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Population density and larval development of range caterpillar (Hemileuca oliviae Cockerell) were influenced by topographic position. Larval populations on ridges and upper slopes were in earlier developmental stages and had lower densities than populations on swales and lower slopes. Plant species composition, vegetative productivity, ground cover, and soil surface temperatures were suggested as habitat factors that might explain these differences. The range caterpillar, Hemileuca oliviae Cockerell, is a major insect pest on approximately 7.7 million ha of shortgrass prairie in New Mexico and adjacent areas of Colorado, Oklahoma, and Texas (Ainsley I9 10, Watts and Everett 1976). Blue grama (Bouteloua gracilis (H.B.K.) Lag. ex Steud.), the dominant forage species of these prairies, appears to be the preferred food source of H. oliviae, although it also feeds on other grasses (Capinera 1978, Wildermuth and Caffrey 19 16). Since 1885 there have been at least four major range caterpillar outbreaks. The most recent outbreak was first noticed around 1960 and has been characterized by population fluctuations due to natural mortality factors and insecticide suppression programs (Watts and Everett 1976). The impact of the range caterpillar on rangeland is expressed in three major ways (Huddleston et al. 1976). (1) The larvae, which grow from 5.0 mm to almost 100 mm in 8 to 12 weeks, consume large amounts of foliage. While feeding, larvae waste additional forage when they cut off parts of leaves without eating them. (2) Grass not eaten by the caterpillars is rendered unpalatable to livestock by the irritating spines on the larvae and on their cast skins, which remain attached to the grass throughout the remainder of the growing season and into the winter. (3) The damaged rangeland is subject to soil erosion by wind and water, which has long-term implications. Watts and Everett (1976) suggested that environmental factors such as soil type, plant productivity, effective precipitation, and ground cover affect population density of range caterpillar. Field researchers conjectured that plant species composition, plant productivity and ground cover are related to topographic position in the rolling terrain of east central New Mexico. Field researchers also observed that there were more range caterpillar larvae in swale and lower slope areas than on adjacent ridges and upper slopes. Additionally, larvae in swale and lower slopes were observed to be in later developmental stages than larvae on upper slopes and ridges. This study was designed to confirm these observations Beavis, Owens, Bellows, and Huddleston are with the Department of Entomology and Plant Pathology, New Mexico State University, Las Cruces, New Mexico 88003; Ortiz is with the Department of Experimental Statistics and Ludwig with theDepartment of Biology of the same university. This article was submitted as Journal Article 774, New Mexico Agricultural Experiment Station, Las Cruces, New Mexico. It reports on work supported in part by USDA/SEA/CR Research Grant 801-15-35. Manuscript received June 4, 1980. JOURNAL OF RANGE MANAGEMENT 34(5), September 1981 relating vegetation and range caterpillar populations to topography.

Density and Developmental Stage of Range Caterpillar Hemileuca oliviae Cockerell, as Affected by Topographic Position