The Influence of Larval Dispersal in the Cinnabar Moth (Tyria jacobaeae) on Predation by the Red Wood Ant (Formica polyctena): An Analysis Based on the Proportional Hazards Model

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Abstract
(1) The influence of the spatial distribution in plant populations, and migration activity of the larvae of Tyria jacobaeae on the risk of an attack by Formica polyctena was studied. (2) To this end, larvae were confronted with ants in two experimental set-ups: Experiment I, in artificial plant populations near an ant nest; and Experiment II, on ant tracks. Moments of attack or exit from the observation area were recorded. (3) The analysis of the data was based on the proportional hazards model. The procedure enables us to assess the influence of several variables on the attack risk simultaneously. The model is that all larvae have an equal basic risk of attack which is altered in proportion to the values of the influencing variables. The method can take into account variables that are fixed, e.g. distance of a plant with larvae to an ant nest, as well as time-varying factors, e.g. behaviour of the larvae. Moreover, it takes account of larvae that leave the observation area or remain unattacked during the observation. (4) Of all the studied factors, the behaviour of the larvae had the largest effect on the risk of attack. The attack rate on larvae in plants or grass leaves was less than 0.34 times the attack rate on larvae on the ground. The spatial distribution also had some influence: the risk of attack increased with the mean number of larvae per plant and there are indications that larvae in clumped distributions (on the spatial scale of a plant population) had a lower risk of attack than larvae in regular distributions. (5) The method of analysis can be used in the study of a variety of ecological processes such as, for instance, survival under diverse conditions. We show how the model is made suitable for ecological research and useful generalizations in this regard are given.

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