Date of Award:

5-1-1979

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Biology

Department name when degree awarded

Life Sciences:Biology

Committee Chair(s)

Ivan G. Palmblad

Committee

Ivan G. Palmblad

Committee

Ben Norton

Committee

James MacMahon

Abstract

Labops hesperius Uhler abundance and ground dispersal in relation to vegetation were studied on three semiarid range sites in central Utah during 1977 and 1978. The objectives of this study were to: 1) verify quantitatively a common observation that L. hesperius populations reach infestation levels within reseeded range but not within native or unimproved range; 2) investigate activity levels of potential arthropod predators to determine if predation might account for the observed differences in L. hesperius abundance; and 3) examine the role which various vegetation components play in shaping this distribution of L. hesperius. Sampling with a modified "D-Vac" technique demonstrated that L. hesperius abundance within the reseeded range dramatically exceeded that within unimproved range. This trend was consistent regardless of vegetation which dominated the unimproved range. Labops hesperius population decline on Site 1, Sterling Ranch, Utah Co., Utah, between 1977 and 1978 is attributed to extensive egg mortality caused by excessively cold temperatures and an inadequate depth of insulating snow. To investigate the ground activity levels of potential arthropod predators, pitfall traps were maintained in different vegetation areas of Site I during the entire L. hesperius lifespan. Carabids and spiders, which constituted the bulk of predators captured, failed to reveal any consistent trends with respect to vegetation. It is tentatively concluded that predation is not the primary reason for the observed distribution of L. hesperius. Sweep net samples of L. hesperius on Site II, Mattson Ranch, Sevier Co., Utah, revealed an edge effect during 1978. Labops hesperius abundance within the reseeded range increased with distance from the boundary with unimproved range, regardless of the dominant vegetation within the unimproved range. Possible explanations for this edge effect are discussed. Significant, positive correlations between grass biomass and L. hesperius abundance on Sites II and III, Wales Canyon, Juab Co., Utah, demonstrate that L. hesperius populations respond to the distribution of food resources, and suggest that grass biomass levels are a major factor determining population levels during infestations. Male L. hesperius exhibit a stronger response to this resource than females. In general, L. hesperius populations fail to reveal any relationship with debris biomass and forb biomass. Female L. hesperius demonstrate a strong positive relationship with debris biomass which is attributed to ovipositional activity. To monitor ground dispersal with respect to vegetation, two dispersal sites were located on the boundary between reseeded and sagebrush-dominated range. Two additional dispersal sites within each vegetation type served as controls. Approximately 6000 bugs were released at each site. Ground dispersal of L. hesperius, as monitored by pitfall captures, is much more rapid and extended than suggested by previous studies and observations. Male L. hesperius disperse independently of females, and exhibit greater activity levels and less net displacement from the release point than do females. Dispersal generally fails to follow random dispersal, consistent direction, and food resource concentration models. On those sites with big sagebrush (Artemisia tridentata Nutt.) pitfall captures demonstrate a strong negative spatial correlation with the distribution of big sagebrush. On boundary sites, L. hesperius actively move away from the big sagebrush dominated portion. Thus, big sagebrush functions as a repellent plant for L. hesperius. The possible nature of this repellent effect is discussed. Reduced apparency of grasses resulting from this associational resistance with big sagebrush is concluded to have a major effect on L. hesperius distribution. The implications of these findings with respect to the emergence of L. hesperius as a pest species on reseeded range and current reseeding practices are discussed. Ideas for future research are presented.

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