Nine populations of cinnabar moth, introduced to North America as biological control agents of tansy ragwort, were studied for 4 to 6 years. We tested the hypothesis that good quality of the food plant, measured as percent protein, would destabilize the moth populations. A positive correlation occurred between the percent nitrogen in the plants and the coefficient of variation of moth population density. Moths tended to be larger and produce more eggs in areas with food plants of better quality. Larval survival was also correlated with the quality of the food and the fluctuation of the population density. Food plants with high nitrogen levels increase larval survival and moth fecundity and allow the moth populations to periodically overexploit their food supply, thus accentuating population fluctuations. This finding is discussed in relation to other studies of cinnabar moth.
Summary
Weed species in arable ecosystems are adapted to the conditions produced by agricultural management. In order to give preventive and curative guidelines to weed management one needs to predict the composition of the weed vegetation after certain (soil) cultivation measures. Dormancy mechanisms of seeds in the seedbank and the environmental conditions in the furrow before and after cultivation are usually such that the vegetation composition is heterogeneous. Multivariate analyses enable us to describe and classify weed vegetation and to analyse its relationship to environmental factors.
spell will stimulate germination of S. arvensis seeds from early spring to late autumn. The data presented good explanations for the cosmopolitan character and the serious weediness of this species. Its classification as a summer or winter annual is discussed.
Summary: Resume: ZusammenfassungChanges in dormancy of Spergula arvensis seeds were studied during pre-incubation at constant temperatures and under a temperature regime that condensed the annual temperature cycle into 73 days. Each day in the regime represented the mean day and night temperatures and day lengths of 5 successive days of an average year in The Netherlands, Incubation occurred in water or loamy sand, in darkness. Germination of the seeds was tested in water or KNO? over a range of temperature. Seeds were irradiated with saturating doses of red light. In half of the treatments, pre-incubated seeds were dehydrated at the transfer to the conditions of the germination test. Breaking ofdormaney occurred under eonditions of "spring". It did not depend on exposure to low 'winter' temperatures, but was induced by rising "spring' temperatures. Seeds developed secondary dormancy in late "autumn". The expression of the changes in dormancy that were induced during pre-incubation depended on the conditions of the germination test. Light, nitrate and dehydration stimulated germination. The experiments predicted that field emergenee from nitrate-poor soils that have not been dehydrated will be restricted to a short period in autumn, whereas disturbance of nitrate-rich soils followed by a dry Correspondence: C, M, Karssen.
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