Event-driven competing risks

The California red scale Aonidiella aurantii (Maskell) (Homoptera: Diaspididae) is a major pest of California citrus, with infestations causing growers significant financial losses. It has recently developed resistance to traditional insecticide sprays. An alternative suppression tactic is the release of a biological control agent, Aphytis melinus DeBach (Aphelinidae: Hymenoptera) that feeds on red scale. Although many aspects of the red scale–Aphytis interaction are now understood, it is difficult to differentiate the effects of temperature and population fluctuations in the field. To investigate such complex interactions, we propose a new stochastic modeling technique, based on event-driven competing risks, that incorporates details of life histories as well as the host–parasitoid interaction. Our continuous-time, individual-oriented modeling approach quantifies relationships among individuals and describes the resulting coupling between the interacting populations. The event-structured simulation drives time in contrast to the usual time-driven stochastic dynamic programming. Our system, developed in the public domain using the R statistical package, allows for different biological clocks, since both red scale and Aphytis development respond to temperature (degree-days) while searching female Aphytis follow a diurnal time schedule, contingent upon temperature-dependent egg maturation.