, Rhodacaridae, and Amerosiidae) feed on larvae and eggs. The fly Celatoria diabroticae (Shimer) (Diptera: Tachinidae) parasitizes rootworms, but like the predators, it seems relatively unimportant in population regulation.
Life Cycle and Description. There is a single generation per year. The egg is the overwintering stage. Hatching occurs in the spring, followed by a larval feeding and pupation period of 1-2 months. Under ideal temperatures of 21-24° C, rootworms can progress from the egg to the adult stage in about 30 days. Adult emergence occurs between July and August in the corn belt, but during May-June in Texas (Cocke et al., 1994). Adults remain active and deposit overwintering eggs through September or October, depending on the onset of cold weather.
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Western corn rootworm larva.
Adult. The adults are yellowish to yellowish brown with black markings, and measure about 6 mm long. The beetle's head often is black, though it sometimes is pale. A common pattern on the elytra is a black stripe centrally, accompanied by another on each elytron. These normally are females. The alternate pattern commonly found in western corn root-worm is black elytra, with some yellow at the tips of the elytra and its margins. These normally are males. Mexican corn rootworm, in contrast, is green with a black head, lacking black on the elytra except for a small amount at the base. The adults of both subspecies display a fairly protracted pre-ovipositional period ranging from about 12-30 days, and then are thought to live for an additional 30-60 days in the ield, though they can survive and continue to produce eggs for over 90 days under laboratory conditions. Oviposition rates peak at about 10-15 days after initiation, and then taper off. Western corn rootworm beetles are quite active, and adults disperse readily from ields with mature corn to ields that are silking or pollinating. Females are particularly vagile. Adult dispersal occurs principally during periods of no wind or low wind speed, and beetles fly mostly at heights of less than 4 m (Vanwoerkom et al., 1983). (See color figure 108.)
The review of rootworms by Chiang (1973) is a helpful introduction to rootworm biology. Rearing methods for larvae generally use sprouted corn as a food source (Jackson, 1986; Branson et al., 1988); adult maintenance is described by Guss et al. (1976). Separation of western corn rootworm larvae from northern corn rootworm and spotted cucumber beetle was described by Mendoza and Peters (1964), but Krysan (1986) reported that the characters used in this key are imperfect. A female sex pheromone has been identified (Guss, 1976). Krysan and Branson (1983) discussed the biosystematics of Diabrotica spp. Mexican corn rootworm was described by Krysan et al. (1980) and Branson et al. (1982). A key to the adult rootworms was presented by Krysan (1986).
The principal injury caused by rootworms is destruction of the roots by larvae. The larvae irst feed on the rootlets and then burrow into the roots, often causing complete destruction of the root system. Plants with damaged roots are stunted, and tend to blow over (lodge) under conditions of high moisture and wind. Corn plants are quite tolerant to larval feeding, however, and sometimes support hundreds of larvae without great yield reduction. In the absence of adequate soil moisture, such plants may display
greater yield loss, or may die (Chiang et al., 1980). Root damage ratings are the preferred method of assessing larval feeding (Branson et al., 1980).
The adults also cause injury, though this stage generally is considered less damaging than the larval stage. The adults feed on the surface of foliage, particularly young foliage and leaf tips, causing the leaf tissue to die and to turn brown. More importantly, they feed on the young corn silk, sometimes completely clipping the silks to the tip of the ear and interfering with pollination. Such plants have ears with kernels irregularly placed or missing, particularly near the ear tip. When beetles are abundant, they consume all the silk and feed directly on the kernels at the ear tip. They also are attracted to openings in the ear husk caused by such insects as grasshoppers and caterpillars, which allow the beetles ready access to the kernels.
Western corn rootworm beetles are implicated in disease transmission. They have been found contaminated externally and internally with Fusarium spp., causative agents of corn stalk rot (Gilbertson et al., 1986). They also have been implicated in the transmission of maize chlorotic mottle virus (Jensen, 1985).
Considerable research has been conducted on insecticide-containing baits for beetle control. Dry baits have been formulated from corn cob grit impregnated with cucurbitacin and carbamate insecticides (Metcalf et al., 1987) and starch matrices impregnated with cucurbitacin or volatile semiochemicals (Lance and Sutter, 1990; Weissling and Meinke, 1991a,b). Liquid formulations of cucurbitacin and insecticide seem preferable over dry formulations, however, due to ease of application. Also, vial traps containing cucurbitacin, volatiles, or pheromone have been used to suppress populations (Lance, 1988). However, these techniques have been limited primarily to area-wide suppression efforts.
Cultural Practices. Several cultural practices affect western corn rootworm survival. Conservation tillage (no-till) results in delayed emergence of beetles rela tive to conventional tillage, but survival is not markedly affected (Gray and Tollefson, 1988a,b). Planting of corn between previous rows is preferable to planting within previous rows, presumably because larvae must disperse farther to find food in the first instance (Chiang et al., 1971). Earlier planting dates also favor rootworm survival, so it is advisable to delay planting in heavily infested soil if insecticides are not used (Musick et al., 1980; Fisher et al., 1991). High moisture levels resulting from flooding can reduce adult emergence, but the level of soil saturation necessary to harm insects is detrimental to corn growth (Reidell and Sutter, 1995). Strip cropping, while deleterious to some species with a narrow host range, seems not to be a deterrent to feeding by western corn rootworm beetles, as they were more numerous on one-, two-, and four-row corn plots alternated with beans than on 16-row corn plots (Capinera et al., 1985). The most important cultural practice is crop rotation. Western corn rootworm populations are suppressed or eliminated by rotation to a non-host crop such as soybean, so continuous corn cropping is discouraged. However, there are reports from Texas that Mexican corn root-worm survives rotation with sorghum (Stewart et al., 1995), and there is preliminary evidence that western corn rootworm in the midwestern states is beginning to display similar behavior. It is not yet apparent whether corn rootworm has adopted a multi-year diapause, as has northern corn rootworm in some areas, or it has adapted to feeding on other hosts. Corn root-worms are not a problem on muck soils, and they do not cause appreciable injury on sandy soils unless irrigated.
Biological Control. Entomopathogenic nematodes (Nematoda: Steinernematidae and Heterorhabditi-dae) have been evaluated for control of western corn rootworm larvae (Wright et al., 1993; Jackson and Hesler, 1995, Ellsbury et al., 1996; Jackson, 1996). Steiner-nema spp. nematodes can provide good suppression, though not as effective as some insecticides. Nema-todes are readily applied through irrigation systems and generally require high volumes of water for effective application. Effective suppression also require high rates of application, which can be a considerable expense.
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