Spatiotemporal Characteristics of Rangeland Grasshopper (Orthoptera: Acrididae) Regional Outbreaks in Montana

CIGLIANO, M.M.; KEMP W.P.; KALARIS, T.M.

JOURNAL OF ORTHOPTERA RESEARCH

The Orthopterist's Society

Año: 1995 vol. 4 p. 111 - 126

1082-6467

ABSTRACT.-A study was conducted to examine the spatiotemporal characteristics of two regional outbreaks of rangeland grasshop- pers in Montana during the periods 1959-1966 and 1984-1992, and to evaluate the temporal relationship of grasshopper abundance to both vegetation types and geomorphological regions. Results showed that grasshopper outbreaks, in Montana, can be characterized as follows: 1). Outbreaks are aperiodic and short-lived. 2). Over the long-term, the extent of the areas exhibiting high densities fluctuates to the extremes. 3). Areas of high densities are present every year, although they may be geographically restricted. 4). Areas of high densities can remain stable, decline, expand, or collapse from one generation to the next. 5). High densities can arise or fall simultaneously over wide areas. 6). Densities generally vary inversely with distance away from the edge of high density areas. 7). New regional outbreaks do not appear to be the result of grasshopper influx from active infestation areas, although migration may occur. 8). If areas exhibiting high densities expand, the extension of the boundaries or the appearance of separate new high density areas beyond those boundaries does not follow any specific pattern. 9). Although no chronically high density areas could be detected, some vegetation types appeared to be more inclined to high densities. 10). The Western Sedimentary Plains was identified as the most affected region during outbreak years (1965 and 1986-1987). 11). Outbreaks appeared not to be self-perpetuating. In terms of the prevailing hypotheses concerning insect outbreaks, the geographical behavior of high density areas prevented their being classified as either gradient or eruptive; they appeared to exhibit behaviors of both. Our results suggest that there is a need for an extension of the existing paradigm concerning insect outbreaks. [KEY WORDS: Insecta, outbreak dynamics, grasshoppers]