Diabrotica barberi Smith and Lawrence Coleoptera Chrysomelidae

Natural History

Distribution. Northern corn rootworm is a native species, and was irst discovered attacking corn in Colorado. It has since spread eastward, principally to the corn belt in the midwestern states. Range expansion was caused primarily by a change in crop production practices: culture of corn continuously in the same fields. In the United States, northern corn rootworm occurs in the Great Plains region from North Dakota to Oklahoma, and east to the Atlantic Ocean. As its common name suggests, this insect is largely absent from the south, but is present in Tennessee and the northern regions of Alabama and Georgia. In Canada, northern corn rootworm is found in southern Ontario and Quebec. Northern corn rootworm and western corn rootworm, Diabrotica virgifera LeConte, can both be found in the same fields, but western corn root-worm has displaced northern corn rootworm, or at least reduced its abundance, in some areas. Displacement has been attributed to greater insecticide resistance in western corn rootworm, but western corn rootworm also has a much higher reproductive rate than does northern corn rootworm.

Host Plants. Northern corn rootworm has a rather restricted host range. The larvae develop only on plants in the family Gramineae. Corn is the only crop regularly attacked by larvae, but development can also occur to a lesser extent on millet, rice, and spelt (a primitive type of wheat). Also, some survival occurs on rangeland forage grasses such as foxtail, Setaria spp.; wheatgrass, Agropyron spp.; weeping lovegrass, Eragrostis curvula; and Canada wildrye, Elymus canadensis (Branson and Ortman 1967a, 1971). Adults consume several types of food, with corn kernels, corn silk and corn tassel tissue favoring survival and reproduction, though corn leaf tissue is inadequate. Blossoms from goldenrod, Solidago canadensis; squash; and sunflower also are suitable (Lance and Fisher, 1987; Siegfried and Mullin, 1990). Additionally, adults feed on pollen from plants in the families Gramineae, Compositae, Leguminosae, and Cucurbitaceae.

Natural Enemies. Several beneficial organisms have been found associated with northern corn root-worm, but they seem to be of little consequence. Predators include ground beetles (Coleoptera: Carabi-dae), soldier beetles (Coleoptera: Cantharidae), and mites (Acari: Laelaptidae, Rhodacaridae, and Amero-siidae). Probably the most important parasitoid is Celatoria diabroticae (Shimer) (Diptera: Tachinidae), a species that also attacks other Diabrotica spp., but its relationship with northern corn rootworm is not clearly known. Pathogens such as protozoa, fungi, and bacteria have been isolated from rootworm larvae. The soil-inhabiting fungus Beauveria bassiana can cause epizootics in larval populations, but this occurs infrequently.

Life Cycle and Description. Generally there is a single generation per year, with the egg stage overwintering, but sometimes eggs pass through more than one winter before hatching occurs. Eggs hatch in late spring, larvae feed until about June, and adults begin to emerge in July. Development is not synchronous, so emergence is protracted. The temperature threshold for development of the various stages is 10-11°C. The mean period required for northern corn rootworm to develop from hatching to emergence of the adult is about 100,47, and 30 days when reared at 15°, 21°, and 27°C (Woodson and Jackson, 1996).

  1. The eggs are oval and whitish. Eggs are small, measuring only about 0.6 mm long and 0.4 mm wide. The surface of the eggs, when examined microscopically, are marked with a polygonal (mostly hexagonal) pattern, with each polygon containing small pits. The presence of pits serves to differentiate northern corn rootworm eggs from those of western corn rootworm, which lack pits. Eggs of spotted cucumber beetle, Dia-brotica undecimpunctata Mannerheim, also contain polygons with pits, but the pits are proportionally larger (Atyeo et al., 1964; Rowley and Peters, 1972; Kry-san, 1986). Eggs are deposited in soil cracks or other breaks in the soil that allow the female to access moist soil. Thus, in irrigated fields the eggs are frequently found in the furrows (Weiss et al., 1983). The female commonly deposits eggs in loose clusters of 3-10 eggs. Females select corn as an oviposition site over other crops and weeds (Boetel et al., 1992). Eggs survive temperatures of 4-5°C for brief periods, but survival decreases markedly at —10°C. The temperature threshold for embryonic development is about 11 °C. The number of days required for hatch of eggs in the spring is about 21, 32, and 79 days when held at 25°, 20°, and 15°C, respectively (Apple et al., 1971; Woodson et al., 1996). Duration of egg diapause is somewhat variable (Fisher et al., 1994), and some members of the population pass through more than one winter before hatching (Landis et al., 1992; Levine et al., 1992).
  2. The larva is elongate and cylindrical in shape, tapering toward the head. The body is white, bearing relatively few hairs or spines. The head capsule, and thoracic and anal plates, are yellowish brown. The three pairs of legs are brownish, and terminate in a single claw. The posterior end of the body bears a single retractile extension or tubercle. Larval development time at variable temperatures was determined by Golden and Meinke (1991) to be about 47 days. However, Woodson and Jackson (1996) reported mean total larval development periods of 73, 32, and 21.5 days at 15°, 21°, and 27°C, respectively. Mean development time of the three instars was 6.5, 6.5, and 19 days, respectively, at 21°C. Head capsule widths average 0.22, 0.33, and 0.49 mm, respectively, for instars 1-3. Larvae attain a length of about 7 mm at maturity.

Northern corn rootworm larva.

Northern corn rootworm larva.

  1. The larvae prepare small cells in the soil for pupation. Pupation often occurs within the upper 5 cm of soil. The pupa is white, except for the reddish brown eyes. It measures about 4.5 mm long and 2.5 mm wide. In general form, it resembles the adult except that the wings are reduced in size and twisted ventrally. Also, the pupa bears a pair of hooks at the tip of the abdomen. Duration of the pupal stage is about 27, 12, and 7 days when reared at 15°, 21°, and 27°C, respectively.
  2. The adults usually are yellowish or yellow-green, and lack the broad-black stripes or extensive black pigmentation found on the elytra of western corn rootworm. However, northern corn rootworm beetles in the northern Great Plains are sometimes striped. Northern corn rootworm beetles generally have yellowish to brownish antennae, tibiae, and tarsi, though in the eastern states they can be blackish. They measure about 6 mm long. The pre-oviposition period of beetles is about 14 days, followed by an oviposition period of 40 -60 days. Adults can survive up to nearly three months, and produce clutches of 20-30 eggs at about seven-day intervals. Total fecundity averages about 185 eggs per female on a good diet, but considerably less on suboptimal diets. However, if optimal temperatures are also provided, mean fecundity can increase to 274 eggs per female (Naranjo and Sawyer, 1987).

The biology of corn rootworm was given by Forbes (1892) and Chiang (1973) and some descriptive information by Krysan et al. (1983). A key to the adult, Diabrotica, and pictures of eggs are found in Krysan (1986). Mendoza and Peters (1964) provided a key to the principal mature rootworm larvae.

Damage

Newly hatched larvae feed on the root hairs and outer tissue of roots, but as they increase in size and appetite they burrow into the roots, often consuming them entirely. Root damage can inhibit plant growth, but also reduces the ability of the plant to resist wind, especially when the soil is wet. Plants that topple, or lodge, owing to damaged-root systems display reduced growth potential. Lodged plants are difficult to harvest. Larval feeding also can allow entrance of Fusarium fungi, resulting in stalk rot. Feeding by adults on corn silks occasionally is enough to interfere with pollination, and beetles sometimes cause direct injury by feeding on the kernels at the tip of the ear. Foliage feeding by adults of this species is rare, though it is quite common with the related western corn rootworm. Maize chlorotic mottle virus may be transmitted by rootworm beetles (Jensen, 1985). Adult root-worm densities of one per plant or greater suggest damaging larval densities during the next year if corn is planted in the same field.

Management

Sampling. A great deal of effort has gone into development of improved sampling protocols for root-worms, though the principal concern is the vast acreage of grain corn, rather than the modest acreage of sweet corn. With grain corn, large quantities of insecticide are applied at great expense to a crop with a modest profit margin, so there is ample incentive to improve decision-making relative to insecticide application.

Sampling has been investigated for most rootworm life stages. The egg and larval sampling has been studied extensively (Weiss and Mayo, 1983; Hein et al., 1985; Fisher and Bergman 1986; Ruesink, 1986; Tollef-son 1990), but remains mostly a research protocol due to the high labor requirements. Thus, most field-level sampling involves adult population monitoring, with its densities used to predict larval damage during the following year. Visual assessment of adult densities can be made by whole-plant or ear-zone counts, and sequential sampling protocols have been developed (Foster et al., 1982; McAuslane et al., 1987). However, traps are usually preferred because they can be left in the field for longer periods and are less affected by beetle movement and short-term weather phenomena. A simple and popular trap is the yellow sticky trap (Hein and Tollefson, 1984; Tollefson, 1986; Hesler and Sutter, 1993). Chemical attractants also have been investigated. Chemical arrestants such as cucurbitacin, lures including sex pheromone, and other chemicals isolated from the Cucurbitaceae such as eugenol, isoeugenol, 2-methoxy-4-propylphenol and cinnamyl alcohol have been found successful (Shaw et al., 1984; Ladd, 1984; Levine and Gray, 1994; McGovern and Ladd, 1990; Hoffman et al., 1996a).

Insecticides. Corn producers generally rely on soil-applied insecticides to protect their crops from larval rootworm damage. Either liquid or granular formulations may be applied to the root zone, and it can be applied at planting time or after the corn is partly grown. Where corn is cropped continuously, insecticide resistance and enhanced microbial degradation of insecticides have been noted (Levine and Oloumi-Sadeghi, 1991). Suppression of adults with foliar insecticides could be made to prevent damage during the subsequent year, but in many cases this is not done owing to the vagility of adults. Adult control is also needed occasionally to protect corn silks and ear tips from injury. There is considerable use of insecticide-containing bait to accomplish adult control with greatly reduced insecticide application rates in midwestern states (Lance, 1988).

Cultural Practices. Levine and Oloumi-Sadeghi (1991) provided an excellent review of how cropping practices affected corn rootworm biology. Planting synchrony is among the cultural factors affecting corn rootworm populations. Beetles depend on the availability of green silks and pollen resources that are available only briefly. Late planting of crops delays and extends the development of the insect population, and reduces survival, possibly owing to deprivation of corn roots to hatching larvae (Musick et al., 1980; Bergman and Turpin, 1984). Thus, late plantings do not require insecticide treatments for this insect, but are likely to be heavily infested in the following year owing to attraction of beetles to late-planted corn. Availability of corn flowers is important for adult survival. Beetles tend to disperse to early-flowering corn until it senesces, then disperse to later-flowering fields (Naranjo and Sawyer, 1988).

The most important cropping practice is crop rotation, which normally results in destruction of root-worms owing to the inability of larvae to survive on crops other that corn. Thus, corn routinely is rotated with a non-host such as soybean. This apparently has led to increased incidence of rootworm populations that diapause through two years before hatching, potentially reducing the effectiveness of this practice (Levine et al., 1992; Steffey et al., 1992). As yet, most areas have not experienced a serious problem with two-year life cycles, and crop rotation remains a preferred management practice.

If corn is to be planted into a field that previously supported corn, tillage and disking will have few effects on insect survival (Gray and Tollefson,

1988a,b). However, planting new rows of corn between the old rows is often desirable, because the eggs often are concentrated at the base of the old corn plants, and young larvae will experience difficulty in dispersing such distances (Chiang et al., 1971). Application of irrigation water and additional nitrogen help to offset loss of roots.

Biological Control. Effective biological controls have yet to be developed for corn rootworms. Considerable research has been conducted on the use of entomopathogenic nematodes (Nematoda: Steinerne-matidae and Heterorhabditidae) for larval suppression. Results have not been consistent, and the cost of this approach remains prohibitive, but good progress is being made on use of these biotic agents (Thurston and Yule, 1990; Wright et al., 1993; Jackson and Hesler, 1995).

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