George B. Hewitt

Response of needle-and-thread and western wheatgrass to defoliation by grasshoppers.

Field and greenhouse studies were conducted to. measure changes in plant growth resulting from grasshopper defoliation. All data indicated that as grasshopper grazing intensity on needleand-thread grass increased, total root weight decreased. A greenhouse study with western wheatgrass showed that heavy grazing (80% removal of top growth) for a 16-day period reduced top growth 82%, root growth 85%, crown growth 81%, rhizome growth 180%, and depth of root penetration 4%. Field obserrations indicated that most grasshopper defoliation of needle-andthread grass and western wheatgrass occurs after seasonal growth has been completed. Most plants that benefit mankind also provide food and shelter for many species of insects. Grasshoppers have long been considered the major pests inhabiting rangeland throughout the western United States and Canada, and since they feed on most forage plants, especially grasses, they have the potential to increase to outbreak numbers in any year and at many locations (Hewitt 1977). Species distribution, biology, food and habitat preferences, and economic importance have been determined for most grasshopper species inhabiting western rangeland. However, very little is known about the effects of insect defoliation. The effects of clipping or grazing plants with livestock have been studied for several plant species, but the effects of grasshopper grazing have never been adequately documented. Grasshoppers lower the production of rangeland forage bydefohating plants, part of which is consumed and part falls to the ground as litter. The effect of this grazing or defoliation upon plant development depends upon the intensity, frequency, selectivity, and season of use. In this study, we measured changes in plant growth resulting from grasshopper defoliation. The development, vigor, and survival of needle-and-thread grass (Stipa comata Trin. & Rupr.) and western wheatgrass (Agropyron smithii Rydb.) were measured under different grasshopper density infestations in both the greenhouse and the field.

Forage losses caused by the grasshopper Aulocara elliotti on shortgrass rangeland.

Highlight: A field-cage study was conducted in 1973 and 1974 to determine the amount of forage (mainly grasses) destroyed by different population densities of the grasshopper Aulocara elliotti. The amount offorage consumed during the third instar and through the adult stage averaged 34.5 mg offorage per grasshopper per day. Thus, an estimated loss of 23.1 lb offorage per acre will resultfrom a density of one Aulocara/m2 if the grasshopper lives for 75 days (45 days as a nymph and 30 days as an adult). Based on total available forage (standing dead and new growth), a 63% forage loss was recorded in 1973 at one site and losses of 26% and 29% at two sites in 1974 resulting from about 20 grasshoppers/iM2. Severe grazing by grasshoppers also resulted in reduced production offorage during the subsequent (1974) season.

Plant phenology as a guide in timing grasshopper control efforts on Montana rangeland.

The flowering of 28 forb species at two locations was correlated with grasshopper development in 1977 and 1978. Indicator plants whose flowering phenologv was associated with grasshopper hatching included: Zygadenus elegans, A llium textile, Delphinium bicolor, Oxytropis sericea, Erysimum asperum, Leucocrinum montanum, and Astragalus gilviflorus. The ideal time for controlling grasshoppers (when most of the population is in the 3rd instar) was associated with the flowering phenology of the following indicator plants: Yucca glauca, Helinathus petiolaris, Opuntia polyacantha, Sphaeralcea coccinea, Antennaria dimorpha, Tragopogon dubius, Cryptanthe celosioides, A ilium textile, Delphinum bicolor, Zygadenus elegans, and Erysimum asperum. cut for control of the alfalfa weevil (Hyperapostica), and to Plant growth and development on the northern Great Plains is governed by environmental variables such as soil temperature, determine the best time to begin spring grazing on native air temperature (heat accumulation), soil fertility, soil moisture, and photoperiod. In addition, site rangeland. characteristics, such as slope and exposure, may affect growth and time of flowering of some species of prairie This paper reports on another use of plant phenology plants. Grasshopper hatching and development too are regulated to a large extent by environmental factors. Thus soil temperature and moisture appear to influence grasshopper hatching; air temperature (heat accumulation) has great influence on the rate of grasshopper development (Hewitt 1978, Parker 1930); cool rainy weather prolongs grasshopper development; and hot dry weather causes a reduction in duration of nymphal stages. Thus both plant and grasshopper phenology are regulated to a large degree by seasonal weather patterns in the grasshopper ecosystem. Phenological observations have been applied to agriculture for a variety of purposes (Hyder and Sneva 1955; Caprio 1966): to aid in selecting crops for specific locations, to determine the best time to spray herbicides on big sagebrush (Artemisia tridentata), to determine when alfalfa should be mixed prairie of Montana. Methods and Procedures Seven observation sites, numbered I through 7, were originally chosen to record plant and grasshopper development; however only two sites, numbers 6 and 7, are reported on since grasshoppers didn’t develop in significant numbers on five of the sites. Site #6 (elevation-l,058 m) was located 40 km N. of Billings, Mont. The observation area was about 0.5 ha in size and sloped gently to the east. The most abundant plant species were needleandthread (Stipa comata) and western wheatgrass (Agropyron smithii). Site #7 (elevation-l ,217 m) was located 3.2 km N. of Reedpoint, Mont. The observation area was about 1 ha in size and sloped westward. This site contained many species of forbs and grasses and no one species appeared to be the most abundant. About 36air km separated the sites. Phenological observations were made weekly, or sometimes more often, during the spring and summer months in 1977 and 1978 at each site from April 20 to June 20, 1977 and from April 25 to July 7 in 1978. Plant development was recorded as pre-bloom stage, beginning to bloom, peak bloom, last stages of blooming, and post-bloom stage. The ground was carefully scrutinized for newly hatched grasshoppers and collections were made with a sweep net once hatching was observed. The number of individuals within each instar of each species was recorded. Plant phenology was correlated with grasshopper hatching and with the time when most of the predominant species of grasshopper were in the 3rd instar and hatching was mostly completed, that is, with the time recommended for control. Results and Discussion the forbs flowered in May and June during the nymphal The flowering time was recorded for 28 species of forbs, Table 1, only 10 of which were common to both sites. Some forbs such as Phlox hoodii, Lomatium spp., and dandelion period of grasshopper development. Hatching began at both (Taraxacum spp.) were in bloom in early April before any grasshoppers hatched at either location. However, most of sites on May 8 in 1977 and on May 20 in 1978. When hatching began in 1977, one plant species was in bloom at which should benefit both ranchers and land managers within State and Federal Governments who make decisions on the timing of grasshopper control programs. The time of grasshopper hatching and the time when controls should be applied are related to the flowering of common forbs on the The author is with the Rangeland Insect Laboratory, Agriculture Research, Science and Education Administration, U.S. Dep. Agr., Bozeman, Montana 59717. The author wishes to express particular thanks to Dr. John H. Rumely, Montana State University, Bozeman, Montana for identifying many of the plant species listed. No manuscript received March 20, 1979. site A and two at site B. In 1978, four plant species were in bloom at site A when hatching began and 11 at site B. Fourteen indicator plants, that is, those whose flowering coincides with the time of grasshopper hatching or the recommended time of control, were selected on the basis of three criteria: (1) they were abundant and widespread throughout the mixed prairie of Montana; (2) at least some species were familiar to ranchers, extension personnel, and land managers who make decisions about grasshopper control; and (3) flowering during both years at each site JOURNAL OF RANGE MANAGEMENT 33(4), July 1980 297 Table 1. Observations on plant phenology at two sites in central Montana, 1977-1978.‘

Control of grasshoppers on rangeland in the United States--a perspective.

Asteraceae (Xompositae) *Antennaria dimorpha (Nutt.) T.& G. Aster cunescens Pursh Erigeron ochroleucus Nutt. * Helianthus petiolaris Nutt. Hymenoxys acaulis (Pursh) Parker Microseris cuspidata (Pursh) Schultz-Bix Senecio cunus Hook. * Tragopogon dubius Boraginaceae *Cryptanthe celosioides (Eastw.) Payson Cactaceae *Qvuntia polycantha Haw. Cruciferae *Erysimum asperum (Nutt.) DC. Lesquerella alpina (Nutt.) Wats. Sisymbrium altissimum L. Fabaceae (=Leguminosae) *Astragalus gilviflorus Sheld. Astragalus missouriensis Nutt. Oxytropis sericea Nutt. Psoralea esculenta Pursh Vicia americana Muhl. Liliaceae *Ailium textile Nels. & Macbr. Calochortus nuttallii Torr. *L.eucocrinum montanum Nutt *Yucca glauca Nutt. * Zygadenus eleians Pursh Linacea Linum perenne L. Malvaceae *Sphaeralcea coccinea (Pursh) Rydb. Ranunculacea *Delphinium bicolor Nutt. Scrophulariaceae Penstemon albidus Nutt. Violaceae Viola nuttallii Pursh 1977 Site No. 6 Site No. 7 1978 1977 1978 Plant species End No. of End No. of End No. of End No. of Beg. of days in Beg. of days in Beg. of days in Beg. of days in bloom bloom bloom bloom bloom bloom bloom bloom bloom bloom bloom bloom