Plant nitrogen and fluctuations of insect populations: A test with the cinnabar moth-tansy ragwort system.

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.