NOTE

Black Grass Bug (:) Damage to Intermediate Wheatgrass Forage Quality

Sue L. Blodgett, 2 Andrew W. Lenssen 3 and S. Dennis Cash

Department of and Range Sciences, Montana State University, Bozeman, Montana 59717 USA

J. Entomol. Sd. 41(1): 92-94 (January 2006) Key Words rangeland pest, forage quality, pest management

The black grass bug, Labops spp. (Hemiptera: Miridae), is native to the western U.S. and was first observed damaging range grasses in the western U.S. coincident with the increase in seeded acreage of wheatgrasses (Agropyron and Thinopyrum spp. (Poaceae)) (Bohning and Currier 1967, J. Range Manage. 20: 265-267). Black grass bug refers to a complex of species belonging to genus Labops spp. and (rb/s/a spp., though in this study Labops was the predominant genera represented. Differ- ences in grass species susceptibility to black grass bug damage have been studied (Bohning and Currier 1967, Todd and Kamm 1974, J. Range Manage 27: 453-458; Higgins et al. 1977, J. Range Manage. 30: 380-384). Black grass bug has been documented damaging crested wheatgrass (Agropyron cristatum (L.) Gaertn.) (Boh- ning and Currier 1967), mixed crested and intermediate wheatgrass (Elytrig/a inter- media (Host) Nevski (Poaceae)) (Todd and Kamm 1974), wheat (Triticum aest/vum L.) fields in Montana (Bohning and Currier 1967), pubescent wheatgrass (formerly Agropyron trichophorum) = intermediate wheatgrass (formerly Agropyron interme- dium) now Thinopyron intermedium ( Host) Barknight and Dewey, and other wheat- grasses (Bohning and Currier 1967). Higgins et al. (1977) ranked susceptibility of several grasses to black grass bug feeding (most to least susceptible): intermediate wheatgrass, Kentucky bluegrass (Poa pratensis L. (Poaceae)), slender wheatgrass (Elymus trachycaulus (Link) Gould ex Shinners (Poaceae)), orchardgrass (Dactyl/s glomerata L. (Poaceae)), smooth brome (Bromus inermis Leyss. (Poaceae)), moun- tain brome (Bromus marginatus Nees ex Steud. (Poaceae)), and a mixture of native grass species including Lettermans needlegrass (Achnutherum letterman/i (Vasey) Barkworth (Poaceae)), sheep fescue (Festuca ovina L. (Poaceae)), and mountain muhly, (Muhlenbergia montana (Nutt.) A. S. Hitchc. (Poaceae)). Black grass bug overwinters as an egg within grass stems, which hatch in the spring, completing one generation per year. Piercing-sucking mouthparts of immature

Received 22 August 2005: accepted for publication 22 September 2005. Address inqljrles (email [email protected] ). USDA ADS Norl,ern Plains Agricultural Research Laboratory, Sidney. MT USA 59270.

92 BLOOGETT et al.: Black Grass Bug 93 and adult bugs cause foliar damage that appears as small white or yellow feeding punctures called stippling when pigment-containing cell contents are removed at the feeding site. Black grass bug plant damage has been reported as reduced yield (Bohning and Currier 1967, Todd and Kamm 1974, Dickerson 1978, J. Range Man- age. 31: 398-399), plant height (Dickerson 1978), seedhead production (Malechek et al. 1977, J. Range Manage. 30: 128-131), and seedhead height (Ansley and McKell 1982, J. Range Manage. 35: 586-590). Forage quality parameters such as crude protein, acid detergent fiber, and cell wall constituents have been determined for paired insecticide treated and untreated control plots (Todd and Kamm 1974). Al- though black grass bug populations were assessed, little effort was made to relate either bug populations or damage rating to yield or quality. Relatively little is known about the relationship between black grass bug populations and plant damage, or the impact of environmental factors on black grass bug, making damaging black grass bug populations difficult to predict. A better understanding of the relationship between black grass bug damage and forage quality would provide much needed manage- ment information and the basis for management guidelines. An opportunity to de- scribe the relationship between black grass bug damage ratings and forage quality was available in 2002. Five pastures of predominantly intermediate wheatgrass with a few alfalfa plants (<1 m 2) were located 26 km east of Townsend, MT, in 2002. Sweep samples col- lected in one pasture averaged 12 adult black grass bugs per sweep; severe stippling of plant foliage was evident. Because bugs were primarily in the adult stage at that time and had already caused substantial damage, plant damage was rated, rather than insect density. One hundred leaf blades were randomly selected from each pasture on 6 July, placed in a paper bag and returned to the laboratory for insect damage rating and forage quality analysis. Leaves were rated for the characteristic black grass bug stippling on a scale of 0-5 (0 = no damage, 1 = up to 20%, 2 21-40%, 3 = 41-60%, 4 = 61-80% and 5 = greater than 80% of the leaf stippled). Leaf samples in each category were maintained separately, dried at 55°C for 72 h, and then ground to pass a 0.5-mm mesh sieve. Percent crude protein, acid detergent fiber (ADF), neutral detergent fiber (NDF), in vitro digestible dry matter (IVDDM), percent sulfur and percent phosphorus were determined using a NIRS System 4500 (Silver Springs, MD) equipped with InfraTech 2 software (NIRS Systems, Silver Springs, MD). ADF represents cellulose, lignin and ash and is inversely related to digestibility. NDF is the total cell wall component and is inversely related to intake. Phosphorus and sulfur are important livestock nutrients obtained from grazing pastures rather than fed hay crops and are standard for forage analysis. Relationships between black grass bug damage rating and forage quality parameters were analyzed using regres- sion analysis (SAS Institute, Inc. 2000, Cary, NC). The maximum black grass bug damage recorded (damage rating = 5) resulted in a 42.4% reduction in crude protein compared with undamaged tissue with a signifi- cant reduction in percent crude protein with increasing black grass bug damage (y = 25.62 - 2.1 x, r2 = 0.38, P= 0.04). Both ADF and NDF were positively correlated with black grass bug damage rating (ADF: y = 25.3 + 1.7x, r2 = 0.77, P = 0.02; NDF: y = 46.1 + 2.8x, r2 = 0.83, P = 0.01). Percent sulfur and phosphorus concentrations were inversely related to black grass bug damage ratings (sulfur: y = 0.24 - 0.02x, r 2 = 0.68, P = 0.05; phosphorus: y = 0.28 - 0.01 x, r2 = 0.64, P = 0.05). There was no significant relationship between black grass bug damage and IVDDM. Black grass bug damage ratings correlated with intermediate wheatgrass forage 94 J. Entomol. Sd. Vol. 41, No. 1(2006)

quality characteristics. There was a significant decrease in percent crude protein and increases in both ADF and NDF as black grass bug damage increased, indicating that black grass bug had an important impact on forage quality. Similar trends were noted by Todd and Kamm (1974) who reported, a 6% increase in cell-wall constituents, a 5% increase in acid-detergent fiber, 2% increase in ash, and 13% decrease in dry weight yield in untreated plots compared with insecticide (dimethoate) treated plots. Black grass bug populations were described, but were related to the forage yield and quality as presence (untreated with pesticide) and absence (pesticide treated). They estimated an overall 2% reduction in forage quality at forage maturity due to black grass bug, although they acknowledged midseason rains stimulated regrowth reduc- ing the impact of black grass bug by the end of their study. They concluded that the impact of black grass bug feeding on rangeland productivity varied with rainfall and time of forage utilization, suggesting that greater losses may be expected in drought years. Malechek et al. (1977) found small differences in forage quality between a black grass bug untreated control and insecticide (malathion) treated plots. Slight increases in crude protein concentration, slight decreases in cellular contents, no effect on yield and 56% decrease in intermediate wheatgrass seedhead production were found at densities of 156 black grass bugs per m2. Our study was conducted during a drought that had begun by mid-2000 and persisted through the months that preceded this study, conditions that increase the impact of black grass bug feeding (Todd and Kamm 1974). We report significant increases in concentrations of acid and neutral detergent fiber with increasing black grass bug damage, indicative of reduced forage digestibility and ruminant intake, consistent with the ranchers observations of forage yield reduction and reduced intake by grazing cattle. Malecheck et al. (1977) stated that stockmen have observed that black grass bug infestations decrease forage palatability although they did not detect a significant depression in intake. Todd and Kamm (1974) noted 18% loss of forage value following black grass bug infestation. We found no significant relation- ship between black grass bug damage and digestibility (IVDDM), in agreement with the sheep digestion-balance trial conducted by Malechek ét al. (1977) in which dif- ferences in daily dry-matter digestibility and dry matter consumption (intake) were not significant.

Acknowledgments

The authors thank Mr. Nelson Wert who brought this situation to our attention and allowed us to collect samples from his ranch.