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The dusky grasshopper ranges widely in western North America from Canada to Mexico. The species inhabits several kinds of grasslands including the mixedgrass, shortgrass, bunchgrass, and desert prairies. It is most abundant in the northern mixedgrass prairie, and is the dominant species on certain rangelands of Saskatchewan. There it favors moist areas of rich grass and sedge growth interspersed with bare ground. In its southern distribution, mesic swales and roadsides dominated by western wheatgrass and prairies with Houston black clay soil dominated by Texas needlegrass afford favorable habitats.
The dusky grasshopper has been reported as a minor pest in alfalfa in Arizona and North Dakota and as a pest in fall wheat in Nebraska. In the latter case, adults migrated from depleted rangeland into the adjacent wheat crop.
The dusky grasshopper is a medium-sized species with marked sexual dimorphism in body size. Live weight of males averages 168 mg and of females 468 mg (dry weights: males 50 mg, females 135 mg).
In an unusual habitat, such as an alfalfa field or a roadside, the dusky grasshopper may feed on a mixture of forbs and grasses. Of 52 specimens collected from an alfalfa field in North Dakota, 30 had ingested alfalfa. In laboratory tests, starved individuals limited to single food plants fed well on certain forbs and refused others. Well-eaten plants included scarlet globemallow, prairie coneflower, dandelion, and a vetch, Vicia sparsifolia.
The dusky grasshopper attacks host plants in several ways. Sitting head-up on a leaf of needleandthread grass, an adult may begin feeding halfway up the leaf, cut through it, hold onto the cut section with the front tarsi, and consume it to the dry tip. Occasionally an adult will turn head-down on a grass or sedge and feed on green basal tissue. On short host plants such as needleleaf sedge, an adult from a horizontal position on the ground may reach up with its mouthparts and begin to feed on a green leaf 3/4 inch above its base, cut through it, hold onto the cut section, and consume it as far as the dry tip. From a horizontal position on the ground, adults may feed on recumbent green leaves or stubs of host plants. A noteworthy observation was made of the feeding of a nymph (instar IV) perched head-up on a 4-inch leaf of needleandthread. The nymph did not feed on the leaf on which it rested but reached out with its mouthparts and fed on the tip ends of three younger, 2-inch leaves. Dusky grasshoppers appear to be thrifty feeders as they consume most of whatever they attack eating all of the green and dropping chiefly the dry brown parts of leaves. They have been observed, however, to feed on dry plant litter and on dry cow dung.
Another example of fall dispersal by the dusky grasshopper is that of a population in western Nebraska that moved from depleted rangeland into adjacent winter wheat on 27 September 1967. Adults were also found in winter wheat in southeast Wyoming during October 1992, indicating earlier dispersal from surrounding mixedgrass prairie.
In flushed flight, the dusky grasshopper softly crepitates and travels 3 to 9 feet at heights of 3 to 12 inches. The flight is usually straight, with an occasional turn near the end.
The nymphs are identifiable by their structures, color patterns, and shape (Fig. 1-5).
1. Head. Face moderately slanting; antennae short and weakly clavate (club-shaped) in instars I to III, filiform in instars IV and V, compound eyes brown with pale tan spots, light line runs from base of antennae diagonally across middle of eye.
2. Pronotum with distinct median carina and plainly evident lateral carinae, disk sloping but not tectate; in green individuals usually a reddish brown to purple band runs down middle of pronotum and continues onto mesonotum, metanotum, and abdomen.
3. Hind femur with outer medial area green, tan, or grayish brown; instar I with medial area often pink in distal half. Tibia and femur of fore- and midlegs carinate (ridged) longitudinally; four ridges visible on outer face, often with black lines between the ridges.
4. Body color green, tan, or grayish brown.
The early instars (I to III) of the dusky grasshopper and of Chortophaga viridifasciata appear similar; both are usually green and structurally similar. They may be separated by a few characteristics that differ. The antennae of the dusky grasshopper are clavate while those of C. viridifasciata are ensiform. The outer faces of the femur and tibia of the fore- and midlegs of the dusky grasshopper have four distinct longitudinal ridges, which usually have black lines between them. Those of C. viridifasciata have two distinct ridges, which are the upper and lower carina, and a third weak ridge between them with no black lines between the ridges. Instar I of the dusky grasshopper has the medial area of the hind femur pink in the distal half, and that of C. viridifasciata is entirely green. Instars IV and V are identifiable by the shape of the pronotum. The disk of the dusky grasshopper slopes moderately, and that of C. viridifasciata is tectate (steep roof-like); the posterior angle of the disk in the dusky grasshopper is obtuse, and that of C. viridifasciata is acute.
For oviposition, females select bare ground that is interspersed among the native grasses of their habitat. Females test the soil by boring into the ground several times before finally depositing a clutch of eggs. When given a choice of five Texas soil types, ranging in coarseness from clay to gravel, ovipositing females preferred Houston black clay; they laid 111 of 204 egg pods in this soil. Males often attend an ovipositing female and may contact her, but they are kicked away by the female. After completing oviposition and withdrawing her ovipositor from the soil, the female brushes particles of soil and litter over the aperture of the hole with her hind tarsi. The pods are 3/4 inch long, slightly curved, and contain from 14 to 20 eggs (Fig. 10). Eggs are tan and 4.0 to 4.4 mm long.
An assemblage of 19 species of grasshoppers occupied the site during the period of study from 1967 to 1971. In 1968 the dusky grasshopper made up approximately 74 percent of the population, reaching a peak density of 12 early instar nymphs per square yard. Mortality of young nymphs was high in both 1968 and 1969. Daily mortality decreased after the grasshoppers reached instar IV. The average daily mortality from instar IV through the adult stage was 3 percent in 1968 and 5 percent in 1969. The population gradually declined from 1968 to 1971. The unusually luxuriant growth of vegetation in 1970 and 1971 suggested that the cause of grasshopper decline was decreased soil temperature due to shading of the soil surface. These physical factors delayed the development and maturation of the dusky grasshopper, resulting in a decrease of egg production. Additionally, because the dusky grasshopper oviposits in bare ground, a reduction of such sites probably incited adults to emigrate. Circumstantial evidence for such movement was observed after lightning caused a fire in part of the study area in August 1969. All grasshoppers perished in the burned area, but the next year dusky grasshoppers migrated from the unburned area, which supported a heavy growth of vegetation, to the burned area, which supported light growth and contained an abundance of bare ground.
Observations in Montana, Texas, and Saskatchewan indicate that the dusky grasshopper finds a favorable habitat in sites of heavy clay soil covered by a good growth of wheatgrasses or needlegrass and prevalent bare ground for basking and oviposition. Nevertheless, the species also inhabits areas of fine sandy loam soil in the mixedgrass prairie. As a case in point, in southeast Wyoming (Laramie County), sites dominated by blue grama, needleandthread, and needleleaf sedge support assemblages in which the dusky grasshopper is a subdominant with densities of 0.1 to 1 young adult per square yard. In a few roadside sites, with sparse western wheatgrass and abundant bare ground, the dusky grasshopper becomes dominant and reaches peak densities of approximately ten young adults per square yard.
Later in the day the grasshoppers bask for a second time until sunset, after which they enter nighttime shelters.
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Bailey, C.G. and P.W. Riegert. 1973. Energy dynamics of Encoptolophus sordidus costalis (Scudder) (Orthoptera: Acrididae) in a grassland ecosystem. Canadian J. Zool 51: 91-100.
Isely, F.B. 1937. Seasonal succession, soil relations, numbers, and regional distribution of northeastern Texas acridians. Ecol. Monog. 7: 318-344.
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