Great Basin Naturalist
Volume 56 Number 1 Article 3
1-31-1996
Consumption of diffuse knapweed by two species of polyphagous grasshoppers (Orthoptera: Acrididae) in southern Idaho
Dennis J. Fielding University of Idaho, Moscow
M. A. Brusven University of Idaho, Moscow
L. P. Kish University of Idaho, Moscow
Follow this and additional works at: https://scholarsarchive.byu.edu/gbn
Recommended Citation Fielding, Dennis J.; Brusven, M. A.; and Kish, L. P. (1996) "Consumption of diffuse knapweed by two species of polyphagous grasshoppers (Orthoptera: Acrididae) in southern Idaho," Great Basin Naturalist: Vol. 56 : No. 1 , Article 3. Available at: https://scholarsarchive.byu.edu/gbn/vol56/iss1/3
This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Great Basin Naturalist 56(1), © 1996, pp. 22-27
CONSUMPTION OF DIFFUSE KNAPWEED BY TWO SPECIES OF POLYPHAGOUS GRASSHOPPERS (ORTHOPTERA: ACRIDIDAE) IN SOUTHERN IDAHO
Dennis J. Fielding1,2, M. A. Brusven1, and L. P. KishI
ABSTHACT.-Consumption of diffuse knapweed (Centaurea dif.{usa Lam.) by 2 polyphagous grasshopper species, Melanoplus sanguinip€s (E) and Oedaleonotus enigma (Scudder), was studied using microhistological analysis of grasshopper crop contents. Grasshoppers were confined to cages containing C. diffusa and Sisymbrium altissimum L., a member of the mustard filmily known to be rcadily eatcn by these 2 grasshopper species. Preference indices for knap weed were lower than for S. altissimutn in 4 of5 trials. An uncagcd population ofM. sanguinipes on a knapweed~infested sitc consumed only small amounts of knapweed until late summer when most other plants werc senescent. Results sug gest that diffuse knapwced's low palatability to generalist herbivores may confer to it a competitive advantage over other rangeland plants.
Key words: Centaurea diftusa Lam., diffuse knapweed, herbivory, insects, competition.
Diffuse and spotted knapweed, Centaurea sanguinipes (F). and Dedaleonotus enigma diffusa Lam. and C. rnaculosa Lam., respec (Scndder), are capable of attaining very high tively, were introduced to the Pacific North densities (>30/m2). Both species feed npon a west around 1900 (Watson and Renney 1974). broad range of forbs (Brusven and Lamley Since then they have rapidly spread through 1971, Banfm and Brusven 1973, Sheldon and out the area (Fig. 1; Forcella and Harvey 1981). Rogers 1978). pfadt (1992) suggested that an Heavy infestations of knapweed reduce produc increase in introduced weeds is a factor lead tion ofmore desirable species offorage plants, ing to outbreaks of 0. enigma. Fielding and thus reducing the value of rangeland for graz Brusven (1993) found that both species prefer ing and wildlife habitat. Several specialist disturbed rangeland habitats dominated by insect herbivores have been introduced in exotic annual plants. This study assessed the attempts to control knapweed (Story and Ander utilization of diffuse knapweed as food hy these son 1978, Maddox 1979). To date, no studies 2 grasshopper species to determine if knap have reported on the consumption of knapweed weed represents a significant and expanding by polyphagous insect herbivores. resource for grasshoppers and if grasshopper Cnicin, a sesquiterpene lactone, is produced herbivory may be a constraint to knapweed by spotted and diffuse knapweed (Drodz 1966, Locken and Kelsey 1987). Picman (1986) sug populations. gested that sesquiterpene lactones have toxic Previous studies (Brusven and Lamley 1971) effects on many herbivores and may function have shown Sisyrnhrium altissimum L., an intro as deterrents to herbivory. Locken and Kelsey duced annual forb, to be preferred by many (1987) suggested that nonpalatability of knap forb-feeding grasshoppers. Both species of weeds may afford them a competitive advantage weeds initiate growth as a basal rosette of over many other plant species by protecting leaves and later develop erect, sparsely leaved them from herbivory. Grasshoppers (Orthoptera: stems: that bear flowers. Because C. diffusa is Acrididae) are a conspicuous and important usually a biennial, it does not develop beyond class of herbivores on rangeland in the west tbe basal rosette until the 2nd year. Sisymbrium ern u.s. altissirnurn constituted a large proportion of Rangeland grasshopper popnlations in south the forbs present in this study; therefore utili ern Idaho occasionally reach outbreak propor zation of C. diffusa and S. altissirnurn was tions. Two species in particular, Melanoplus compared.
IDepartment "I' Piau!, Soil, and EntOlTIolngieal Sciences, University ofIdaho, Mo~eow, 1D 8.'3844-2339. 2Presont addnJss; PO B"x 75010, University ofAlaska, l'~lirlmllh. AK 99775-0102,
22 1996] KNAPWEED CONSUMPTION BY GRASSHOPPERS 23
1920 1980 1990
\
Fig.!. Idaho counties reporting infestations ofdiffuse knapweed, Cen/aurea diffusa.
MATERIALS AND METHODS fresh weight was obtained within 4 h of clip ping. Clipped plants were then allowed to air The study site is about 3 km south of Sho dry until they quit losing weight (W-15 d), shone. Idaho (Lincoln County). in a knapweed after which dry weights were ohtained (to the infested area that had been seeded with crested nearest 0.1 g). Percent moisture of above wheatgrass (Agropyron enstatum [L.] Gaertn.) ground portions ofeach plant species was then in 1975. Grasshopper food preferences were determined. identified by microhistological analysis ofgrass Plants were rated after each trial according hopper crop contents (Brusven and Mulkern to phenology as follows: 1, vegetative growth 1960, Sparks and Malechek 1968. Fielding and only; 2. flowering; 3, seed set: 4. seed maturity: Brusven 1992). Grasshoppers were confmed to 5. senescent or dormant (USDA-Soil Conser cages so that relative amounts of different vation Service 1976). Crass species present plant species could be precisely determioed. included Poa sandbergii Vasey, 8romus tecto Five trials were conducted during the summer rom L., and Agropy"on cn.statum. Centaurea. of 1989: 0. enigma 4th- and 5th-instar nymphs dijJ'u,!a and S. altissimum composed about 97% in early June: 0. enigma adults and M. san of aboveground biomass of forbs. Both Ist guinipes 4th and 5th instars in late June: M. and 2nd-year C. dijJ'u,!a were present in each sanguinipes adults in July and again in August. ofthe cages. Other forbs present were Helian For each trial, 4 wire-mesh (5-mm pore size). thus annttus L., Lactuca seniola L .• and Epilo conical cages covering 0.5 m2 each were placed bium L. sp. in the field such that at least 1 plant each of C. Grasshopper crops were removed and the diffusa and Sisymbrium altissimum L., along contents mounted on glass slides in glycerin with assorted common grasses, OCCUlTed within and safranin stain. Plant fragments in the crops each cage. Twelve to 15 grasshoppers of a sin were identified by comparing them with refer gle species were placed in each cage. Grass ence slides made from fragments of known hoppers used in the tests were collected from plants collected at the study site. similar to the rangeland and placed in the cages within 20 h methods described by Fielding and Brusven of collection. A 4-d interval was estimated to (1992). Frequency counts were made for eacb be sufficient to completely void previous meals plant species by determining their presence or and to accurately assess preferences in choice absence in 20 microscope viewing fields per tests. After 4 d, 10 grasshoppers were removed grasshopper crop. Trichomes, hairs. and pollen from each cage and immediately preserved in were not counted. Frequencies from the 10 95% ethanol for crop analysis. grasshoppers per cage were summed. Relative Species composition of plants in each cage frequency was calculated by dividing the fre was determined on an air-dry basis by clipping quency of a plant species by the total fre and sorting by species ahoveground portions quency of all plant species (Sparks and ofplants in each cage after each trial. Clipped Malechek 1968. pfadt and Lavigne 1982). plants were stored in air-tight plastic bags. and HoJecheck and Gross (1982) demonstrated the 24 GREAT BASIN NATURALIST [Volume 56
near equivalence ofrelative frequency to actual Plant species composition at the site was dry weight percentage ofplants consumed. determined by visual estimates, in 5% incre Relative availability ofdifferent plant species ments, of the ground cover of each plant within an area has been shown to influence species in forty 0.1-m2 quadrats, arranged in 4 diet composition in many grasshopper species transects of 10 quadrats each. Ground cover (Ueckert et al. 1972, Mitchell 1975). To account estimates were made in July and again in for the effect of availability on consumption, October after precipitation caused abundant preference values for plant species constitut gennination of cheatgrass. Because accurate ing more than 10% of either cage or crop con estimates of food availability (biomass) in the tents were calculated by dividing relative fre field were not availahle, preference values were quency of a plant species in the crops by that not calculated and the results are presented species' percentage of tbe dry-weight of all for comparative purposes only. plants within the cages (Ueckert and Hansen 1971). A preference value > 1 indicates feed RESULTS ing in greater proportion to the plant's avail ab~ity, whereas a preference value <1 indi Cages were placed such that C. diffusa was cates low preference in relation to a plant's equally as abundant as or more abundant than availability. S. altissimum in each trial (Table 1). Percentage Possibly, total dry weight ofa plant may not moisture of both species of weeds declined accurately portray the amount of plant mater throughout the season (Table 1). Sisymbrium ial available to grasshoppers, thus introducing altissimum tended to be slightly more advanced bias into the preference values. In this study phenologically than C. diffusa throughout the our observations indicated that both species of season, partly due to the presence of 1st-year weeds had similar ratios of leaves to stems. rosettes of c. diffusa in the cages, but also Also, we have observed grasshoppers feeding because of earlier flowering by S. altissimum on stems of both weed species. Because we (Table 1). hud no way to determine more precisely Although C. diffusa constituted a substan exactly what proportion of the plant was avail tial percentage (10-46%) of the caged grass able as food to the grasshoppers, we used total hoppers' diet, preference values for C. diffusa aboveground biomass as a reasonably objec were < 1 in every trial, indicating that it was tive measure of availahility. The presence of not consumed in proportion to its dry weight 1st-year rosettes of C. diffusa in the cages composition within the cages (Table 1). Prefer ensured that each replication included a rep ences values for S. altissimum were> 1 in each resentative choice ofplant material. trial, indicating that it was consumed in pTO· Differences between plant species in rela portions greater than its relative availability. tive frequency and preference values were After flowering in July, a large portion of tested using the Wilcoxon 2-sample test (PROC the C. diffusa plant material in the crops ofM. NPARIWAY, SAS 1985), \vith each cage rep sanguinipes consisted of floral parts (44% and resenting 1 replication. Comparisons between 30% of the C. diffusa material consumed, in plant species were made for each trial ofa sin the July and August trials, respectively). Other gle grasshopper species and \vith data from forbs represented in in situ caged trials were different trials pooled hy grasshopper species. not present in sufficient quantity to ade The same statistical methods were also used to quately assess their preference values. test for dilIerences in relative frequency and More S. altissimum than C. diffusa was con preference values between grasshopper sumed by grasshoppers in 3 of the 5 trials species for C. diffusa and S. altissimum. (Table 1). Preference values for S. altissimum Food selection was monitored in an uncaged were greater than those for C. diffusa in 4 of population of M. sanguinipes near the cage the trials (Table 1). Combining data from the 3 study. Thirty to 50 individuals were collected trials with M. sang-uinipes, crop contents and on each of5 dates from Juue through October preference values for S. altissimum, 42% and from an area of ca 1 ha infested with knap 2.0, respectively, were greater than for C. dif weed. Food preference in this population was fusa, 16% and 0.5, respectively (Wilcoxon test, determined by microhistological methods de P < 0.01 for both tests). For 0. enigma, the scribed above. overall preference value for S. altissimum, 3.5, 1996J KNAPWEED CONSU~fPTlONBY GRASSHOPPERS 25
TABLE L Relative availability and consumption by grasshoppers ofplant species. Plant Percent Merm Relatiw Relativt:l Mean phenological moisture dry weight availability frequency preference Plant species st.'4?:e1 "fplants in cages in C'.tges2 m crops index 4th· and 5th-instar Oedaleonotus etligma nymphs on 6 June 1989 Ccntatwea dijfusa 1 77 1304 23 lOa3 0.3& Sisymbrium altissimum 1 81 8.8 15 48b 5.06b Other Forbs 1 85 0.6 1 <1 Agropyron cristatum 1-2 57 18.1 31 <1 <0.05 Poa sandbcrgii 4 24 4.7 8 2 Bromu.s tectorum 4 21 12.8 22 32 1.42 Detritus 6 adult oedo1eonouu enigma on 26 June 1989 ~ntaurea diffusa 1 64 59.0 61 46a 0.760. Siaymbrium ultissimurn 1-2 67 27.1 28 48a 1.93>1 Other forbs 1-2 79 1.0 1 1 A..,aropymn l,'1istatum 3-4 45 6.8 7 0 Paa sandbergii 5 15 0 0 0 Bromm tectorwn 5 1.2 3.6 3 1 - Detritus 4 4th- and 5th-instm' Melanoplus satlgtJin~s nymph.~ on 26 June 1989 Cenlaurea diffusa 1. 64 60.8 59 100 0.25•• Sisymbrium altissimtnn 1-2 67 25.8 2S 74b 3.00b Other Forbs 1-2 79 2.1 2 0 Agropyron ~tatum 3-4 45 6.2 6 0 Poa SOlldhergii 5 15 4.1 4 5 8mrnus tectO'rum 5 J2 3.1 3 4 Detritus 2 adult Melanoplus sanguinipes on 21 July 1989 CentlJurea diffusa 1-2 63 2.5.4 29 16a 0.560 SisymbriWIl altissimum. 2-3 55 24.5 28 44b l.55b Other forbs 1-2 75 8.8 1 3 Agropyron cristatum 4 45 7.0 8 4 Pou sandbergii 5 9 17.5 20 3 0.17 Brpmus tectorum 5 14 llA 13 27 2.52 Detritus 3 adult Melnnoplu., sanguinipes on 25 August 1989 Centaur-en diffusa 1.3-4 22 38.8 38 23a O,70a Sisymbrium ultissimum 4-5 II 1804 18 24a lASb Other Coms 2-3 65 8.2 8 5 Agropyron cristatum 4 18 15.3 15 3 0.37 POll sarulbergii. 5 8 4.1 4 5 Bromus tectorum 5 7 13.3 13 2S 2.83 Detritus 16
II. Yegetalive growth only; 2, Howering; 3, seed St.-I; of, seed 1I'1.1tu!ily;5, seuescent ordonnlln[ IIM.....n (N • 4) pen"""t>l&<" ofabovegron",1 pbnl biOlll3»ll (Hil~dry basis) within (\lIgct 3MeilrlS for c. diffusa and S. aJtil;~~lIlum within columns ofeachtrilll followed by difft:rent letters are ~jSlTlHlclllltlydi!J;"""'II, P < 0.05, WjlUll.'lnn 2-sil.mple h,s!. was greater than for C. diffusa, 0.6 (Wilcoxon centage of air-dry biomass. Even though 0. test, P < 0.05). There was no difference in con enigma is generally considered to be a forb sumption by 0. enigma between S. altissimum feeder (Sheldon and Rogers 1978, pfadt 1992), and C. dijftlSa, 48% and 27%, respectively B. tectol'um constih,ted 32% of the ruet of 0. (Wilcoxon test, P > 0.05). There were no dif erngma in early June (Tahle 1). Adult 0. enigma ferences between the 2 species nf grasshop pers in relative frequency or preference values in late Jnne ate very little B. tectorum. Melanop for either S. altissimurn or C. diffusa (Wilcoxon lus sanguinipes consumed B. tect.o1'tLtn through test, P > 0.10 for both comparisons). out the summer, with 4-27% of its diet com Of the grass species, only Bromm tecWrum posed of B. tectomm, even though the grass was eaten in greater proportion than it'i per- was completely senescent by 26 June (Table 1). 26 GUEAT BASIN NATURAL1ST [Volume 56
TABLE 2. Relative frequency of food items in crops ufM. StJrli,'Uinipes on 5 dales and percentage ground Gover in July and October 1989.
Pen..'tmtage Relative frequency ofcrop componenls ground cover 30 20 14 6 13 July October June July Aug Sep Oet Sisytllnrium ulti.>'simum 46 23 22 7 6 2 1 Cenlaurea diffusa IS 30 32 .oS 1 6 4 Other forbs" 19 25 7 24 6
Knapweed was the most common forb grow duce the quality of S. altissimllm relative to C. ing on the site where the uncaged population difftlSa, especially when 1st-year rosettes, coo ofM. sanguinipes was studied {Tahle 2). In June, sisting mostly of leaves, are considered. S. altissimllm was the largest single fond item, Locken and Kelsey (1987) reported that but consumption declined as the season pro cnicin concentrations in C. maculosa vary con· gressed. Knapweed was a substantial food item, siderably within and among individual knap especially in August and September when it weed plants. enicin is sIDred within glandular remained sUL"Culent alier other forbs had dried. trichomcs on the surface of knapwecd tissues. After rainfall stimulated germination of B. tee Highest concentrations of cnicin were found tarum in late September and October (Table in leaves surrounding the inflorescence. Only 2), it became the primary food item for M. san· trace quantities were found by Locken and guinipes, and Forbs constituted only a minor Kelsey (1987) in the stem epidermis and flow portion ofthe diet. ers. Leaf concentrations were lowest in spring and increased with flowering. We assume that DISCUSSION cnicin concentrations in C. dijJusa follow much the same pattern. Variability in cnicin concen The evolutionary history of an herbivorous tration may result in selective consumption by species, by shaping its food habits and other grasshoppers of knapweed tissues with low life history traits, determines its present rela cnidn concentrations. Our results suggest that tionships with exotic plant ,-pecies. The 2 grass this is the case: In late-summer trials much of hopper species in this study consume a wide the Imapweed tissue consumed by grasshoppers variety of plants, especially forbs (Banflll and consisted of llowers. This implies that during Brusven 1973, Sheldon and Rogers 1978, pfadt years of high grasshopper densities, feeding 1992), and will readily accept exotic plant hy grasshoppers, especially 00 the flowers, species. Melanop/w; sangtlinipes is a very oppor could result in a modest reduction in seed tunistic feeder. Egg hatch in this species is often production in this plant. spread out over a long period, resulting in a Results of this study provide support for the large proportion ofa population maturing dur hypothesis that Imapweed is protected from ing the dry periods typical of late summer in herbivory by its chemical constituents (Picman the intermountain region. At such times many 1986, Locken and Kelsey 1987). When com late·maturing plants that still retain some suc~ pared to S. altissimllm, dilIuse knapweed was a culence, such as rabbithrush, sagebrush, and 2nd-choice food item for these generalist some lupine species, are primary food items grasshopper species. Its low palatability may for M. sanguinipes. The results of this study confer a competitive advantage to knapweed indicate that this was the case with C. difftlSa; when herbivory is a strong selection factor. even though it was not highly preferred by M. Although it is conG'Civable that at higll densities sanguinipes, it was a major food item in late grassboppers may consume significact amounts summer when most other plants were dry. of Imapweed and reduce seed production, Sisymbrium aWssimm" tended to become sene many otber plant.' would be afIected to a greater scent earlier thao C. diffusa, which would reo degree, tho:; reducing competition ID Imapweed. 1996] KNAPWEED CONSUMPTION BY GRASSHOPPERS 27
Grasshopper species used in this trial are southern Idaho rangeland. Environmental Entomol the dominant species contributing to outbreaks ogy 22,71-81. FORCELLA, E, AND S. J. IIARVEY. 1981. New and exotic in southern Idaho. It appears that increasing weeds of Montana. Volume II: Migration and distri knapweed infestations do not represent a sig bution of 100 alien weeds in northwestern U.S.A., nificant increase in food resources for these 1880-1980. Montana Department of Agriculture, grasshoppers. However, because knapweed Helena. stays green longer during the summer than HOLECHECK, J. L., A.'