Wyoming distribution map
The finned grasshopper inhabits the shortgrass prairie and to a limited extent the desert prairie and the western edge of the mixedgrass prairie, ranging from Wyoming and Nebraska to central Mexico. Vegetation of these grasslands consists of the dominant plant, blue grama grass, several subdominant grasses, forbs, small shrubs, and much bare ground. During daylight hours, the finned grasshopper rests and conducts its activities on blue grama plants and on bare ground; at night it rests on bare ground and ground litter under canopies of blue grama.
Collected from a mixedgrass prairie site in southeast Wyoming, live weight of six males averaged 202 mg and six females 461 mg (dry weight: males 59 mg, females 140 mg). The average dry weight of males and females are similar to those of the bigheaded grasshopper, Aulocara elliotti, a large and injurious rangeland species. Assuming an equal impact by grasshoppers of the same size and a density of one young adult finned grasshopper per square yard, we can estimate an annual loss of 20 pounds (dry weight) of forage per acre. During an outbreak, rangeland sites harbor assemblages of grasshopper species in which the finned grasshopper may be present as a subdominant member. Even the low density of the finned grasshopper during an outbreak contributes to serious damage of forage.
Observations of feeding behavior of the finned grasshopper were made in a laboratory cage (1 ft3) provided with sod translocated from the mixedgrass prairie of southeast Wyoming. A typical observation was made of a female starved for 19 hours and placed on bare soil in the cage. She crawled around on the bare soil for three minutes before she contacted a blue grama plant. She raised up diagonally on the plant and fed on a green leaf three inches long from tip to base, handling the leaf with her front tarsi while resting on bare soil with mid and hindlegs. She repeated this behavior five times. The sixth leaf she cut at one inch level, held onto it, fed on all of the green portion, and dropped the dry yellow tip. Finally she fed on the stub of blue grama. The time expended searching and feeding on six leaves and one stub was 28 minutes. Another female fed in the same manner, except for picking up an inch section of a fresh blue grama leaf from the soil surface and consuming all of it. She took 18 minutes from start to end of feeding.
The nymphs are identifiable by their shape, structures, and color patterns (Fig. 1-5).
1. Head: face nearly vertical; compound eye with bottom
third dark brown and black, upper two-thirds tan with three dark, diagonal
stripes; antennae filiform, proximally gray, distally black.
2. Pronotum: median carina distinct, incised twice; posteroventral angle of lobe approximating a right angle.
3. Hind femur with inner face mainly black, one pale yellow bar in front of knee (Fig. 8); in instar I the outer surface of the hind femur is strikingly black except for a light area proximally (Fig. 1); hind tibia usually black, sometimes gray.
4. Body short and robust; color brown with black markings, venter generally yellow or pale tan, and unspotted.
Nymphs of the finned and Kiowa grasshoppers occupying the same habitat present a problem of identification. The easiest way of telling the nymphs apart is to examine the inner face of the hind femur. Nymphs of the finned grasshopper have the inner face chiefly black with one pale bar in front of the knee, while the inner femur of the Kiowa grasshopper is less black and has two pale bars. Also the phenologies of the two species are different; the finned grasshopper hatches and develops about four weeks later in the season, but some of the instars overlap.
Oviposition was not observed in nature and rarely in cages, presumably because of the secretive behavior of females. In a cubic foot cage into which blue grama sod had been translocated from the natural habitat, a female was observed drilling and attempting to oviposit for 60 minutes into a small bare area (2 sq in) among grama plants. Search of the soil, however, revealed no eggs. At the end of life of six pairs that had been held individually in six cages in the laboratory, the sod and soil were examined for eggs. Production ranged from 1 to 48 eggs per female and averaged 30. The females laid in the small bare areas between grama plants at depths of 1/4 inch (top eggs of a clutch) to 1 1/4 inches (bottom eggs). No pod was formed, but a very light coating of froth enveloped some of the eggs. Number of eggs in a clutch ranged from 12 to 18. Eggs were 4.6 to 5.2 mm long and yellow to dark brown, some were two-toned yellow and dark brown (Fig. 10).
An interesting result of the cage studies was the lack of oviposition by females in the summer of 1999 when they were exposed daily to 12 hours of light and heat by a 25 watt incandescent bulb and to daylight entering through south-facing windows. In 2000, the 25 watt bulb of each cage was programmed for 7 hours on and 17 hours off with light coming through south-facing windows. Under these conditions each female produced eggs. We venture that like Melanoplus devastator, maturation of T. aspera is triggered by declining day lengths of late summer. Apparently the stimulus of declining photoperiod is also required by Cordillacris crenulata to develop eggs and oviposit, as this species did not produce eggs under 12 hours of daily light during adulthood.
Distribution of the finned grasshopper is spotty in the north of its
range, with populations inhabiting limited acreage of shortgrass prairie.
In the study sites in southeast Laramie County, Wyoming, the population
was localized in an apparently favorable habitat of several acres. In contrast
to other species of grasshoppers, such as Trachyrhachys kiowa which
inhabited a much larger area outside this favorable site, T. aspera
was notably absent. A New Mexico dot map indicates that near the center
of its geographical range this grasshopper is widely distributed over shortgrass
prairie, however even here densities have remained low.
Calculated daily mortality rates of adults were low (approximately 2 percent) allowing this stage to survive into late fall—through September and October. Why the species remains numerically low and a subdominant member of the grasshopper assemblage is an interesting and important question. One possibility is that females have a low fecundity in nature like in our cage study. Another possibility is that high egg mortality may occur during the period following deposition, a time of minimal rainfall. Because the eggs lie loosely in the soil unprotected by a pod, they may be at great risk of desiccating.
Finned grasshoppers appear to have low heat tolerance. On clear days they begin to stilt at 10:40 am (ground temperature 115°F, air 73°F). Soon afterwards they climb on top of grama grass, approximately 1 inch above the soil surface; then face the sun directly so that only the front of the head is impinged while the rest of the body is shaded. They were observed to avoid high temperatures from morning to late afternoon (10:40 am to 4:30 pm). During this time no other voluntary activities were observed. The question arises: when do these grasshoppers feed? A female translocated onto rangeland grama grass fed after sunset (6:56 pm, 14 September 2000, soil temperature 68°F, air temperature 67°F). Evidently finned grasshoppers can make adjustments in the time of their feeding to the temperature flux of their habitat. Intensive investigation will be required to discover all the details of their daily activities.
Joem, A. 1979. Resource utilization and community structure in assemblages of arid grassland grasshoppers (Orthoptera: Acrididae). Trans. Amer. Entomol. Soc. 105: 253-300.
Joern, A. 1982. Vegetation structure and microhabitat selection in grasshoppers (Orthoptera: Acrididae). Southwestern Naturalist 27: 197-209.
Pfadt, R.E. and R. L. Lavigne. 1982. Food habits of grasshoppers inhabiting the Pawnee site. Wyoming Agr. Exp. Stn. SM 42.
Richman, D.B., D.C. Lightfoot, C.A. Sutherland, and D.J. Ferguson. 1993. A manual of the grasshoppers of New Mexico, Orthoptera: Acrididae and Romaleidae. New Mexico State University Handbook 7.
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