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GRASSHOPPERCO~TY
RESPONSES TO SHRUB LOSS,
ANNUAL GRASSLANDS, AND
CRESTED WHEATGRASS
SEEDINGS: MANAGEMENT
IMPLICATIONS
Dennis J. Fielding
MerI~ A. Brusven
ABSTRACT
Grasshopper density and species composition were sampled
at 42 sites arrayed along a disturbance gradient. Grasshopper density was lowest and species diversity was highest in
vegetation types with shrub cover. Annual grasslands had
the highest grasshopper densities and the lowest species diversity~ and were dominated by generalist species with
wide diet breadths. Management concerns that arise from
the different characteristics (food habits~ migratory propensity) of the dominant grasshopper species associated with
the various plant communities were discussed. Management ofgrasshopper populations by habitat manipulation
may be a viable alternative strategy.
INTRODUCTION
Grasshopper populations periodically reach outbreak proportions in the Intermountain region (Hewitt and Onsager
1983). In 1985, during a massive grasshopper outbreak,
about 2.5 million ha of rangeland across southern Idaho
were treated with broad-spectrum insecticides. This type
of sledge-hammer approach to the control of insect pests
applied on a landscape scale is becoming less acceptable.
The undesirable aspects of broad-spectrum biocides (effect
on nontarget arthropods, expense) make it compelling to
find ways to manage grasshopper populations so that outbreaks are less frequent and of smaller extent. This project
was undertaken to assess the role that range management
actions have on grasshopper populations and to provide insights into the management of grasshoppers through habitat manipulation.
Like many areas of the Intermountain region, southcentral Idaho has suffered extensive habitat degradation
and shrub loss due to increased fire frequency associated
with the invasion of cheatgrass. The diminished resource
values that result from this process of shrub loss have been
documented by many studies. The response of rangeland
grasshoppers to the conversion of native vegetation to annual grasslands has not been studied in detail.
There are well over 100 species of grasshoppers in the
Intermountain region. Only four or five species attain
very high densities and account for most of the outbreak
populations. One of these, Melanoplus sanguinipes L., is
particularly troublesome. In south-central Idaho, where
there is an extensive interface between publicly owned
rangeland and privately owned irrigated cropland, migration of grasshoppers from rangeland to cropland is a major problem. Melanoplus sanguinipes is well known for
its propensity to migrate (McAnelly and Rankin 1986)
and its broad range of food plants (Mulkern and others
1969) makes it a threat to a wide variety of crops, as well
as rangeland forage species. Another abundant species
in the Intermountain region is Aulocara eUiotti. It differs
from M. sanguinipes in several respects: it is restricted to
feeding on grasses only (Mulkern and others 1969) and
is seldom found in cultivated crops.
The objective of the present study is to identify patterns
of grasshopper species composition among different vegetation types, and to determine whether the conversion of
native plant communities to annual grasslands has an affect on the abundance of the major grasshopper pest species in south-central Idaho.
MATERIALS AND METHODS
Forty-two sites were sampled for plant and grasshopper
species composition. These sites were all within the Davis
Mountain SW USGS 7.5' quadrangle map (north of Bliss,
ID). This area was selected because a wide spectrum of
plant communities, from relatively undisturbed to dominance by exotic annual grasses (cheatgrass, Bromus tectorum, and medusahead, Taeniantherum asperum) and
introduced perennial grasses (primarily crested wheatgrass, Agropyron cristatum), were all present within a
small geographic area. The small scale of the study was
intended to minimize the effects of local weather patterns
on grasshopper community composition.
The current year's standing crop by plant species was
estimated by the weight-unit method (USDA-SCS 1976).
Ten, 1-m2 quadrats were estimated at each site in July of
1990 and 1991. Both years' data were averaged for subsequent analysis.
Paper presented at the Symposium on Ecology, Management, and Restoration of Intermountain Annual Rangelands, Boise, ID, May 18-22, 1992.
Dennis J. Fielding is Postdoctoral Fellow and Merlyn A Brusven is Professor of Entomology, Department of Plant, Soil and Entomological Sciences, University of Idaho, Moscow, ID 83843.
162
Grasshoppers were sampled twice per year in June and
late July and August. Densities were estimated by counting the number of grasshoppers flushed from 50, O.l-m2
quadrats. Species composition was determined by capturing and identifying at least 30 grasshoppers at each site.
Density of individual grasshopper species was estimated
by multiplying the species' proportions by overall density
on the site. Pooled grasshopper data from the four sampling dates were used for subsequent analysis.
Plant data were summarized and primary gradients identified using detrended correspondence analysis (DCA) (Hill
1980), an eigenvector ordination technique. Primary gradients in plant species composition were identified by nonparametric correlation (Spearman's r., Zar 1984) of shrub
biomass, native perennial grasses (excluding Poa spp.), annual vegetation, and the percent exotic plant species with
plant DCA axis-! scores for the 42 sites. Trends in grasshopper community composition were also examined by nonparametric correlation with the plant DCA a.xis-1 scores.
Table 1-Mean plant aboveground biomass (gram dry weight/
square meter) by vegetation type
Vegetation~~
Artr/
Agsp
Artr/
Brte
Artr/
Agcr
Agcr/
Brte
Taas/
Brte
Bromus tectorum
Bromus japonicus
Taeniantherum asperum
Agropyron cristatum
Poa sandbergii
Agropyron spicatum
Other native grasses1
Artemisia tridentata
Chrysothamus spp.
Annual and biennial forbs
Perennial forbs
5.2
2.8
<.1
.4
.8
14.9
4.8
34.4
1.9
.2
.7
11.5
1.5
1.4
.7
1.5
<.1
.8
33.6
3.2
.7
<.1
3.0
.5
.3
19.0
4.8
<.1
<.1
26.5
4.8
.2
<.1
5.4
<.1
1.8
21.8
5.7
<.1
.3
2.2
.5
1.3
.3
15.0
1.0
22.2
.4
2.5
<.1
<.1
.3
.2
3.4
<.1
Number of plant species
12.1
9.3
5.6
8.0
7.6
Plant
11ncludas
E/ymus clnereus, Sitsnlon hystrlx, Slips thurberiana, and Agropy-
ronsmithli.
RESULTS
The DCA ordination of the vegetation is shown in figure 1.
The first axis, which accounted for 53 percent of the variation in plant data, may be interpreted largely as a disturbance gradient. The biomasses of shrubs and native grasses
were negatively correlated with the plant DCA axis-! scores
(r. =-0.79 and --0.72, respectively, N =42, P < 0.01), indicating less disturbed plant communities at the low end of
axis-! (fig. 1). Biomass of annual vegetation and the percentage of aboveground biomass represented by exotic plant
species were positively correlated (r. 0.59 and 0.91, respectively, N 42, P < 0.01) with the plant DCA axis-1, indicating the dominance by introduced plant species at the high
end of plant DCA a.xis-1.
Five somewhat subjective but nonoverlapping vegetation
types were delineated on the ordination diagram and were
labeled according to the two plant species with the greatest
mean aboveground biomass within the vegetation type.
Table !lists the mean composition of the five vegetation
types.
Grasshopper species composition also differed among
the vegetation types (table 2). Melanoplus sanguinipes
showed a strong affinity for the annual grassland sites
where it accounted for 66 percent of all grasshoppers collected (table 2). Melanoplus sanguinipes did not comprise
more than 15 percent of the population in any of the other
vegetation types. Density of M. sanguinipes was strongly
=
=
Table 2-Mean density and percentages of grasshopper species
collected by vegetation type
. - - - - - · ---------------------.
.....
260
• ·.
200.
Artr/Agcr
C\1 150
.• ' /
..\
Agcr/Brte
•
" ' ......
~
/.
tr/Agspt •
<
50 ··· ... ·-. ~... :.~,~
:.
Overall density (per m2)
•
··.··.···I·····-..~··-. ·. . ·.~·.·. -.·.~- -· /
100
Density and
species
•
~·· .. \
Percentage of population:
Agensotettix deorum
Amphitomus coloradus
Aulocara elliotti
Clrcotettix undulatus
Cordillacris occipitalis
Cratypedes neglectus
Dlssostelra spurcata
Hespsrotettix viridis
Melanoplus cinereus
Melanoplus sanguinipss
Oedaleonotus enigma
Phoetaliotes nebrascsnsis
Spharagemon equale
Stenobothrus shastanus
Trachyrachys kiowa
Trimerotropis gracilis
Trimerotropis psuedofasciata
.···· • ........ ······~···a;
,./
• .
./_..-·· Ta;/Br.·te ~-··'
•
.......
I ~r/Brte
_./
•
oL---~----J-~·~=~·'----~·-~·~··--~--~----~
0
50
100
150
200
25'0
300
350
Axls1
Figure 1-Detrended Correspondence Analysis
ordination of 42 sites based on plant species
aboveground biomass. Vegetation types are
labeled according to the dominant plant species:
Artr, big sagebrush (Artemisia tridentata); Agsp.
bluebunch wheatgrass (Agropyron spicatum);
Agcr. crested wheatgrass (Agropyron cristatum);
Brte. cheatgrass (Bromus tectorum); Taas. medusahead wildrye (Taeniantherum asperum).
163
Vegetation type
Artr/ Artr/ Artr/ Agcr/ Taas/
Agsp Brte Agcr Brte Brte
0.22
0.31
0.68
1.18
16
25
5
14
6
2
34
2
5
50
4
63
8
3
9
3
10
9
•
15
13
7
6
15
5
66
14
2
4
5
•
3
•
1
1
13
8
1
•
•
5
2
1
2
..
1.63
•
•
13
2
•
•
•
DISCUSSION
correlated with the plant DCA axis-1 scores (fig. 2).
Aulocara elliotti dominated the crested wheatgrass sites
(table 2). Density of A elliotti also was positively correlated with axis-1 scores, although it reached its highest
densities near the middle of plant DCA axis-1, where the
Agcr/Brte sites were located (fig. 3).
Both overall density and diversity (Shannon's H') of
grasshoppers were strongly correlated with the plant
DCA axis-1 scores (figs. 4 and 5). Sites high on plant
DCA axis-1, the sites lacking sagebrush cover, had the
highest grasshopper densities and the lowest diversity.
In the lesser disturbed Artr/Agsp sites, 13 species comprised 95 percent of the grasshoppers collected from these
sites (table 2). Progressively fewer species comprised 95
percent of the grasshoppers in the vegetation types arrayed from left to right along plant DCA axis-1 (fig. 1).
Only four species accounted for 95 percent of the grasshoppers collected from the annual grassland sites.
Striking differences were observed in the grasshopper
assemblages associated with natural and introduced plant
communities in south-central Idaho. The pattern of reduced
biodiversity associated with the conversion to exotic annual
grasslands (Whisenant 1990; T. Rich, these proceedings)
was reflected in the grasshopper assemblages within the
study area. Grasshopper assemblages were composed of
progressively fewer species along the primary disturbance gradient in the plant communities. Sagebrush was
probably the single most important factor affecting grasshopper community structure. The vegetation types with
sagebrush were all characterized by low overall density
and high diversity without any single species dominating,
except A elliotti in the Artr/Agcr sites.
Much of the observed patterns were the result of the distribution of two of the most common species, M. sanguinipes
2.5
4
r8
E
~
Cl)
rs • 0.68
N
E
3
Q)
1.5
~
~
CD
1
•
::l
z
•
0.5
Q)
.c 2
•• • •
E
••
::l
150
200
250
300
••
0
350
•
.
.
.
• •• •
.•
0
• '•• I
•
50
100
Vegetation DCA Axis 1
r s • 0.53
CD
c.
2
~
-
•
• •••
>-
·a;
E
~
•
~ 1.6
~
1.5
i5
~
CD
• ••
::l
0.5.
•
••
•
~
..
•
0
50
100
150
200
250
•
•
. . J . . _ - - L - - - L - - . - - - 1 - - . . L ...
200
250
.c 1
0
•
0
&1S
~
"
•
300
•
•• •
• ••
•• •
•
•
0
._...JJ!..L.JI.1d~_!_l __ !.J_t_~___1_
0
••
150
,.,
• •
• •
c.
c.
•
.•
•
•
300
350
• ••
CD
E
•
ra • -0.70
~
.c
z
••
•
Figure 4-Relationship of overall grasshopper
density to to vegetation DCA axis-1 soores.
2.5
2·
•
Vegetation DCA Axis 1
Figure 2-Relationship of M. sanguinipes density
to vegetation DCA axis-1 scores.
N
•
•
•
•
1.
.--S--~~~~:--~--~----~-~
100
• •
z
•
•
50
•
c.
•
•
•
~
c.
~
0.72
•
2
N
•
•
•
•
•
•
• •
•
• ••
0.6
350
0
Vegetation DCA Axis 1
50
100
150
200
260
Vegetation DCA Axis 1
Figure 3-Relationship of A. e/liotti density to
vegetation DCA axis-1 scores.
Figure 5-Relationship of grasshopper diversity
to vegetation DCA axis-1 scores.
164
300
350
all vegetation types. Examination of nymphal survey data
from 1985, a year of extremely high densities, indicates
that, although densities were high across south-central
Idaho, the relationship between annual grasslands and
higher grasshopper densities prevailed (Fielding and
Brusven in press). In the Shoshone Bureau of Land Management district in 1985, annual grasslands averaged 41
grasshoppers/m2 compared to about 221m2 in sagebrushcovered areas (Fielding and Brusven in press). It may be
argued that when densities exceed a certain threshold it
does not matter whether there are 20 or 40 grasshoppers
per square meter, it will be enough to cause hardships for
farmers and ranchers. However, the evidence to date suggests that outbreaks would be less frequent, less intense,
and cover less area in habitats with shrub cover than on
frequently burned, cheatgrass-dominated landscapes.
If further research confirms that annual grasslands do
experience more frequent grasshopper outbreaks, then rehabilitation of annual grasslands with shrubs and perennial grasses should be considered as a means of noncatastrophic management of grasshoppers. Because migration
of M. sanguinipes from rangeland to irrigated croplands is
a primary rationale for control operations in south-central
Idaho, some of the highest priority areas for rehabilitation
would be those areas adjacent to croplands. Although the
costs of grasshopper control alone may not justify the expense of planting shrubs and perennial grasses over large
areas, the benefits to game birds such as pheasant (Sands,
these proceedings), other wildlife, and livestock may be
enough to justify rehabilitation of high-priority areas. The
management of grasshoppers by the manipulation of vegetation has the advantage of being an environmentally
sound, long-term strategy that could benefit many other
resources.
and A elliotti. The annual grasslands were dominated by
M. sanguinipes. Aulocara elliotti dominated the crested
wheatgrass seedings. The relative abundance of their preferred host plants may account for much of their observed
habitat preferences. Both species can subsist largely on
cheatgrass early in the season, then switch to other foods
as the cheatgrass dries. Crested wheatgrass is readily
accepted by A elliotti later in the season, while M. sanguinipes feeds largely on weedy forbs later in the season
(Fielding and Brusven 1991). Annual and biennial forbs
were most abundant on the annual grasslands (table 1).
The different life history strategies exhibited by M. sanguinipes and A elliotti result in different management implications for these species. Aulocara elliotti is more of
a specialist adapted to exploit a perennial grass resource.
As a member of the grass-feeding subfamily Gomphocerinae, it is restricted in its host range to grasses. Phenologically it is well adapted to the perennial grasses of the
area, maturing at about the same time as the plants. It
appears that the life history strategy ofA elliotti is to specialize on a perennial resource, remain in a resource patch
and tolerate conditions during adverse periods. Because
A elliotti tends to mature at about the same time as its
host plants, it is less likely to migrate off the rangeland
in search of more suitable habitat. In contrast, M. sanguinipes is a much more opportunistic feeder, and populations hedge their bets with a wide spread in hatching
dates. A large proportion of the population of M. sanguinipes will mature well into the summer when most plants
have dried, leading to a situation where they will be much
more likely to migrate to irrigated cropland. Therefore,
where migration to cropland is a concern, then high populations of M. sanguinipes may be considered undesirable.
However, if destruction of forage grasses is the primary
concern, then populations ofA elliotti will compete directly
with livestock for available forage grasses, whereas M. sanguinipes will tend to feed first on less desirable weedy
forbs.
Because populations ofA elliotti are usually more tightly
synchronized than populations of M. sanguinipes (Onsager
1987), it is easier to assess the potential for damage from
A elliotti early in the season instead of waiting for hatching to be completed, as is the case forM. sanguinipes where
a substantial proportion of the population may already be
in the fourth instar (the ideal time for treatment) before
all hatching has been completed.
Because of its high reproductive potential, it is probable
that M. sanguinipes will be the dominant species in most
vegetation types during outbreak years. However, in those
habitats with a more equitable distribution of species,
M. sanguinipes may be not be able to attain its full potential for explosive population growth. Few studies have
found evidence of direct competition between grasshopper
species; it seems especially unlikely that populations of
M. sanguinipes will be inhibited by other grasshopper species in habitats where M. sanguinipes already dominates.
These results indicate that areas with shrub cover and
an understory of perennial grasses will have lower overall
grasshopper densities with a lower proportion of pest species. These data were taken during years of low grasshopper density; it may be expected that during outbreak years
grasshopper density may exceed treatment thresholds in
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