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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 grasshoppers to the conversion of native vegetation to an­ nual grasslands has not been studied in detail. Grasshopper density and species composition were sampled There are well over 100 species of grasshoppers in the at 42 sites arrayed along a disturbance gradient. Grasshop­ Intermountain region. Only four or five species attain per density was lowest and species diversity was highest in very high densities and account for most of the outbreak vegetation types with shrub cover. Annual grasslands had populations. One of these, Melanoplus sanguinipes L., is the highest grasshopper densities and the lowest species di­ particularly troublesome. In south-central Idaho, where versity~ and were dominated by generalist species with there is an extensive interface between publicly owned wide diet breadths. Management concerns that arise from rangeland and privately owned irrigated cropland, migra­ the different characteristics (food habits~ migratory propen­ tion of grasshoppers from rangeland to cropland is a ma­ sity) of the dominant grasshopper species associated with jor problem. Melanoplus sanguinipes is well known for the various plant communities were discussed. Manage­ its propensity to migrate (McAnelly and Rankin 1986) ment ofgrasshopper populations by habitat manipulation and its broad range of food plants (Mulkern and others may be a viable alternative strategy. 1969) makes it a threat to a wide variety of crops, as well as rangeland forage species. Another abundant species INTRODUCTION in the Intermountain region is Aulocara eUiotti. It differs from M. sanguinipes in several respects: it is restricted to Grasshopper populations periodically reach outbreak pro­ feeding on grasses only (Mulkern and others 1969) and portions in the Intermountain region (Hewitt and Onsager is seldom found in cultivated crops. 1983). In 1985, during a massive grasshopper outbreak, The objective of the present study is to identify patterns about 2.5 million ha of rangeland across southern Idaho of grasshopper species composition among different vege­ were treated with broad-spectrum insecticides. This type tation types, and to determine whether the conversion of of sledge-hammer approach to the control of insect pests native plant communities to annual grasslands has an af­ applied on a landscape scale is becoming less acceptable. fect on the abundance of the major grasshopper pest spe­ The undesirable aspects of broad-spectrum biocides (effect cies in south-central Idaho. on nontarget arthropods, expense) make it compelling to find ways to manage grasshopper populations so that out­ MATERIALS AND METHODS breaks are less frequent and of smaller extent. This project was undertaken to assess the role that range management Forty-two sites were sampled for plant and grasshopper actions have on grasshopper populations and to provide in­ species composition. These sites were all within the Davis sights into the management of grasshoppers through habi­ Mountain SW USGS 7 .5' quadrangle map (north of Bliss, tat manipulation. ID). This area was selected because a wide spectrum of Like many areas of the Intermountain region, south­ plant communities, from relatively undisturbed to domi­ central Idaho has suffered extensive habitat degradation nance by exotic annual grasses (cheatgrass, Bromus tec­ and shrub loss due to increased fire frequency associated torum, and medusahead, Taeniantherum asperum) and with the invasion of cheatgrass. The diminished resource introduced perennial grasses (primarily crested wheat­ values that result from this process of shrub loss have been grass, Agropyron cristatum), were all present within a documented by many studies. The response of rangeland 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 Paper presented at the Symposium on Ecology, Management, and Res­ estimated by the weight-unit method (USDA-SCS 1976). toration of Intermountain Annual Rangelands, Boise, ID, May 18-22, 1992. 2 Dennis J. Fielding is Postdoctoral Fellow and Merlyn A Brusven is Pro­ Ten, 1-m quadrats were estimated at each site in July of fessor of Entomology, Department of Plant, Soil and Entomological Sci­ 1990 and 1991. Both years' data were averaged for subse­ ences, University of Idaho, Moscow, ID 83843. quent analysis.

162 Grasshoppers were sampled twice per year in June and Table 1-Mean plant aboveground biomass (gram dry weight/ late July and August. Densities were estimated by count­ square meter) by vegetation type ing the number of grasshoppers flushed from 50, O.l-m2 quadrats. Species composition was determined by captur­ Vegetation~~ ing and identifying at least 30 grasshoppers at each site. Artr/ Artr/ Artr/ Agcr/ Taas/ Plant Agsp Agcr Density of individual grasshopper species was estimated Brte Brte Brte by multiplying the species' proportions by overall density Bromus tectorum 5.2 11.5 3.0 5.4 15.0 on the site. Pooled grasshopper data from the four sam­ Bromus japonicus 2.8 1.5 .5 <.1 1.0 pling dates were used for subsequent analysis. Taeniantherum asperum <.1 1.4 .3 1.8 22.2 Plant data were summarized and primary gradients iden­ Agropyron cristatum .4 .7 19.0 21.8 .4 Poa sandbergii .8 tified using detrended correspondence analysis (DCA) (Hill 1.5 4.8 5.7 2.5 Agropyron spicatum 14.9 <.1 <.1 <.1 <.1 1980), an eigenvector ordination technique. Primary gradi­ Other native grasses 1 4.8 .8 <.1 .3 <.1 ents in plant species composition were identified by non­ Artemisia tridentata 34.4 33.6 26.5 2.2 .3 parametric correlation (Spearman's r., Zar 1984) of shrub Chrysothamus spp. 1.9 3.2 4.8 .5 .2 biomass, native perennial grasses (excluding Poa spp.), an­ Annual and biennial forbs .2 .7 .2 1.3 3.4 nual vegetation, and the percent exotic plant species with Perennial forbs .7 <.1 <.1 .3 <.1 plant DCA axis-! scores for the 42 sites. Trends in grass­ Number of plant species 12.1 9.3 5.6 8.0 7.6 hopper community composition were also examined by non­ 11ncludas E/ymus clnereus, Sitsnlon hystrlx, Slips thurberiana, and Agropy- parametric correlation with the plant DCA a.xis-1 scores. ronsmithli. RESULTS The DCA ordination of the vegetation is shown in figure 1. Five somewhat subjective but nonoverlapping vegetation The first axis, which accounted for 53 percent of the varia­ types were delineated on the ordination diagram and were tion in plant data, may be interpreted largely as a distur­ labeled according to the two plant species with the greatest bance gradient. The biomasses of shrubs and native grasses mean aboveground biomass within the vegetation type. were negatively correlated with the plant DCA axis-! scores Table !lists the mean composition of the five vegetation (r. =-0.79 and --0.72, respectively, N =42, P < 0.01), indi­ types. cating less disturbed plant communities at the low end of Grasshopper species composition also differed among axis-! (fig. 1). Biomass of annual vegetation and the percen­ the vegetation types (table 2). Melanoplus sanguinipes tage of aboveground biomass represented by exotic plant showed a strong affinity for the annual grassland sites species were positively correlated (r. =0.59 and 0.91, respec­ where it accounted for 66 percent of all grasshoppers col­ tively, N =42, P < 0.01) with the plant DCA axis-1, indicat­ lected (table 2). Melanoplus sanguinipes did not comprise ing the dominance by introduced plant species at the high more than 15 percent of the population in any of the other end of plant DCA a.xis-1. vegetation types. Density of M. sanguinipes was strongly

.-----· ------. Table 2-Mean density and percentages of grasshopper species collected by vegetation type 260 ..... • ·. Agcr/Brte Vegetation type . '/ Artr/ Artr/ Agcr/ Taas/ 200. Density and Artr/ Brte Artr/Agcr • • species Agsp Brte Agcr Brte 0.22 0.31 0.68 1.18 1.63 C\1 150 "' ...... Overall density (per m2) ~ /. ..\ • Percentage of population: < tr/Agspt • .···· • ...... ······~···a; Agensotettix deorum 16 6 2 • 100 ··.··.···I·····-..~··-...· .. ·.~·.··.. -.·.~---··/ • ,./ • .. Amphitomus coloradus 25 2 5 4 ./_..-·· Ta;/Br.·te ~-··' Aulocara elliotti 5 34 50 63 13 50 ··· ... ·-. ... :. .. \ • ~ :.~,~ ~·· ...... Clrcotettix undulatus 14 • .. • I ~r/Brte _./ • Cordillacris occipitalis 8 9 3 oL---~----J-~·~=~·'----~·-~·~··--~--~----~ Cratypedes neglectus 3 0 50 100 150 200 25'0 300 350 Dlssostelra spurcata 2 Axls1 Hespsrotettix viridis 1 10 9 • Melanoplus cinereus 1 • Figure 1-Detrended Correspondence Analysis Melanoplus sanguinipss 13 15 7 15 66 ordination of 42 sites based on plant species Oedaleonotus enigma 8 13 6 5 14 Phoetaliotes nebrascsnsis 1 • • aboveground biomass. Vegetation types are • labeled according to the dominant plant species: Spharagemon equale 2 4 5 Artr, big sagebrush (Artemisia tridentata); Agsp. Stenobothrus shastanus 5 • bluebunch wheatgrass (Agropyron spicatum); Trachyrachys kiowa 3 • Agcr. crested wheatgrass (Agropyron cristatum); Trimerotropis gracilis 2 2 Brte. cheatgrass (Bromus tectorum); Taas. me­ Trimerotropis psuedofasciata 1 dusahead wildrye (Taeniantherum asperum).

163 correlated with the plant DCA axis-1 scores (fig. 2). DISCUSSION Aulocara elliotti dominated the crested wheatgrass sites (table 2). Density ofA elliotti also was positively corre­ Striking differences were observed in the grasshopper lated with axis-1 scores, although it reached its highest assemblages associated with natural and introduced plant densities near the middle of plant DCA axis-1, where the communities in south-central Idaho. The pattern of reduced Agcr/Brte sites were located (fig. 3). biodiversity associated with the conversion to exotic annual Both overall density and diversity (Shannon's H') of grasslands (Whisenant 1990; T. Rich, these proceedings) grasshoppers were strongly correlated with the plant was reflected in the grasshopper assemblages within the DCA axis-1 scores (figs. 4 and 5). Sites high on plant study area. Grasshopper assemblages were composed of DCA axis-1, the sites lacking sagebrush cover, had the progressively fewer species along the primary distur­ highest grasshopper densities and the lowest diversity. bance gradient in the plant communities. Sagebrush was In the lesser disturbed Artr/Agsp sites, 13 species com­ probably the single most important factor affecting grass­ prised 95 percent of the grasshoppers collected from these hopper community structure. The vegetation types with sites (table 2). Progressively fewer species comprised 95 sagebrush were all characterized by low overall density percent of the grasshoppers in the vegetation types ar­ and high diversity without any single species dominating, rayed from left to right along plant DCA axis-1 (fig. 1). except A elliotti in the Artr/Agcr sites. Only four species accounted for 95 percent of the grass­ Much of the observed patterns were the result of the dis­ hoppers collected from the annual grassland sites. tribution of two of the most common species, M. sanguinipes

2.5 4 r • 0.72 0.68 8 rs • 2 • N 3 N E • • ~ E Q) ~ 1.5 c. Cl) • c. ~ • Q) ~ .c 2 • CD • • • E ~ 1 • ::l • •• ::l • z • • • • • • z • • • 1 . • 0.5 • • • • . • .. . • • • • .• • '•• I .--S--~~~~:--~--~----~-~ 0 • • • • • • ..J.._--L---L--.---1--..L.• .. 50 100 150 200 250 300 350 0 50 100 150 200 250 300 350 Vegetation DCA Axis 1 Vegetation DCA Axis 1 Figure 2-Relationship of M. sanguinipes density Figure 4-Relationship of overall grasshopper to vegetation DCA axis-1 scores. density to to vegetation DCA axis-1 soores.

2.5 • • r • -0.70 r s • 0.53 2 •• a 2· • ~ • •• N >- • E ·a; • ,., ~ ~ - • • CD 1.5 ~ 1.6 • c. • • • ~ i5 • CD ~ • .c CD • • E c. • ::l • • • • c. • z .c0 • • • • 0 1 • 0 • • • 0.5. &1S • • • ~ ~ • • • • • • . • • . . • • • 0 ._...JJ!..L.JI.1d~_!_l __!.J_t_~___1_ " 0.6 0 50 100 150 200 250 300 350 0 50 100 150 200 Vegetation DCA Axis 1 260 300 350 Vegetation DCA Axis 1

Figure 3-Relationship of A. e/liotti density to Figure 5-Relationship of grasshopper diversity vegetation DCA axis-1 scores. to vegetation DCA axis-1 scores.

164 and A elliotti. The annual grasslands were dominated by all vegetation types. Examination of nymphal survey data M. sanguinipes. Aulocara elliotti dominated the crested from 1985, a year of extremely high densities, indicates wheatgrass seedings. The relative abundance of their pre­ that, although densities were high across south-central ferred host plants may account for much of their observed Idaho, the relationship between annual grasslands and habitat preferences. Both species can subsist largely on higher grasshopper densities prevailed (Fielding and cheatgrass early in the season, then switch to other foods Brusven in press). In the Shoshone Bureau of Land Man­ as the cheatgrass dries. Crested wheatgrass is readily agement district in 1985, annual grasslands averaged 41 accepted by A elliotti later in the season, while M. sangui­ grasshoppers/m2 compared to about 221m2 in sagebrush­ nipes feeds largely on weedy forbs later in the season covered areas (Fielding and Brusven in press). It may be (Fielding and Brusven 1991). Annual and biennial forbs argued that when densities exceed a certain threshold it were most abundant on the annual grasslands (table 1). does not matter whether there are 20 or 40 grasshoppers The different life history strategies exhibited by M. san­ per square meter, it will be enough to cause hardships for guinipes and A elliotti result in different management im­ farmers and ranchers. However, the evidence to date sug­ plications for these species. Aulocara elliotti is more of gests that outbreaks would be less frequent, less intense, a specialist adapted to exploit a perennial grass resource. and cover less area in habitats with shrub cover than on As a member of the grass-feeding subfamily Gomphocer­ frequently burned, cheatgrass-dominated landscapes. inae, it is restricted in its host range to grasses. Pheno­ If further research confirms that annual grasslands do logically it is well adapted to the perennial grasses of the experience more frequent grasshopper outbreaks, then re­ area, maturing at about the same time as the plants. It habilitation of annual grasslands with shrubs and peren­ appears that the life history strategy ofA elliotti is to spe­ nial grasses should be considered as a means of noncata­ cialize on a perennial resource, remain in a resource patch strophic management of grasshoppers. Because migration and tolerate conditions during adverse periods. Because ofM. sanguinipes from rangeland to irrigated croplands is A elliotti tends to mature at about the same time as its a primary rationale for control operations in south-central host plants, it is less likely to migrate off the rangeland Idaho, some of the highest priority areas for rehabilitation in search of more suitable habitat. In contrast, M. san­ would be those areas adjacent to croplands. Although the guinipes is a much more opportunistic feeder, and popu­ costs of grasshopper control alone may not justify the ex­ lations hedge their bets with a wide spread in hatching pense of planting shrubs and perennial grasses over large dates. A large proportion of the population of M. sangui­ areas, the benefits to game birds such as pheasant (Sands, nipes will mature well into the summer when most plants these proceedings), other wildlife, and livestock may be have dried, leading to a situation where they will be much enough to justify rehabilitation of high-priority areas. The more likely to migrate to irrigated cropland. Therefore, management of grasshoppers by the manipulation of veg­ where migration to cropland is a concern, then high popu­ etation has the advantage of being an environmentally lations of M. sanguinipes may be considered undesirable. sound, long-term strategy that could benefit many other However, if destruction of forage grasses is the primary resources. concern, then populations ofA elliotti will compete directly with livestock for available forage grasses, whereas M. san­ REFERENCES guinipes will tend to feed first on less desirable weedy forbs. Fielding, D. J.; Brusven, M. A.1990. Historical analysis Because populations ofA elliotti are usually more tightly of grasshopper (Orthoptera: Acrididae) population re­ synchronized than populations of M. sanguinipes (Onsager sponses to climate in southern Idaho, 1950-1980. Envi­ 1987), it is easier to assess the potential for damage from ronmental Entomology. 19: 1786-1791. A elliotti early in the season instead of waiting for hatch­ Fielding, D. J.; Brusven, M.A. 1992. Food and habitat ing to be completed, as is the case forM. sanguinipes where preferences of Melanoplus sanguinipes and Aulocara a substantial proportion of the population may already be elliotti (Orthoptera: Acrididae) on southern Idaho range­ in the fourth instar (the ideal time for treatment) before land. Journal of Economic Entomology. 85: 783-788. all hatching has been completed. Fielding, D. J.; Brusven, M. A. [In press]. Spatial analysis Because of its high reproductive potential, it is probable of grasshopper density and ecological disturbance on that M. sanguinipes will be the dominant species in most southern Idaho rangeland. Agriculture Ecosystems & vegetation types during outbreak years. However, in those Environment. habitats with a more equitable distribution of species, Hewitt, G. B.; Onsager, J. A 1983. Control of grasshoppers M. sanguinipes may be not be able to attain its full poten­ on rangeland in the United States-a perspective. Jour­ tial for explosive population growth. Few studies have nal of Range Management. 36: 202-207. found evidence of direct competition between grasshopper Hill, M. 0.; Gauch, H. G., Jr. 1980. Detrended correspon­ species; it seems especially unlikely that populations of dence analysis: an improved ordination technique. M. sanguinipes will be inhibited by other grasshopper spe­ Vegetatio. 42:47-58. cies in habitats where M. sanguinipes already dominates. McAnelly, M. L.; Rankin, M. A.1986. Migration in the These results indicate that areas with shrub cover and grasshopper Melanoplus sanguinipes (Fab.). I. The ca­ an understory of perennial grasses will have lower overall pacity for flight in non-swarming populations. Biologi­ grasshopper densities with a lower proportion of pest spe­ cal Bulletin. 170: 368-377. cies. These data were taken during years of low grasshop­ Mulkern, G. B.; Pruess, K. P.; Knutson, H.; Hagen, A. F.; per density; it may be expected that during outbreak years Campbell, J. B.; Lambley, J.D. 1969. Food habits and grasshopper density may exceed treatment thresholds in

165 preferences of grassland grasshoppers of the north cen­ Whisenant, S. G. 1990. Changing fire frequencies on Idaho's tral Great Plains. Bull. 481. Fargo, ND: North Dakota Snake River Plains: Ecological and management implica­ State University Agricultural Experiment Station. 32 p. tions. In: McArthur, E. Durant; Romney, Evan M.; Smith, Onsager, J. A. 1987. Integrated management of rangeland Stanley D.; Tueller, Paul T., compilers. Proceedings­ grasshoppers. In: Capinera, J. L., ed. Integrated pest symposium on cheatgrass invasion, shrub die-off, and management on rangeland. Boulder, CO: Westview Press: other aspects of shrub biology and management; 1989 196-204. April 5-7; Las Vegas, NV. Gen. Tech. Rep. INI'-276. Ogden, U.S. Department of Agriculture, Soil Conservation Service. UT: U.S. Department of Agriculture, Forest Service, In­ 1976. National range handbook. Washington, DC: U.S. termountain Research Station: 4-10. Department of Agriculture, Soil Conservation Service. Zar, J. H. 1984. Biostatistical analysis. Englewood Cliffs, [n.p.] NJ: 2d ed. Prentice-Hall. 718 p.

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