Section VI: Decision Support Tools (6 of 11)

VI.6 Relative Importance of Rangeland Grasshoppers in Western North America: A Numerical Ranking From the Literature

Richard J. Dysart

Introduction
Reviewing the Literature
Pest-Status Categories
Scientific Names
My Findings
Conclusions
References

Introduction

There are about 400 species of grasshoppers found in the 17 Western States (Pfadt 1988). However, only a small percentage of these species ever become abundant enough to cause economic concern. The problem for any rangeland entomologist is how to arrange these species into meaningful groups for purposes of making management decisions. The assessment of the economic status of a particular grasshopper species is difficult because of variations in food availability and host selectivity. Mulkern et al. (1964) reported that the degree of selectivity is inherent in the grasshopper species but the expression of selectivity is determined by the habitat. To add to the complexity, grasshopper preferences may change with plant maturity during the growing season (Fielding and Brusven 1992). Because of their known food habits and capacity for survival, about two dozen grasshopper species generally are considered as pests, and a few other species have been called beneficials (Watts et al. 1989).

Between these extremes are more than 350 grasshopper species that are of little or no economic concern. However, while most species alone never cause serious economic loss, together an assemblage of minor species can inflict serious damage to rangeland. Through the years, the pest grasshoppers have received the greatest attention. Grasshoppers of the family Acrididae surpass all other arthropods in their destructiveness to rangeland (Watts et al. 1982). Although few in number, the pest grasshoppers cause losses to western rangeland estimated at $393 million per year, based on 1977 dollars (Hewitt and Onsager 1983).

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Reviewing the Literature

Several authors have made estimates of the relative importance of the major pest grasshoppers on western rangeland, but the work by Hewitt (1977) is probably the most thorough and the most cited. To my knowledge, however, no estimates have been made on the relative importance of the minor, occasional, and nonpest grasshoppers. The purpose of this chapter is to score and rank the western grasshopper species, in terms of relative economic importance, on the basis of remarks made by many grasshopper experts in their reports and publications. It is important to point out that these estimates represent merely the opinions of those involved, not conclusive proof. By including a large number of articles and authors that cover most of the literature on the subject, I hope that the resulting compilation will be a consensus from the literature, without introduction of bias on my part.

This review is restricted to grasshoppers found in 17 Western United States (Arizona, California, Colorado, Idaho, Kansas, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oklahoma, Oregon, South Dakota, Texas, Utah, Washington, and Wyoming) plus the 4 western provinces of Canada (Alberta, British Columbia, Manitoba, and Saskatchewan). Furthermore, only grasshoppers belonging to the family Acrididae are included here, even though many research papers reviewed mentioned species from other families of Orthoptera.

In my evaluation, I have emphasized the impact of grasshoppers on rangeland rather than on cropland. In cases where authors made comparative remarks, such as this species is of major importance to crops, but only of minor importance to range ..., I used only the rangeland remarks to assign a pest-status category to that species.

For my review of the North American grasshopper literature, I selected only articles in which the authors had grouped or characterized a number of grasshopper species according to their importance. Because of this limitation, several important taxonomic analyses (Brooks 1958, Handford 1946, and Otte 1981 and 1984) could not be used for my purposes.

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Pest-Status Categories

Grasshoppers are important herbivores, and any pest classification is based on whether they compete with or benefit human activities. Many articles I reviewed contained proof that a species actually caused measurable injury to rangeland, but many did not. Also, most articles which claimed that certain grasshopper species were beneficial presented no data to support the claim.

In my review, I have used the authors's remarks regardless of the evidence presented. In most instances, it was not difficult to assign species to one of my pest-status categories because the authors had made clear statements concerning the relative importance of grasshopper species in a study. However, it was sometimes necessary to make an interpretation from somewhat vague statements, such as ... occasionally common on rangeland. After reviewing each article, I translated the authors' remarks on a particular grasshopper species into one of five categories:

Serious pest species (S) Authors usually made clear statements about grasshoppers in this category, such as ... frequently causes major damage to rangeland, or ... one of the 10 most destructive species in our study.

Minor pest species (M) Authors categorized such species with phrases like ... this species occasionally causes injury to forage grasses, or ... populations may require control treatment in specific areas.

Innocuous species (I) Authors' remarks often contained phrases like ... of no economic importance, or ... this species was rarely encountered in the study area. Also, if an author categorized 10 grasshopper species as serious pests and another 10 species as minor pests but then discussed 10 additional species without mention of economic importance, I classified the latter species as innocuous.

Possibly beneficial species (b?) In this and the next category I included grasshoppers that feed to some extent on undesirable rangeland plants, such as the perennial snakeweeds (Gutierrezia spp.). I also assigned species to the possibly beneficial category when the authors' remarks were either uncertain or intentionally ambiguous, for example ... possibly beneficial since it feeds on noxious forbs.

Beneficial species (B) In these cases the author's remarks were clear and unequivocal: ... this grasshopper is a beneficial insect.

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Scientific Names

In this chapter, grasshopper names follow the usage recognized by the following sources, by subfamily:

Acridinae-Otte (1981)
Cyrtacanthacridinae-Arnett (1985), Helfer (1987)
Gomphocerinae-Otte (1981)
Melanoplinae-Arnett (1985), Helfer (1987)
Oedipodinae-Otte (1984)

Also the scientific names of all grasshoppers discussed were checked for proper usage by Dan Otte (Academy of Natural Sciences, Philadelphia) while this chapter was still in manuscript form. However, I am responsible for the accuracy of all names as printed here. In general, I have tried not to use names of subspecies, but in several instances that was unavoidable.

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My Findings

My review of the literature yielded 69 articles (table VI.6-1) in which the authors provided opinions of the relative pest status for the grasshopper species in their studies. In the articles selected, a total of 377 different grasshopper species were discussed by 77 different authors and coauthors over a period of 70 years (1924-93). When these authors' opinions were translated into my five pest-status categories, there were a total of 2,731 rankings on the 377 species. The 2,731 rankings broke down into the five categories as follows:

	Percent
Serious pest species	17.4
Minor pest species	15.7
Innocuous species	65.7
Possibly beneficial species	0.5
Beneficial species	0.7

Table VI.6-1 - Summary of pest-status rankings of 377 western rangeland grasshoppers from 69 articles.

¹ S=serious, M=minor, I=innocuous, b?=possibly beneficial, B=beneficial

The 377 grasshoppers (table VI.6-2) included species in the following five acridid subfamilies: Acridinae (1), Cyrtacanthacridinae (8), Gomphocerinae (63), Melanoplinae (185), and Oedipodinae (120). Also listed in table VI.6-2 are the status category tally counts for each species. In order to make calculations, I assigned points for each status category, as follows: Serious = +2, Minor = +1, Innocuous = 0, Possibly beneficial = -1, and Beneficial = -2.

The total score for each grasshopper species was calculated by multiplying the category tally count times the respective point values for each pest-status category. The rank number was determined by the magnitude of the total score for each species. In cases of tie scores, the species with the highest frequency of mentions as a serious and minor pest was given the higher rank.

Table VI.6-2 - Summary of pest-status rankings of 377 western rangeland grasshoppers from 69 articles.

¹ S=serious, M=minor, I=innocuous, b?=possibly beneficial, B=beneficial
² A=Acredinae, C=Cyrtacanthacidinae, G=Gomphocerinae, M=Melonoplinae, O=Oedipodinae

Each of the 377 species is represented (in order of overall score and rank) in the bar graph shown in figure VI.6-1. From left to right, it displays 111 grasshopper species with scores above zero (pests), 254 species with a score of zero (innocuous), and 12 species with scores below zero (possibly beneficial or beneficial).

Pest Species.-A total of 114 different grasshoppers were categorized as either a serious or a minor pest in at least one paper, but only 111 species had total scores above zero. In table VI.6-3, I have listed 38 of the highest ranked pest species, those with scores of 10 and above. As expected, the migratory grasshopper (Melanoplus sanguinipes) was ranked as the number 1 pest, with the highest total score (112 points) of the 377 grasshopper species.

Table VI.6-3-List of the 38 most serious "pest" grasshoppers on western rangeland (those listed have scores of 10 and above)

¹ S=serious, M=minor, I=innocuous, b?=possibly beneficial, B=beneficial
² G=Gomphocerinae, M=Melonoplinae, O=Oedipodinae

Innocuous Species.-There were 254 grasshopper species with a total score of zero. Within this group, higher rank numbers were assigned to species having the highest frequency of mention. Several species, including Acrolophitus hirtipes, Pseudopomala brachyptera, and Spharagemon campestris, were mentioned frequently but were never described as either a pest or a beneficial. For innocuous species with only a single ranking, the rank number has no significance; it was assigned due to the alphabetical arrangement of scientific names.

Beneficial Species.-Overall, 19 different grasshoppers were categorized by at least one author as either beneficial or possibly beneficial, but only 12 species had total scores below zero. The highest ranked beneficial grasshoppers are listed in table VI.6-4. Although 12 species were scored as beneficial, only 2 were mentioned as such with any frequency: Hesperotettix viridis Thomas, a grasshopper commonly associated with snakeweed (Gutierrezia spp.), and Hypochlora alba Dodge, which prefers to feed on sagebrush (Artemisia spp.).

Table VI.6-4-List of the 12 highest ranked "beneficial" grasshoppers on western rangeland (those listed all have scores below zero)

¹ S=serious, M=minor, I=innocuous, b?=possibly beneficial, B=beneficial
² G=Gomphocerinae, M=Melonoplinae, O=Oedipodinae

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Conclusions

In his 1977 review, Hewitt divided the western rangelands into three different regions: Great Plains, Intermountain, and Pacific Coastal. The literature I reviewed covered a cross section of these same regions, but the reader should be aware that not all of the 377 grasshoppers listed here are common to all regions. Indeed, one limitation of my scoring scheme is that wide-spread species are cited more frequently and thus accumulate higher total scores than species with a more restricted distribution. A serious pest that occurs in a small geographic area would not be such a pest in the big picture. Three such species, listed in table VI.6-3, are Dissosteira longipennis, Melanoplus devastator, and Oedaleonotus enigma.

Figure VI.6-1-Graphic display of total scores of 377 western range grasshoppers arranged (left to right) by pest-status rank number. Graph is plotted from data shown in table VI.6-2.

The graph in figure VI.6-1 offers a view of the whole spectrum of western grasshoppers and should provide some perspective when evaluating their relative importance as pests and as beneficials. From the graph it seems clear that nearly one-third (111) of the western grasshopper species are at least occasionally classified as pests. Again I must stress that damage to rangeland is rarely caused by only a single pest species but usually by an assemblage of several grasshopper species.

About two-thirds (254) of the western grasshoppers are thought to be of no economic importance, and only 12 species are considered to be of possible benefit to the rangeland. This small number of beneficial grasshoppers, amounts to only 3 percent of the 377 species involved in this review, which is several orders of magnitude less than the recent estimate of 10 percent claimed by Lockwood (1993). The grasshopper most frequently called a beneficial is Hesperotettix viridis. Although often seen feeding on snakeweed, it also feeds on more than 30 other rangeland plants (Pfadt 1988). Another grasshopper, Hypochlora alba, is highly ranked as a beneficial because of its preference for sagebrush. But the value of sagebrush on rangeland is widely debated. As a strong competitor with desirable forage plants for domestic livestock, it is considered by some as an undesirable weed. Others consider sagebrush a beneficial plant because it comprises an important portion of the diet of mule deer, antelope, and the sage grouse (Watts et al. 1982).

Concerning the relative importance of the major pest grasshoppers, I believe that the rankings shown in table VI.6-3 represent a good consensus of opinions from the North American literature. Although experts differ over the ranking of individual species, most agree that there are about 2 dozen western grasshoppers that should be categorized as pests. I believe that a statement by Watts et al. (1989) summarized the pest issue quite well: About a dozen species frequently occur in high densities, and . . . an additional 12 species occasionally occur in high densities. Readers are free to compare their own opinions with the species listed and the pest-status rankings shown.

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