Can there be an escalating arms race without coevolution? Implications from a host-parasitoid simulation

Abstract

Under a restricted set of conditions, predator-prey or parasite-host systems may exhibit an escalating arms race over several generations that is not coevolutionary. Preconditions for such a process include high correlation between prey/host quality and defensive capability, and phenotypic plasticity in predator/parasite-counter defenses that responds to quality. We present simulation models based on the parasitoid waspEurytoma gigantea, which lays its egg in the goldenrod gall induced by the flyEurosta solidaginis. For the parasitoid to successfully lay an egg, the gall walls must be thinner than the parasitoid's ovipositor is long. Wall thickness is highly correlated with gall size, so probability of successful attack declines with gall size. However, since the parasitoid eats the gall tissue, individuals developing in small galls have little food and mature with shorter ovipositors than those which develop in large galls. The simulation showed that the population mean parasitoid size is set by mean gall size. Since small galls are more frequently parasitized, there is a selection pressure on the gallmaker to induce larger galls. But, an additional simulation showed that since parasitoid ovipositor length depends on gall size, an evolutionary increase in gall size will also result in a non-evolutionary increase in parasitoid body size and ovipositor length over several generations.