Wyoming distribution map
The obscure grasshopper is widely distributed in the grasslands of North America. It thrives in habitats of short grasses and mid grasses, preferring blue grama, a short grass, for food and patches of mid grasses for roosting and basking. On stems of the latter, grasshoppers rest vertically head up for the greater part of the day. In the tallgrass prairie, scattered populations may occupy upper ridges vegetated by short grasses.
Of the three weight divisions of range grasshoppers, the obscure grasshopper is in the lightest group. Live weights of males from the mixedgrass prairie of eastern Wyoming average 66 mg and females 143 mg (dry weight males 21 mg and females 48 mg).
The obscure grasshopper feeds chiefly on the green leaves of grasses. The usual method of attack is to sit vertically, head up, on a leaf of blue grama, cut it near the middle, hang on to the cut portion with the front tarsi, and devour it to the tip. From a horizontal position on bare ground, this grasshopper may also feed on a recumbent leaf of blue grama. Feeding toward the base, it leaves a narrow section of leaf. Occasionally a grasshopper may feed head down on a green leaf of needleandthread, but this orientation is not maintained long before it turns around to its usual head up direction.
Evasive flight is straight and silent for distances of 2 to 5 feet at heights of 3 to 6 inches. Flying grasshoppers may alight on vegetation or on the ground surface.
The nymphs are identifiable by their shape, external structures, and color (Fig. 1-5).
1. Head with face strongly slanted, antennae ensiform; lateral foveolae triangular, invisible from dorsal view.
2. Pronotum with distinct, parallel lateral carinae and distinct median carina. The median carina is entire (uncut) in instars I to III; slightly cut in instar IV and more so in instar V.
3. General color pale tan, markings present but pale with little contrast to the general color. Dark markings becoming more evident in instars IV and V. Hind femur with dark stripe on upper part of medial area, stripe faint in earlier instars. Hind tibia pale yellow; gray in front in instars IV and V.
Early instars of three species, Opeia obscura, Eritettix simplex, and Psoloessa delicatula, often occur together and may cause a problem in identification. They hatch late in the season and appear superficially similar. O. obscura, however, begins to hatch a month earlier than the other two species. A few distinct characters separate them readily. Nymphs of P. delicatula have the face nearly vertical, while the faces of O. obscura and of E. simplex are strongly slanting. The latter two can be separated by the differences in color patterns of the pronotal disk; E. simplex has a dark velvet band along each side, while O. obscura is entirely pale tan.
No special study of the obscure grasshopper's mating behavior has been made. In a laboratory terrarium, one observation was made of a male mounting a female and attempting, unsuccessfully, to engage his genitalia with the female's.
Females deposit their eggs deep in the soil and form a pod with weak earthen walls (Fig. 10). The eggs themselves are held together tightly and covered by cementing secretions of the female. An observation of oviposition was made of a caged female in a laboratory terrarium that contained mixedgrass sod and bare ground. The female chose bare ground beneath a 15-watt incandescent bulb. She worked her ovipositor into the soil to the full length of the abdomen and took 100 minutes to complete oviposition, after which she took 20 minutes to withdraw her abdomen and 90 seconds to brush loose soil over the hole with her hind tarsi. Following this parental care she walked away.
Pods of the obscure grasshopper are seven-eighths inch long and contain eight to ten small (4 to 4.3 mm long), pale tan eggs. The eggs overwinter, but no study of their embryology has been made. The species has one generation annually.
The fluctuations of population densities of the obscure grasshopper were recorded in an assemblage of 19 species in the mixedgrass prairie of eastern Wyoming (Table 1). An increase took place over a two-year period just before a sudden five-fold increase occurred from 1973 to 1974. The assemblage tripled at the same time, indicating favorable conditions for nearly all of the species. The response of the obscure grasshopper and the dominant species of the assemblage, Ageneotettix deorum, followed Parker's model of gradual increase for a period of a few years, then a large increase of numbers in the following year. The assemblage, however, was decreasing until the final year, but the rate of increase of some species was great enough to cause the whole population to go from noneconomic to economic without reaching outbreak proportions (25 or more young adults per square yard).
On the shortgrass prairie of northeastern Colorado, increases in a population of the obscure grasshopper, as the dominant species in an assemblage of five species, did not appear to follow Parker's model. Rather, this species increased by 1.7 to 2.7 fold over a period of four years to reach a peak density of 2.2 adult grasshoppers per square yard. On the other hand, this peak density may be less than the density possible under more favorable conditions.
Prevalence of the obscure grasshopper in the mixedgrass prairie of eastern Wyoming is moderate as indicated by the results of the 1991 grasshopper survey. The species was found in 77 of the 419 sites examined (18 percent). It occurred chiefly as a subdominant in assemblages of grasshoppers. In the mixedgrass prairie of Montana, a study of the grasshopper fauna in 1953 and 1954 showed that the obscure grasshopper was present in ten sites out of 38 (26 percent) and dominant in one of these. In the latter site, the assemblage consisted of nine species with a total density of three per square yard.
Because of their habit of sitting vertically on grasses, they are prone to remain inactive. When nymphs are flushed from their perches, they usually jump onto another plant. Occasionally they land on bare ground but quickly climb a grass leaf or stem. Adults may jump and behave like the nymphs or fly evasively.
High soil temperatures of 110° to 130° and air temperatures at or above 90°F induce the grasshoppers to seek protection. They react in three main ways: they climb up to 4 inches on midgrasses, or they take diagonal positions on blue grama facing the sun to expose less of the body surface, or they rest in the shade under a canopy of vegetation above ground level on blue grama.
Their day ends with basking in the evening rays of the sun while resting vertically on grasses. As sunlight dims and temperatures cool, they eventually assume their nighttime positions.
Anderson, N. L. 1973. The vegetation of rangeland sites associated with some grasshopper studies in Montana. Montana Agr. Exp. Stn. Bull. 668.
Anderson, N. L. and J. C. Wright. 1952. Grasshopper investigations on Montana range lands. Montana Agr. Exp. Stn. Bull. 486.
Capinera, J. L. and D. C. Thompson. 1987. Dynamics and structure of grasshopper assemblages in shortgrass prairie. Can. Entomol. 119: 567-575.
Larsen, J. C., J. A. Hutchason, and T. McNary. 1988. The Wyoming Grasshopper Information System. Cooperative Agricultural Pest Survey, University of Wyoming, Laramie.
Mulkern, G. B., K. P. Pruess, H. Knutson, A. F. Hagen, J. B. Campbell, and J. D. Lambley. 1969. Food habits and preferences of grassland grasshoppers of the North Central Great Plains. North Dakota Agr. Exp. Stn. Bull. 481.
Pfadt, R. E. 1977. Some aspects of the ecology of grasshopper populations inhabiting the shortgrass plains. Minnesota Agr. Exp. Stn. Tech. Bull. 310: 73-79.
Pfadt, R. E. and R. J. Lavigne. 1982. Food habits of grasshoppers inhabiting the Pawnee Site. Wyoming Agr. Exp. Stn. Sci. Monogr. 42.
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