Cinnabar Moth and its Food Plant, Ragwort: Further Analysis of a Simple Interaction Model

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Abstract
A model of the interaction between a cinnabar moth population and its food plant, ragwort, developed by Dempster and Lakhani (1979), is further analyzed by studying the effects of changing various parameters on the model populations and by testing the model with data from localities other than Weeting Heath. In the model, the ragwort increases with increasing rainfall, but the moth numbers cannot increase indefinitely because of density-dependent dispersal. The observed mean rainfall at Weeting Heath [England, UK] (1966-1977) is just below that required to maintain the ragwort population. Variation in rainfall may be an advantage enabling the plant to multiply during a good year, thus increasing its chance of surviving periods of low rainfall. The cinnabar moth numbers in the model are robust to variations in egg mortality and potential fecundity, but sensitive to changes in larval and pupal survival. Since survival is food-limited, the fluctuations in the moth numbers are dominated by the pattern of rainfall. The close relationship between food (rainfall) and moth numbers is further demonstrated by studying the response of the moth population to arbitrary levels of food for the larvae. Results are presented to show the extent by which the Weeting interaction model appears applicable to cinnabar/ragwort populations at 2 other sites. The relationship between plant numbers and rainfall at Nanaimo [British Columbia] Canada) is similar to that at Weeting, perhaps because this site is similar to Weeting in having well-drained soil and a marked summer drought. There is no marked relationship between rainfall and plant numbers at the 2nd site near Jordan [Oregon] (USA) where the soil is heavier and the rainfall much higher. Food availability appears to be the main factor determining moth abundance in the model, and at all 3 sites reasonable predictions of trends in the moth populations are obtainable from changes in the ragwort biomass.

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