FG Populations

Field Guide to Common Western Grasshoppers: Populations

by Robert E. Pfadt

List of Species Fact Sheets (60 Species)

 

Grasshopper infestations or assemblages consist of the individuals of several species that live together in the same habitat sharing or competing for available food and space. Members of the dominant species outnumber members of other species and may make up more than 50 percent of the assemblage. Occasionally two or three species may become codominants. No evidence has been found for any essential relationship among species that brings them together. The habitat affords the minimum requirements for all the permanent species and ample measure for the abundant.

Grass-feeding species of grasshoppers are the most numerous in grasslands. In a northern mixedgrass prairie site 18 miles northwest of Fort Collins, Colorado, a total of 24 species were recorded during an outbreak in 1981 (Table 2). Of the total, 14 were grass feeders, six were mixed feeders, and four were forb feeders. The number of individuals of grass-feeding species made up 85% of the total population. The dominant grasshopper, Ageneotettix deorum (Scudder), contributed 52% of the population. A second example of an outbreak population in northern mixedgrass prairie was the assemblage inhabiting a site 15 miles north of Hartville, Wyoming, where 16 species were recorded (Table 2). Nine species were grass feeders, one a mixed feeder, and six were forb feeders. The number of individuals of grass feeding species made up 89% of the population. The dominant grasshopper, Aulocara elliotti (Thomas), contributed 74% of the population.

Why was A. deorum dominant in one mixedgrass prairie site and A. elliotti dominant in another? And why was Cordillacris occipitalis (Thomas) the second most abundant in one site and entirely missing from the other? Answers to these questions are not available. An hypothesis for the cause of the observed variations in densities was the differences in habitat in conjunction with differences in requirements of the grasshoppers. Although both sites are part of the northern mixedgrass prairie, the soil, slope, and vegetation of each differ significantly. Grasshopper species vary in densities and dominance depending on the soil, vegetation, topography, and use of a habitat. Because of differential effects of weather, parasites, disease, or insecticidal treatments, the densities of grasshopper species inhabiting a rangeland site may change with time. The abundant species, however, tend to retain their dominant status over the years.

The composition of grasshopper assemblages is characteristic of various grassland types. A scout working in a western state expects particular species to compose economic infestations in certain areas. Table 3 lists species abundant in several grassland types and in disturbed land (crop borders, fence rows, reversions, roadsides). Because the species composition of grasshopper assemblages infesting particular habitats remains almost the same year after year, a scout is aided in identifying nymphs by knowing the species that were present as adults during past years. Widespread species with high biotic potential, such as Aulocara elliotti and Ageneotettix deorum, inhabit many grassland types and become abundant members in various assemblages of grasshoppers. In outbreaks on desert grasslands of Arizona and New Mexico, for example, A. elliotti is often the dominant species (Table 2), as in many infestations of the northern mixedgrass prairie.

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