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PREDISPERSAL SEED PREDATION OF THE PLATTE THISTLE AND ITS EFFECT ON SEED PRODUCTION
In this study, the impact of three native insects on the population dynamics of a native plant is documented. The three insects, Paracantha culta (Wiedemann), Orellia occidentalis (Snow) (both Diptera: Tephritidae), and Homoeosoma stypticellum Grote (Lepidoptera: Pyralidae), feed on the developing seeds of the Platte thistle, Cirsium canescens Nutt. The biology and ecology of this predispersal seed predation of the Platte thistle is described.^ The Platte thistle is a monocarpic perennial that inhabits the Nebraska Sandhills. From 1977 to 1979, the life history and population dynamics of the thistle were studied. The thistle typically grew in small, scattered groups of plants. Seedling establishment was sparse, mostly because of poor moisture conditions. In plots with dense vegetative cover thistle rosettes had a low mortality rate, but required more time for growth. Seed heads developed during May and June and gained most of their dry weight shortly before they matured. Viable seed production averaged 72, 197, and 303 seeds per plant at three sites. Seed production was greatly affected by plant growth and the destruction of developing seeds.^ The life history, seed destruction, and population density of the three insect seed predators were also studied. Each species oviposited during a different developmental stage of the seed head so the larvae fed at primarily different times. Egg and larval distributions of O. occidentalis were negatively correlated with larval distribution of P. culta at one site. P. culta larvae destroyed the greatest number of seeds per larva, whereas O. occidentalis larvae destroyed the least. The seed predators attacked an average of 97% of the seed heads at a site. The predominant insect species varied between sites.^ Spatial patterns of Platte thistle seed production and the three seed predators were investigated between and within thistle patches at Arapaho Prairie, Nebraska in 1978. The insects infested 97% of the seed heads. The significant differences in seed and insect counts between patches were not generally related to density. Within patches, the number of each insect species per plant increased with plant size, but larger plants still produced more seeds. P. culta was concentrated on the seed heads of large plants in dense clumps. Distribution of the other seed predators among seed heads depended more on P. culta distribution than on plant characteristics. The seed production variance, which was greater than the insect infestation variance, masked the effect of insect spatial patterns on seed production. Seed heads that developed before peak oviposition produced the most seeds. Thus, temporal patterns of Platte thistle flowering influenced seed production and predation more than the spatial patterns of insects.^ The predispersal seed predation of the Platte thistle by the three seed predators was quantified and compared to other causes of seed mortality at three Nebraska sites in 1979. Although non-biotic agents damaged 18-31% of the developing seeds, the three insect species damaged at least 21-54% of the seeds. The predominant insect species varied between sites. Only 9-24% of the potential seeds per plant matured. ^
LAMP, WILLIAM OWEN, "PREDISPERSAL SEED PREDATION OF THE PLATTE THISTLE AND ITS EFFECT ON SEED PRODUCTION" (1980). ETD collection for University of Nebraska - Lincoln. AAI8021346.