Western North American Naturalist
Volume 63 Number 2 Article 3
4-30-2003
Effects of grass bug feeding and drought stress on selected lines of crested wheatgrass
Robert S. Nowak University of Nevada, Reno
James D. Hansen Utah State University
Cheryl L. Nowak USDA Forest Service, Rocky Mountain Research Station, Reno, Nevada
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Recommended Citation Nowak, Robert S.; Hansen, James D.; and Nowak, Cheryl L. (2003) "Effects of grass bug feeding and drought stress on selected lines of crested wheatgrass," Western North American Naturalist: Vol. 63 : No. 2 , Article 3. Available at: https://scholarsarchive.byu.edu/wnan/vol63/iss2/3
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EFFECTS OF GRASS BUG FEEDING AND DROUGHT STRESS ON SELECTED LINES OF CRESTED WHEATGRASS
Robert S. Nowak!, James D. Hansen2, and Cheryl L. Nowak"
ABSTRACf.-The sequential effects of feeding by grass bugs (lrbisia padfica [Hemiptera: MiridaeD and of drought stress on the growth of2 crested wheatgrasses (the hybrid Agropyron cri8ta.tum x Msertorum and A. cristatum cv, 'Fair way') were investigated in a controlled greenhouse experiment. Growth rates ofgenotypes that were preViously selected for resistance to grass bug feeding were not consistently greater than those of unselected genotypes when plants were exposed to bug feeding. Thus, the mechanism ofresistance to bug feeding for the selected genotypes does not appear to be "tolerance," Le., rapid growth rates that allow the resistant genotypes to compensate for damage to green leaves caused by bug feeding. IIi addition, previous bug feeding did not exacerbate the effects of drought stress on plant growth rates; droughted plants generally had lower growth rates, independent of the presence or absence of prior bug feeding: Thus, we suspect that the selection process may have inadvertently favored green, robust plants rather than true resistance to bug feeding.
Key wonk: Irbisia pacifica, grass bugs, Agropyron cristatum, Agropyron cristatum X desertorom hybrid, feeding resistant genotypes, plant growth rate, tkought.
Crested wheatgrasses, Agropyron cri.statum Hansen and Nowak (1988) found that growth (L.) Gaertn. and Agropyron desertorum (Fisch. in Great Basin wildrye, Leymus cinereus (Scrib. ex Link) Schult., are often used to rehabilitate & Merr.) Love, was significantly limited by the western rangeland after disturbance from min combination ofgrass bug feeding and drought. ing, overgrazing, or fire. Unfortunately, mono Although the interactive effects of these 2 culture stands of these grasses may be suscep stresses may also affect plant growth of crested tible to grass-feeding mirids (Todd and Kamm wheatgrass, these effects have never been 1974, Ansley and McKell 1982). As part of a measured under either natural or controlled strategy to reduce the impact of these insects, conditions. plants of crested wheatgrass resistant to grass Our research had 2 primary objectives. The bugs have been identified (Hansen et al. 1985). 1st objective was to determine if the mecha Under natural conditions in the Great Basin nism of resistance to grass bug feeding for pre ofwestern North America, a single generation viously selected genotypes is "tolerance" based ofthe grass bug Irbisia pacijU;a (Uhler) (Hemip on rapid growth rates (Painter 1968, Wiseman tera: Miridae) occurs in late spring and early 1985). The null hypothesis was expressed as summer, with the greatest feeding damage follows: the growth rate ofplant genotypes that occurring near the end of May and into June had been previously selected for bug resis (Hansen 1988). These grass bugs are sucking tance would not differ from the growth rate of insects that damage leaves by lacerating cells genotypes that had not been selected. How with stylets, which leads to the development ever, we expected that growth rates of previ of chlorotic areas that may cover>70% of the ously selected, bug-resistant genotypes would leaf area. Droughts are common in the Great be relatively greater than those of unselected Basin throughout the summer (Smith and genotypes during and immediately after hug Nowak 1990). Hence, plant growth previously feeding. Our 2nd objective was to determine impaired by bug feeding may be further the sequential effects of bug feeding and impeded by the lack ofavailable moisture dur drought on plant growth ofcrested wheatgrass. ing the last portions of the growing season. Our null hypothesis was that bug-feeding and
lAuthor for cOfrmpondence: Department of Environmental and Resource Sciences I MS 370, University of Nevada-Reno, Reno, NV 89557. 2USDA-ARS, Crops Research Laboratory, Utah State University, Logan, UT 84322. Present addres,: USDA-ARS, Yakima Agricultural Rf'M.'lrch Laboratory, 5230 Konnowac Pass Road, Wapato, WA 98951. 3USDA Forest Service, Rocky Mountain Research Station, 920 Valley Road, Reno, NV 891H2.
167 168 WESTERN NORTH AMERICAN NATURALIST [Volume 63 drought treatments would not synergistically At the beginning ofthe study, we determined affect plant growth. However, we expected total number of leaves (TL) and number of that previous hug feeding would exacerbate green leaves (GL) for each tiller. In addition, the effects of drought stress on plant growth. length, width, condition, and location of each We tested our hypotheses using wheatgrass leaf on a tiller were measured. Leaf condition cultivars intended for rangeland rehahilitation. was visually estimated as the percent of the leafthat was senescent or damaged (Hansen et METHODS al. 1985); senesced and damaged leaf tissues were not distinguished. Location of the leaf 'Fairway' crested wheatgrass, A. cristatum, was important for tracking individual leaves and a crested wheatgrass hyhrid, A. cristatum through time. By multiplying leaf length by X A. desertarum, served as host plants. For width, we estimated total leaf area (TA). both grasses we used 2 groups of genotypes: Undamaged green leaf area (GA) was calcu "selected" genotypes that were found to be lated by multiplying TA by percent damage (as resistant to grass bug feeding in previouS stud a proportion), then subtracting the product ies (Hansen et al. 1985) and "unselected" geno from TA. During the 5-6 weeks of the study, types that were grown from bulk seed of the 2 we repeated all measurements 5 more times. crested wheatgrass varieties. For the selected To assess plant growth, we computed the genotypes, we vegetatively cloned individual relative growth rate (RGR) of the number of plants for this study by carefully removing 2-3 green leaves per tiller, of the total number of tillers from the previously selected plants and leaves per tiller, of the amount of green leaf planting those tillers in l64-mL cone-shaped area per tiller, and of the total amount of leaf pots (Super-Cell Cone-tainers, Ray Leach Nurs area per tiller. RGR was used rather than ery, Canby, OR) in a greenhouse. Although we absolute growth rates because RGR incorpo intended to have a clone of each previously rates initial plant size and hence allows direct selected genotype in each of the treatments comparison of growth rates among plants of described below, insufficient growth and mor different sizes (Hunt 1978). The classical inter tality of some clones before we began the ex val equation (Chiariello et al. 1989) was used periment precluded this balanced design. to calculate RGRs for each of the 5 time inter Similarly, insufficient growth and mortality of vals between sequential dates when leaf mea plants grown from seed for the unselected surements were taken. The general form of genotypes also occurred prior to the start of the equations is: the experiment. Thus, sample sizes were 31 unselected plants and 36 selected plants for A. cristatum, and 30 plants each of unselected RGR= and selected genotypes for A. cristatum X desertarum. where ml and m2 are measurements of plant These individual plants, which were com leaf number or area at time 1 (tj) and time 2 posed of 1 to 5 tillers, were the experimental (t2), respectively. Although RGR is generally a replicates. After all plants were established, positive number, RGRs of green leaves and of we then caged them separately in a cardboard green area have negative values as leaves on cylinder (17 em high X 9 em diameter) with a the plant senesce. Because the experimental fabric screen tOPi these cages were used to unit was the plant, we averaged all individual contain grass bugs during the bug-feeding tiller measurements from an individual plant treatment. Although the plant was the experi before statistical analyses were performed. mental unit to which treatments were applied, For the bug-feeding treatment, approximate data are expressed on a per tiller basis rather ly half the plants from selected and unselected than per plant because the unequal number of lines of both grass varieties were randomly tillers per plant leads to differences in plant chosen, then exposed to adult Irbisia pacifica. size that are unrelated to treatments. For Insects were collected from a pasture of inter plants with more than 1 tiller, individual tillers mediate wheatgrass, Thinopyron intermedium were treated as subsamples and thus averaged (Host) Barkw. & D.R. Dewey, about 3 km together to derive the per tiller data for that northeast of North Logan, Utab. The stocking plant. rate was 20 bugs per cage. Bugs were removed •
2003] GRASS BUG EFFECfS ON CRESTED WHEATGRASS 169
after severe damage was obvious, which was 6 treatment interaction terms followed the pro days for A. cristatum X desertarum and 11 cedures of Steel and Tonie (1980). The statis days for A. cristatum. tical program SYSTAT (SYSTAT 1997) was used Drought stress was initiated after bug re for the AOVs. moval for approximately half the plants from all treatment categories. Drought stress was RESULTS imposed by withholding water from the plants. Initial Measurements Remaining plants were well watered. The drought-stress phase ofthe experiments lasted A. cristalum X desertarum genotypes that about a month. Although we made no mea were previously selected for resistance to surements of plant water status during the grass bugs had 33% more green leaf area per drought phase of the study, the soil surface tiller than unselected genotypes at the start of around droughted plants was visibly dry by the experiment (F = 5.77, df = 1,58, P < 0.05; the end ofthe study. Fig. 1A). We observed no significant difference Mter performing the last leaf measurement, between the 2 genotypes for the number of we harvested the leaves for protein analysis to green leaves, total number of leaves, and total . indicate nutritional quality. Each leaf was leafarea per tiller. For A. cristatum, previously clipped, placed in a labeled coin envelope, and selected genotypes averaged about 20% greater then dried. Protein concentrations of the 2nd number of green leaves (F = 8.89, df = 1,65, and 4th youngest leaves were determined fol P < 0.01), 20% greater total number of leaves lowing Nowak and Caldwell (1984). These 2 (F = 6.71, df = 1,65, P < 0.01), 120% greater leaf cohorts represent samples of leaves that green leaf area (F = 39.15, df = 1,65, P < were mature at the beginning of the study and 0,01), and 80% greater total leaf area per tiller leaves that were produced during the study. (F = 26.94, df = 1,65, P < 0.01) than unse Because of cost constraints, only leaves from lected genotypes prior to bug feeding (Fig. IE). well-watered plants were analyzed. Each crested wheatgrass variety was ana Plant Growth During lyzed separately. The statistical analyses uti Bug Feeding lized 4 sets of analysis of variance (AOV) for A. cristatum Xdeserlrmttn. RGRs ofnumber each crested wheatgrass variety. The 1st set ofgreen leaves (F = 7.13, df = 1,55, P < 0.05) was I-way AOVs to determine if the initial and of green leaf area per tiller (F = 6.85, df numbyrs and areas of leaves (i.e., number of = 1,56, P < 0.05) were significantly affected green leaves, total number of leaves, green by genotype: unselected genotypes lost green leaf area, and total leaf area per tiller) differed leaves at a greater rate than selected geno between selected and unselected genotypes. types (Fig. 2A). In addition, RGR of green leaf The 2nd set was 2-factor AOVs to determine if area per tiller for plants that were not exposed the RGRs of number of green leaves, total to bug feeding was significantly greater than number of leaves, green leaf area, and total for those exposed to bugs (F = 35.31, df = leaf area per tiller were affected by genotype 1,56, P < 0.01). In contrast, RGRs oftotal leaves or by bug feeding. The 3rd set was repeated and total area per tiller were not significantly measures AOVs to analyze the effects of geno affected by either genotype or by bug feeding. type, previous bug feeding, and water treat A. cristatum. Genotype and bug feeding ment on the RGRs. The last set was 2x2 facto significantly affected the RGR of all 4 growth rial AOVs for each cohort of leaves to deter measurements (green leaves: F = 18.53, df = mine if genotype or prior bug feeding affected 1,62, P < 0.01; total leaves: F = 15.44, df = leaf protein content. Results from AOVs were 1,62, P < 0.01; green area: F = 63.82, df = considered significant if P < 0.05. Where nec 1,61, P < 0.01; total area: F = 30.25, df = 1,62, essary, degrees of freedom were adjusted by P < 0.01). Selected genotypes without bug the Huynh-Feldt procedure (Huynh and Feldt feeding consistently had the greatest RGRs 1976). Ifthe F test for a main effect or interac among all 4 treatment groups, whereas selected tion term was significant, means were com genotypes with bug feeding consistently had pared with a least significance difference (LSD) the lowest RGRs (Fig. 2B). For unselected geno test. Calculation of the LSD value for within- types, RGRs of plants exposed to bug feeding 170 WESTERN NORTH AMERICAN NATURALIST [Volume 63
A. cnstatum X daser/arum A A. cnstatum B jl ., NS ** ~ * ~ NS NS O-m .L E 4 **lllID 20 0 I" ern "I" = T ::> " j T "'\ _l"J " ~ ~ '\ ,::c'\ ~;c o " ~ '\ ~ ~ 2 T "~- 10 '" 1l ~ E < ::J ~ Z ~ ~ ~ ~ 1"\ I"
Fig L Number ofleaves and leaf area per tiller at the beginning ofthe experiment for Agropyron cristatum x deserto rom (A) and A. cristatum cv. 'Fairway' (B). Mean (± standard error) number ofgreen leaves, total number ofleaves, green leaf area, and total leaf area are given for unselected (Unsel) and previously selected (Sel) genotypes. An asterisk ("') indicates that mean values for the 2 genotypes were significantly different (P S 0.05); NS indicates that mean values were not significantly different.
were not significantly different from those with Only 1 whole-plot treatment, genotype, out bugs. was significant in the AOV of RGR of green leaf area per tiller (Table 1). The rate ofloss of Post-feeding Interactions green leafarea per tiller was greater for selected with Drought genotypes of A. cristalum x desertorum than A. cristatum X desertorum. The 3 whole for unselected genotypes over all time inter plot treatments were significant in the AOV of vals (Fig. 3). As with total number of leaves, RGR for number of green leaves per tiller the significant interval effect showed that (Table 1). Over all time intervals, the rate of RGR varied among time intervals. The signifi loss of green leaves from tillers of selected cant interval*genotype interaction term indi genotypes was significantly greater than that cated that significant differences between geno of unselected genotypes (Fig. 3). In addition, types varied among time intervals. Although plants with previous bug feeding lost green the decrease in green leaf area for selected leaves at a faster rate than those without prior genotypes was numerically greater than for bug feeding. Finally, the rate ofloss for plants unselected genotypes for 3 of 4 time intervals tbat were not watered was significantly greater after initiation ofwatering treatments, the dif than those watered. ference between genotypes was significant The genotype and bug-feeding wbole-plot only for the 3rd time interval. treatments were significant terms in the AOV Bug feeding was the only whole-plot treat of RGR for total number of leaves per tiller ment significant in the AOV of total leaf area (Table 1). Growth rates ofleaves for unselected per tiller (Table 1). Overall, RGR was greater genotypes and for genotypes without bug for plants without bug feeding than with feed feeding (Fig. 3) were significantly greater than ing (Fig. 3), but the significant interval'bug selected and with feeding, respectively. The interaction term showed that this difference significant interval effect indicates that over was significant only during the 1st time inter all treatments, mean RGR varied significantly val after initiation of the watering treatments, through time; in this case, RGRs for the first 2 even though RGR for plants without feeding time intervals after initiation of the watering was numerically greater than with feeding for treatments were significantly greater than all 4 time intervals. Finally, the interval'geno those for the last 2 intervals. type interaction term was also significant 2003] GRASS BUG EFFECTS ON CRESTED WHEATGRASS 171
A. cristaturn X desertorurn A 0.04 i- NS NS V I I r/~ 0.00 -~ Fl
-0.16 L b d a e o~ A. cristaturn B 0.04 ab be e a b b e a -.... ~I :t:: 0.00 -= "- ~ I I~ 'X f/j u> en U) 0)0)0)0) ~ ~ ::J ::J ::J :J mcnllllll V ~ -0.04 ~ I + I + IV ~I~ b be e a a; -~ Q.l -~ - Ulco :g'" en en'" 0:: -0.08 :::> <.? :::> 0:: -0.12 ~ b be e a -0.16 r
Green leaves Total Leaves Green Area Total area
Fig 2. Mean (+ standard error) relative growth rates for number of green leaves, total number of leaves, green leaf area, and total leaf area per tiller during the bug-feeding portion of the experiment for Agropyron cristatum X deserto rum (A) and A. cristatum cv. 'Fairway' (B). Results are given for plants that were unselected (UDsel) or previously selected (Sel) genotypes and that were not (- Bugs) or were exposed (+ Bugs) to grass bug feeding. NS indicates that mean values among the 4 treatment means were not significantly different (P > 0.05); treatment means with different lowercase letters were significantly different.
(Table 1). Mean comparisons indicated that treatments for the 1st and 4th time intervals RGR ofleafarea per tiller was not significantly after initiation of the watering treatments. Fbr different between unselected and selected the 3rd interval, the rate of green leafloss for genotypes except for the 2nd time interval: droughted plants was significantly greater than during this interval, the mean RGR was signif for well-watered plants, regardless ofpresence icantly greater for unselected genotypes (0.009) or absence of prior bug feeding. However, re than for selected genotypes (0.005). sults during the 2nd time interval depended A. cristatum. Over all time intervals, green on prior bug feeding: without bug feeding, well leaves were lost significantly faster from un watered plants had significantly greater RGR, watered plants than from watered plants (Table whereas with prior bug feeding, droughted 1, Fig. 3). However, this overall watering effect plants had greater RGR. The interval*geno interacted with time as well as with both time type*bug interaction term was also significant and prior bug feeding. Mean comparisous of (Table 1). Although the whole-plot genotype the interval*bug*water interaction term showed effect was not significant, the rate of green leaf no significant difference between watering loss for unselected genotypes was significantly 172 WESTERN NORTH AMERICAN NATURAUST [Volume 63
TABLE L P-valuesafrom repeated-measures AOVs for relative growth rate ofnumber ofgreen leaves per tiller (R-GL), total number of leaves per tiller (R-TL), green leaf area per tiller (R-GA), and total leaf area per tiller (R-TA) for the last 4 time intervals for genotypes of both A. crlstatum x Msertorum and A cristatum that were previously selected or un- selected for bug-feeding resistance.
A. cristatum X des6f'torum A. cristatum Source oferror df R-GL R-TL R-GA R-TA df R-GL R-TL R-GA R-TA
WHOLE-PLOT TREATMENTS Genotype 1 <0.01 0.02 <0.01 NSb 1 NS NS NS NS Bug feeding 1 0.02 <0.01 NS 0.04 1 NS NS NS 0.04 Water 1 <0.01 NS NS NS 1 0.01 0.05 <0.01 <0,01 Type*bug 1 NS NS NS NS 1 NS NS NS <0.01 lYPe*water 1 NS NS NS NS 1 NS NS NS 0.02 Bug*water 1 NS NS NS NS 1 NS NS <0.01 NS 1)rpe*bug*water 1 NS NS NS NS 1 NS NS 0.05 NS Error A 52 59 REPEATED MEASURES Intetval 3 NS <0.01 <0.01 <0.01 3 NS <0.01 NS <0.01 Interva1*genotype 3 NS NS 0.04 <0.01 3 <0.01 NS <0.01 NS InteIVal*bug 3 NS NS NS <0.01 3 NS NS <0.01 NS IntelVa1*water 3 NS NS NS NS 3 0.01 NS <0.01 0.03 Interval*lype*bug 3 NS NS NS NS 3 <0.01 NS <0.01 0.02 InteIVal*type*water 3 NS NS NS NS 3 NS NS NS NS Interva1*bug*water 3 NS NS NS NS 3 0.05 NS NS NS Interval*type*bug*water 3 NS NS NS NS 3 NS NS NS 0.01 Error B 156 177 ap·values for the repeated-measures terms have been corrected by the H·F procedure (see Methods). bp > 0.05.
greater than for selected genotypes duriug the was greater for plants without prior feeding last time interval. Within each genotype, bug (Table 2). Finally, although RGR over all time feediug also had significaut effects on RGR: intervals was significantly different between previous bug feeding increased the rate of geuotypes only for droughted plants without greeu leaf loss for selected genotypes during prior bug feeding (Table 2), the rate of green the 1st time interval after initiation of the area loss for selected genotypes was siguifi watering treatments and for unselected geno cantly greater than for unselected genotypes types during the last iutervaL during the 1st time interval, but significantly RGR of total number ofleaves per tiller for less during the last time interval. droughted plants was significantly less than RGR of total leaf area for plants without for well-watered plants (Table 1, Fig. 3). The previous bug feeding was greater than for plants only other significant tenn in the AOV was the with previous feeding (Table 1, Fig. 3), but this interval main effect (Table 1). The rate ofaddi trend was significant only for the selected tion ofnew leaves was significantly lower dur genotype (Table 3A) and for all but the last ing the last time interval than during earlier time interval. Effects of the watering treat intervals. meut were significant only for the selected Over all time intervals, plants that were not genotypes (Table 3B), although a drought watered lost green leaf area at a greater rate effect over both genotypes occurred during than those that were watered (Table 1, Fig. 3). the 3rd time interval after initiation of the However, 3 interaction terms with water were watering treatments. also significant and indicated (1) the rate of leaf area loss was significaut ouly during the LeafProtein Content last 2 time iutervals, and (2) the effect ofdrought Leaf protein content for a leaf cohort pro was significant ouly without prior bug feeding duced during the experiment (leaf 4) did not (Table 2). In additiou, a siguificant effect of differ siguificantly between genotypes or be prior bug feediug occurred only for droughted tWeen bug-feeding treatments for well-watered plants. luterestingly, the rate ofgreen area loss plants (Fig. 4). For a leaf cohort that was 2003) GRASS BUG EFFECTS ON CRESTED WHEATCRASS 173
A. cris1atum )( deserlorom 0- 0.00 ~:: f'.- ~- !,,- ~ m.t, -0.02 f:::l ~ c t, -0.02 r::: t'> II ~ .0'" f'.- .1. f( "~!!::.. .1. "- "/ e...... {).06 T[\ -0.03 ",E T.1 ~ - -0.04 " ,£. -0.08 0:: -0.10 "" NS 1s .0.05 *** * 0.008 ~ 0.010 T* NS ~~ NS * NS Cll _ 0.008 r G:l b 0.006 j!-;- 0.006 ~ s'l; .!9;' .9 0 0.004 ~ O.O~ . g;: R, "," ~ 0:-- ~ C> 0.002 " 5 0.002 0: ~ "'- f'.- ~ 0.000 % ~ 0.000 L.-
A. en'statum en 0.00 0.00 ."- ~ • ~ -0.01 ~ ~ - Fig 3. Mean (+ standard error) relative growth rates for Agropyron cristatum x desertomm (top sets ofpanels) and for A. cristatum (bottom sets ofpanels) over the 4 time intervals during the post-feeding, watering-treabnent portion of the experiment. RGRs of number of green leaves, green leaf area, total number of leaves, and total leaf area are averaged over the 2 genotypes, over the 2 bug·feeding treatments. and Dve..- the 2 watering treatments. An asterisk (.) indicates that mean values for the 2 genotypes were significantly different (P S 0.05); NS indicates that mean values were not sig nificantly different mature at the beginning of the experiment DISCUSSION (leaf 2). genotype and feeding effects were not , Effects ofBug Feeding significant for A. cristatUtll X tksertcmrn, but on Plant Growth both the genotype (F = 5.00. df = 1.23. P < 0.05) and the genotype'bug interaction (F = Our results generally are not consistent with 6.74. df = 1.23. P < 0.05) terms of the AOV tolerance. i.e.• greater growth rates during or were significant for A. cristatum (Fig. 4B). For immediately after bug feeding, as the mecha this cohort ofleaves on weU-watered A. crista nism of resistance to bug feeding in selected tum plants, leaf protein content in selected genotypes ofA. cristatum x tksertoru", If tol genotypes with bug feeding was significantly erance had been the mecbanism. then we greater than in those without bug feeding. It would have expected selected genotypes with was also greater than in unselected genotypes bug feeding to have greater relative growth with bug feeding. rates than unselected genotypes with bug 174 WESTERN NORTH AMERICAN NATURALIST [Volume 63 TABLE 2. LSD mean comparisons for the genotype*bug*water interaction tenn from the AOV ofrelative growth rate of green area per tiller for A. cristatum. Unselected Selected Genotype ------Bugs + Bugs - Bugs + Bugs Prior feeding --,- -"----, --,- -'----, --,- -"----, --,- -'----, Water treatment Drought Watered Drought Watered Drought Watered Drought Watered Mean RGR (x 10-')' -6.gb -3.70 -3.Se -4.8be -IO.3a -3.7c -5.2be -6.0be ·Means with the same letter were not significantly diITerent (P > 0,05). Table 3. LSD mean comparisons for the genotype*bug (A) and genotype*water (B) interactions terms from the AOV ofrelative growth rate oftotal area per tiller for A. cristatum. A. GENOTYPE'BUG Unselected Genotype Selected Prior feeding -Bugs + Bugs -Bugs + Bugs Mean RGR (x I B.GENOTYPE'VVATER Unselected Selected Genotype Water treatment Drought Watered Drought Watered Mean RGR (x I()-3) 3.6b 3.gb LBa 5.3b aMeans with the same letter were not significantly different (P > 0.05), feeding. Thus, at the very least, AOV terms tions of the experiment, even though selected with the genotype'bug feeding interaction genotypes without bug feeding often did better should have been significant. For A. cristatum than unselected genotypes. Only during the X desertorum, none of these AOV terms were last time interval ofthe experiment, when plants significant. Because greater RGRs for selected were nearly senescent, did selected genotypes genotypes, regardless of bug feeding, are gen with prior bug feeding have significantly greater erally consistent with a tolerance mechanism, RGRs than unselected genotypes. Thus, the we also considered all AOV tenus that included evidence to support greater growth rates after genotype. Some results, such as those from bug feeding as the tolerance mechanism is the RGR ofnumber and ofarea ofgreen leaves weak for A. cristatum. per tiller during the bug-feeding portion ofthe Two alternative mechanisms may account experiment (Fig. 2A), indicate greater RGRs for resistance to bug feeding in the selected for selected genotypes regardless of bug feed genotypes. First, the selection criteria may ing. However, results from the post-feeding, have inadvertently favored plant "greenness." drought portion of the experiment are con Robust plants with more green foliage (either trary to the mechanism: unselected genotypes more leaves or more green leafarea per tiller) generally had greater RGRs (Fig. 3). Thus, the may appear to be more bug resistant simply preponderance of evidence does not support because the same amount of foliage damaged greater growth rates after bug feeding as the by bugs would be less apparent in these plants tolerance mechanism for A cristatum X deser than in plants with fewer green leaves or torum genotypes. smaller green leafarea. For A. cristatum, selected For A. cristatum, most evidence also did genotypes had significantly greater amounts of not support a growth rate tolerance mecha leaves and leaf area than unselected geno nism. RGRs of selected genotypes with bug types; for A. cristatum x desertorum, only green feeding were often significantly less than those leafarea per tiller was significantly greater for for unselected genotypes during both the bug selected genotypes, although the initial number feeding (Fig. 2B) and drought (Table 3A) por- of leaves and total leaf area were numerically 2003] GRASS BUG EFFECTS ON CRESTED WHEATGRASS 175 8 r--,---,-,-,----,-----:-oc-,--,----,------=:, A. crista/urn X desertarum A A. cris/atum B NS NS _ 6 NS i'- ab a a b o Leaf 2 Leaf 4 Leaf 2 Leaf 4 Fig 4. Mean (+ standard error) leaf protein content for Agropyron oristatum x desertorum (A) and A. cri8tatum CV. 'Fairway' (B). Results for well-watered plants that were unselected (Umel) or previously selected (Sel) genotypes and that were not (- Bugs) or were exposed (+ Bugs) to grass bug feeding are given for 2 cohorts ofleaves Oeaf2, leaf4). NS indicates that mean values among the 4 treatment means were not significantly different (P > 0.05); treatment means with different lowercase letters were significantly different. greater for the selected plants. Thus, our results might influence feeding preference by the black are consistent with this alternative mechanism grass bug, Labops hesperius. Finally, specific of relative greenness. A 2nd alteruative mech chemicals that either attract or repel grass bugs anism is that the bugs have a lower preference have not been identified for these crested for selected genotypes due to some difference wheatgrass varieties. Although Windig et al. in nutritional, morphological, or chemical (1983) reported correlations between the pyro characteristics (Painter 1968). Although during lysis mass spectra of various range grasses and the initial selection experiments bugs were feeding preference by Labops hesperius, spe allowed to freely move among genotypes cific plant compounds were not isolated and (Hansen et al. 1985), during the experiment tested for their effects on feeding preference. reported here they were confined by cages to Clearly, additional studies are needed to validate an individual plant. Thus, bugs were forced to the potential role of these morphological and feed on each genotype, and plant characteris chemical characteristics in feeding deterrence. tics that influence bug preference would not Effects ofBug Feeding and have influenced feeding damage. Increased Drought Stress on Plant Growth damage by [rhisia sericans on Calamagrostis canadensis was correlated with increased leaf For both crested wheatgrass varieties, prior protein content (McKendrick and Bleicher bug feeding did not exacerbate the effects of 1980), but leaf protein contents did not differ drought stress on plant growth. Ifdrought and between selected and unselected genotypes bug feeding had additive effects on plant of A. cristafum x desertorum (Fig. 4A). For A. growth, then RGRs of well-watered plants cristatum, selected genotypes tended to have with prior bug feeding should have been greater greater leaf protein (Fig. 4B), but this trend is than those that experienced both drought stress opposite to our expectations. Thus, differences and bug feeding. However, none of the inter in protein content do not explain bug resis action terms with bug*water were significant tance in selected genotypes of crested wheat for A. cristafum X desertorum (Table 1). For A. grass. Differences in leaf morphology were cristatum, the bug'water and type'bug'water also not apparent between genotypes, although interaction terms for green area indicated that Ling et al. (1985) suggested that ultrastruc drought did not significantly affect RGR for tural leaf characteristics such as trichome size plants with prior bug feeding (Table 2). For 176 WESTERN NORTH AMERICAN NATURALIST [Volume 63 green leaves ofA. eristatum, prior bug feeding LITERATURE CITED appeared to benefit droughted plants during the 2nd time interval after drought initiation. ANSLEY, RM., AND C.M. McKELL. 1982. Crested wheat· grass vigor as affected by black grass bug and cattle In other studies an additive effect of previous grazing. Joumal ofRange Management 35:586-590. bug feeding and drought stress occurred for CHIARIELLO, N.R., H.A. MOONEY, AND K. WILLIAMS. 1989. Leymus einereus, but not for Thinopyrum inter Growth, carbon allocation and the cost of plant tis· medium (Hansen and Nowak 1988). Why L. sues. Pages 327-365 in RW. Pearcy, J. Ehleringer, physi~ einereus is the only one of the 4 species to B.A. Mooney, and P.w. Rundel, editors, Plant ological ecology: field methods and instrumentation, have an additive effect is not known, but it Chapman and Hall, New York. may be related to overall susceptibility of L. HANSEN, J.D. 1986. Differential feeding on range grass cinereus to I. pacifica feeding: feeding damage seedlings by lrbisw pacijWa (Hemiptera: Miridae). to L. cinereus during feeding trials was twice Journal of the Kansas Entomological Society 59: 199-203. that to A. cristatum (Hansen 1986) and can be . 1988. Field observations oflrbisia pacifica (Hemip~ particularly damaging under field conditions --t";""era: Miridae): feeding behavior and effects on host (Watts et ai. 1982). plant growth. Great Basin Naturalist 48:68-74. HANSEN, J.D., K.B. ASAY, AND D.C. NIELSON. 1985. Screen~ ing range grasses for resistance to black grass bugs SUMMARY Labops hesperius and lrbisia pacifica (Hemiptera: Miridae). Journal ofRange Management 38:254-257. Although our experiments did not provide HANSEN, J.D., AND R.S. NOWAK. 1985. Evaluating damage evidence to support tolerance as the mecha by grass bug feeding on crested wheatgrass. South~ nism of bug resistance in the selected geno western Entomologist 10:89-94. ---::c' 1988. Feeding damage by lrbisia pacifica (Hemip~ types, the selected genotypes do offer advan tera: Miridae) on host plant growth: effects of feed~ tages for rangeland rehabilitation. Selected ing and drought. Annals ofthe Entomological Society genotypes of both cultivars initially had more ofAmerica 81:599-604. foliage than unselected genotypes. Without HUNT, R. 1978. Plant growth analysis. Edward Arnold Pub~ lishers Ltd., London. 67 pp. bug feeding and with adequate water, selected HUYNH, H., AND LoS. FEWT. 1976. Conditions under which genotypes generally were more productive mean square ratios in repeated measurement designs than unselected genotypes. Even when attacked have exact F-distribution. Journal of the American by 1. paciftea, selected tillers had either the Statistical Association 65:1582-1589. same amount of green area as unselected ones LATIIN, J.D., A. CHRISTIE, AND M.D. SCHWARTZ. 1995. Native black grass bugs (lrbisia-Labops) on introduced or more area. From a livestock forage perspec wheatgrasses: commentary and annotated bibliogra~ tive, the greater overall productivity of selected phy (Hemiptera: Heteroptera: Miridae). Proceedings genotypes would be a definite advantage, of the Entomological Society of Washington 97: especially when the trend ofincreased leaf pro 90-111. LING, YH., w.F. CAMPBELL, B.A. HAWS, AND K.H. ASAY. tein with bug feeding is considered (Fig. 4; 1985. Scanning electron microscope (SEM) studies Malechek et al. 1977, Hansen and Nowak 1985). ofmorphology ofrange grasses in relation to feeding However, whether the use of selected geno by Labops hesperfus. Crop Science 25:327-332. types for rangeland rehabilitation will reduce MALECHEK, J.C., A.M. GRAY, AND B.A. HAWS. 1977. Yield damage caused by grass bugs more than other and nutritional quality of intermediate wheatgrass infested by black grass bugs at low population densi~ management techniques (Lattin et al. 1995) is ties. Journal ofRange Management 30:128-131. not known and would require additional field McKENDRICK, J.D., AND D.P. BLEICHER. 1980. Observa~ studies. tions of a grass bug on bluejoint ranges. Agroborealis 12,15-18. NOWAK, RS., AND M.M. CALDWELL. 1984. Photosynthetic ACKNOWLEDGMENTS activity and survival of foliage during winter for two bunchgrass species in a cold~winter steppe environ~ Appreciation is extended to S.F. Parker, K.L. ment. Photosynthetica 18:192-200. Bartz, L.D. Kelso, T.E. Dombrowski, and D.C. PAINTER, RH. 1968, Insect resistance in crop plants. Uni~ versity of Kansas Press, Lawrence. Nielson for their assistance in various aspects SMITH, S.D., AND RS. NOWAK. 1990. Ecophysiology of of this study. This research was supported by plants in the Intermountain lowlands. Pages 179-241 the USDA-ARS, Utah Agricultural Experiment in C.B. Osmond, L.F. Pitelka, and C.M. Hidy, edi Station, and Nevada Agricultural Experiment tors, Ecological studies. Volume 80. Plant biology of Springer~Verlag, Station (NAES Publication No. 52031262); RSN the Basin and Range. Heidelberg. STEEL, RC.D., AND J,H, TORRIE. 1980. Principles and also gratefully acknowledges sabbatical leave procedures of statistics. 2nd edition. McGraw Hill, support from the University of Nevada-Reno. New York. 2003] GRASS BUG EFFECTS ON CRESTED WHEATGRASS 177 SYSTAT 1997. SYSTAT for Windows version 7.0.1. SPSS observed in pyrolysis mass spectra of range grasses Inc., Chicago, IL. with different resistance to Labops hesperiw Uhler TODD, lG., AND JA. KAMM. 1974. Biology and impact ofa attack. Joumal of Analysis and Applied Pyrolysis 5: grass bug Labops hesperius Uhler in Oregon range 183-198. land. Journal of Range Management 27:453-458. WISEMAN, B.R. 1985. Types and mechanisms of host plant WATTS, J.G., E.W HUDDLESTON, AND J,e. OWENS. 1982. resistance to insect attack. Insect Science and Appli Rangeland entomology. Annual Review of Entomol cations 6:239-242. ogy 2a83-311. WINDlG, W, H.L.C. MEUZELAAR, B.A. HAWS, WE CAMP Received 13 November 2001 BELL, AND K.H. AsAY. 1983. Biochemical differences Accepwd 8 April 2002 •