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Spatial Analysis of Ecological Factors Related to Grasshopper (Orthoptera: Acrididae) Population Dynamics in Wyoming
By Scott P. Schell
A Thesis Submitted to the Department of Plant, Soil and Insect Sciences and The Graduate School of the University of Wyoming in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE in ENTOMOLOGY Laramie, Wyoming July 1994
I also want to thank Larry DeBrey for opening all of those wire gates and being a friend and Chuck Bomar for his friendship and fly fishing expertise.
Table of Contents
Landform-Soil map of Wyoming.
In North America only one species of grasshopper has ever been refered to as a locust. The Rocky Mountain Locust (Melanoplus spretus Walsh) caused the early settlers of the American and Canadian west and mid-west great hardship (Evans 1971). The Rocky Mountain Locust differed in one major respect from the locusts of the tropics and sub-tropics in that it was univoltine and its eggs had an obligate diapause period (Riley 1891). The widespread destruction of crops caused by the Rocky Mountain Locust was the impetus behind the original government involvement in grasshopper management (Evans 1971). The U.S. Congress, in 1876, established a three-man commission to study the Rocky Mountain Locust and other insect pests of national importance (Evans 1971). However, the last known specimens of the Rocky Mountain Locust were collected in 1902 (Turnbull 1980). It is theorized by Lockwood and DeBrey (1990) that the coming of intensive agricultural practices to river valleys in the Rocky Mountain region (the area that is believed to have been the locusts' permanent breeding ground) led to its extinction.
The extinction of the Rocky Mountain Locust does not mean that humankind's competition with grasshoppers is over North America. In the U.S., there are nearly 400 species of grasshoppers in 17 western states alone (Pfadt 1988). Although most species of grasshoppers are either innocuous or beneficial to our interests, some grasshoppers may reach high densities and cause severe damage to rangelands. In Wyoming, 13 species can cause damage to crops or rangeland (DeBrey et. al. 1993). Of these pest species, none exhibit all of the characteristics that define locusts. However, several species can move long distances when triggered by unknown environmental cues (Pfadt 1988). For example, remains of swarms of the Lessor Migratory Grasshopper (Melanoplus sanguinipes F.) have been found on Knife Point Glacier, Wind River Mountains, Wyoming, at over 3500 m in elevation, which is well outside of their typical habitat (Lockwood et. al. 1994). M. sanguinipes adults were also documented moving several hundred kilometers from where they hatched in the Dakotas during outbreaks between 1938 and 1941 (Farrow 1990).
The grasshopper species that are pests on western rangelands share a general life cycle (Lockwood 1994). Adult grasshoppers mate in late summer. The female grasshoppers force their abdomens into the first 2-5 cm of the soil or root crowns of plants to lay eggs (Pfadt 1988). While ovipositing, the female excretes a frothy glue-like substance which hardens into a pod that protects the eggs. Females can deposit as many as twenty pods and a total of 400 eggs throughout their lives (Pfadt 1988). The eggs must undergo some embryonic development before entering diapause for the winter (Pfadt 1988). The eggs depend on the soil atmosphere for air and moisture, but in most species they are insulated from direct contact with the soil by the egg pod material (Pickford 1970). Diapause is broken when soil temperatures exceed 10o C and embryonic development is completed (Pfadt 1988). The eggs hatch sometime during the spring, with the exact timing depending on species and environmental conditions (i.e., the accumulation of degree-days). Acridids, being hemimetabolous insects, emerge from the eggs looking like miniature, wingless adults (Evans 1984). The newly hatched grasshoppers must develop through 5+1 nymphal instars (depending on the species and/or sex) to reach adulthood (Pfadt 1988).
Grasshoppers are the major above-ground invertebrate herbivore on western rangelands (Cigliano et al. 1994). They have a host of pathogens (fungal, viral, bacterial, and protozoan) and predators (arthropod, reptilian, avian, and mammalian) that attack them (Lavigne and Pfadt 1966). Catastrophic weather events, such as hail storms, can also be major source of mortality in grasshopper populations (Lockwood and Schell 1994). If pest grasshoppers escape these sources of mortality in numbers sufficient to damage crops or out-compete domestic livestock for rangeland forage, various control measures can be undertaken by agriculturalists or government pest managers. In cropland, grasshoppers are fairly easy to control because they are univoltine and the individuals that survive the control measures do not rebuild the population immediately. However, the management of acridids on rangelands in the United States differs from the rest of the world.
In Africa, Asia and Australia, locusts are controlled on rangeland to prevent them from reaching croplands, but in the United States, grasshoppers on rangeland are controlled to preserve the native forage for domestic livestock. In Wyoming, during the last major outbreak period, approximately $22.75 million was spent in 1985-1986 on grasshopper control measures by private, state and federal sources (Lockwood and Schell 1994). Approximately 2 million ha of Wyoming rangeland was sprayed with broad-spectrum insecticides during these two years as part of the control effort. The federal agency now charged with assisting private landowners and government grazing lease holders with grasshopper management is the United States Department of Agriculture's (USDA) Animal and Plant Health Inspection Service (APHIS). In order to manage grasshoppers, the conditions constituting a grasshopper outbreak had to be operationally defined by APHIS. A density of 9.6 grasshoppers per square meter (8 per square yard) over an area of at least 4,016 ha (10,000 ac) is defined as an outbreak (USDA-APHIS 1987).
APHIS also conducts annual adult grasshopper surveys to generate maps for use in predicting areas that may be vulnerable to grasshopper outbreaks the following year. Before 1985, the Wyoming Department of Agriculture (WDA) conducted the survey and provided the results to APHIS for their national grasshopper "hazard map". The survey method used by APHIS requires 25 to 50 stops be made by survey personnel in each county. At these stops, 18 visualized square foot counts are made using a method detailed by DeBrey et al. (1993) to determine grasshopper population densities. Areas of common densities are derived through subjective interpolation of the survey point data and are then hand-drawn on state maps by experienced personnel (Kemp et al. 1989). The exact methodology used by WDA is unknown but believed to be similar. The actual predictive capability of this method has proved inadequate (Davis et. al. 1992). However, the annual survey maps of Wyoming provide a powerful analytical and descriptive tool, as they constitute a spatially related data set covering 31 of the last 34 years. Access to the Cheyenne, Wyoming, USDA-APHIS office archives has recently expanded the database to 50 years, but this thesis will only examine the years 1960-81, 1985-1993.
To adequately characterize the spatial-temporal dynamics of grasshopper outbreaks in the 31 years of data, a geographic information system (GIS) is needed. GIS is a general term applied to a collection of programs that utilize a computer's enormous data handling and computational ability to allow a user to assemble and query layers of data that are related spatially (Liebhold et al. 1993). This technology has only just begun to be applied to ecological questions (Liebhold et al. 1993). However, several studies have used GIS to analyze the relation of insect outbreaks to geographic and ecological covariables. GIS is being used with remotely sensed data (satellite imagery) in Australia and Africa to predict the location of hatching areas of locust species (Bryceson 1989, Tappan et al. 1991). A study by the Canadian Forestry Service using GIS technology found that outbreaks of the Douglas fir tussock moth (Orgyra pseudotsugata McDunnough) were related to biogeoclimatic zones (Shepard et al. 1988). GIS has been used in Colorado to analyze the relation of grasshopper outbreaks to soil texture characteristics (Carter et al. 1992). Two studies conducted in Alberta, Canada, with GIS have shown that grasshopper outbreaks are significantly related to soil type and levels of rainfall (Johnson and Worobec 1988, Johnson 1989). In Montana, regional spatiotemporal grasshopper outbreak characteristics have been examined with geostatistics and GIS (Cigliano et al. 1993). They found that in Montana: 1) outbreaks are irregular and short-lived; 2) areas of high densities fluctuate to the extremes; 3) areas of high densities are present every year; 4) densities vary inversely with the distance away from the perimeter of an area exhibiting high densities; 5) new areas of high densities did not appear to be the result of grasshopper migration; 6) no chronically high density areas could be detected but some vegetation types appear to be more prone to high densities; 7) high densities can arise simultaneously over wide areas; 8) areas of high densities, once initiated, can decline, expand or collapse; 9) if areas exhibiting high densities expand there appears to be no specific pattern to the expansion; 10) outbreaks, at the scale studied, do not appear to be self-perpetuating but sensitive to external environmental variations (Cigliano et al. 1993).
The characterization of grasshopper population dynamics, as was done in the Montana study, holds great interest for insect ecologists and grasshopper managers. Current theory on pest insect population dynamics is based on outbreaks composed of single insect species in forest ecosystems. A rather clear dichotomy of population dynamics may exist in these situations. Berryman (1987) has named the two dynamics eruptive and gradient. Eruptive population dynamics are characterized by having foci from which pest populations build and spread outward from through succeeding generations (Berryman 1987). Gradient population dynamics lack foci and depend on external environmental conditions, such as food resources, which the pest insect tracks (Berryman 1987). If grasshopper outbreak dynamics are eruptive, then survey efforts to find and control eruptive foci while they are still localized could prevent large-scale outbreaks. If, on the other hand, grasshopper dynamics are gradient and outbreaks occur whenever environmental conditions are favorable over large areas, efforts could be directed to identifying environmental predictors of outbreaks. Rangeland grasshopper outbreaks appear to have characteristics of both dynamics at the spatiotemporal scales that have been examined in Wyoming and Montana (Cigliano et al. 1994, Lockwood and Schell 1994). However, Berryman (1987) specifies that his outbreak theory applies to single species dynamics only and rangeland grasshopper outbreaks are, in most cases, composed of several species (Cigliano et al. 1994) so the applicability of current theory to rangeland grasshopper population ecology remains tenuous. By using a GIS to investigate the 31 years of Wyoming grasshopper data this conundrum can be addressed through direct large-scale analysis.
With this background, the creation of a historic grasshopper outbreak frequency map for Wyoming promises to be informative. If patterns of outbreaks are seen in either time or space then perhaps they can be ascribed to related ecological factors and thereby confirm or refute the properties of eruptive and gradient outbreaks. If a particularly revealing ecological factor is encountered within an ecosystem, more detailed analysis can be conducted to characterize habitat susceptibility to outbreaks. The goal of this thesis is to provide information on the spatial properties of grasshopper population dynamics to improve the understanding and management of grasshoppers in Wyoming.
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