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 Follow this and additional works at: https://scholarsarchive.byu.edu/wnan 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 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 Western North American Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Westem North American Naturalist 63(2), ©2003, pp. 167-177 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.