Date of Award:

5-1-1997

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Biology

Committee Chair(s)

Frank J. Messina

Committee

Frank J. Messina

Committee

Edward W. Evans

Committee

Edmund D. Brodie Jr.

Abstract

This research investigates plant- and predator-mediated effects of the bird cherry-oat aphid, Rhopalosiphum padi (L.) on the Russian wheat aphid, Diuraphis noxia (Mordvilko). I combined a suite of population-level experiments with behavioral observations to determine whether the two aphids interacted principally through a common host, crested wheatgrass (Agropyron desertorum [Fisher ex Link] Schultes), or a common predator, larvae of the green lacewing (Chrysoperla plorabunda [Fitch]). The population-level experiments revealed some evidence of host-mediated interspecific competition but only at extremely high aphid densities (>2000 individuals per greenhouse seedling). In addition, some of these experiments suggested that the bird cherry-oat aphid positively affects the rate of increase of the Russian wheat aphid in the presence of predators. Experiments examining predator behavior in response to conspecific and heterospecific aphid aggregations revealed that D. noxia is less susceptible to predators when R. padi are also available, but especially when the former species is highly concealed. This decreased susceptibility to predation is one possible explanation for the facilitative effect of R. padi on D. noxia in the presence of predators in the population-level experiments. Alternatively, the numerical dilution of D. noxia by R. padi may have drawn predators away from the former species. Though D. noxia were less susceptible to predation than were R. padi during the behavioral observations, they were not completely immune from predator attack. Differences in susceptibility to predation likely reflect differences in rates at which the two aphid species were encountered rather than differences in accessibility per se. Lacewing larvae were able to locate and consume Russian wheat aphids in concealed feeding locations when no other prey were available. For this reason, lacewing larvae might be good candidates for release against D. noxia. However, the presence of R. padi might impede any generalist predator's ability to control D. noxia, especially over short time scales.

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