The cinnabar moth, Tyria jacobaeae (L.) (Lepidoptera:Arctiidae), was released in 1959 to control the grassland weed tansy ragwort, Senecio jacobaea L. (Asteraceae), despite evidence that caterpillars of this species can feed on native plants within the genera Senecio and Packera. Previous studies confirmed the moth's ability to develop on the native Senecio triangularis Hook., although no systematic study has been conducted to determine the extent of non-target impact on all potential host species. To address the lack of systematic studies we conducted a regional survey to determine the consequences of exposure of non-target plants to cinnabar moth caterpillars. We also conducted a local field experiment to determine the influence of habitat on the patterns of association of the moth and non-target plants. In the regional survey, we mapped the potential distribution of the cinnabar moth in Oregon to determine the extent of exposure of native Senecio and Packera species, and systematically sampled exposed species to assess the frequency and severity of feeding on these plants. We found that nine of the 20 native non-target species in Oregon were exposed to the cinnabar moth, three of the 10 native Senecio and six of the 10 native Packera. Ten of the native species escaped exposure because they occur east of the Cascade Mountain Range where the cinnabar moth does not occur. We found feeding damage on three of the nine exposed species: Packera cymbalarioides, P. pseudaurea, and S. triangularis were attacked at one of three (33%), two of six (33%), and seven of 15 (47%) sites that supported populations of each species, respectively. Within sites, attack frequency of stems was 33% (of six total stems sampled) for P. cymbalarioides, and ranged from 53% to 56% (of 20 to 108 total stems sampled) for P. pseudaurea and 7% to 64.5% (of 32 to 458 total stems sampled) for S. triangularis. Conditional median damage per site (median of attacked stems only) was 10% in P. cymbalarioides, 5% to 17.5% in P. pseudaurea, and 5% to 37.5% in S. triangularis. The attack rate on non-target plants (7.1 to 64.5 percent of stems attacked at a singe site) was equal to or greater than on the target weed (8.3 to 50.0 percent of stems attacked at a single site). At three sites, caterpillars attacked non-target plants but the target weed was absent, and at one site, the target was present but caterpillars fed on non-target plants only. We conclude that attack frequency and severity on the three species is not high, but equaled or exceeded the level of attack on the target weed. We also conducted a mark-release-recapture experiment to relate habitat preference to patterns of non-target host use in the field. We compared adult moth dispersal patterns and larval development between a meadow habitat and a forest habitat. We found that long-term dispersal distance (spanning days) was similar in both habitats but we recaptured a higher percentage of moths from the meadow (47%) compared to the forest (10%). Short-term displacements, based on direct observations of flights immediately after release, differed between habitats: moths in the meadow flew short distances (8.5m ± 1.5, n=13) at or below the herbaceous canopy (0.8 m ± 0.2, n=13) while moths in the forest flew longer horizontal (22.8 m ± 2.8, n=15) and vertical distances (5.9 m ± 0.9, n=15). We recovered seven fifth instar larvae (of 278 eggs) from the meadow habitat but no larvae beyond the second instar (of 119 eggs) were recovered from the forest habitat. We conclude that the cinnabar moth is limited to meadow habitats because adult moths display movement patterns that remove them from forest habitats (possibly due to disorientation) and larvae are unable to survive on plants growing in the forest. Taken together, the regional survey and the local field-experiment indicate that the cinnabar moth uses only a small proportion of available non-target host plant species. Other species are likely unused because of geographic isolation from the moth, habitat selection by the moth, or phenological differences between the moth and non-target plants. / Graduation date: 2003