Résumé
I employ a mathematical model integrating the population and co-adaptive dynamics of an insect host and its specialist parasitoiod wasp to investigate the competitive evolution of two forms of host resistance: concealment from adult parasitoid location and encapsulation of parasitoid eggs. When only one form of resistance is permitted to evolve, concealment always evolves to frequencies equal to or higher than encapsulation. When both forms of resistance evolve, the outcome depends on the capacity of the parasitoid to evolve counter-measures. Evolution of the host in the presence of the most virulent parasitoid clone results in no differences between the equilibrium frequencies of the two forms of resistance, unless host clone densities are somehow unequally perturbed. When the parasitoid is allowed to co-evolve, the frequency of hosts concealing themselves exceeds the frequency of those capable of encapsulating parasitoid eggs. At equilibrium, there is generally a negative correlation between resistance measures in the host population, and a positive one for counter-measures in the parasitoid population. The mechanism driving the asymmetries between the two forms of host defence is the loss of more reproductive effort by parasitoids when the host encapsulates as compared to when an encounter is missed due to concealment. I predict that hosts should pay greater costs to maintain concealment as compared to encapsulation when they are relatively common and/or parasitism rates relatively low; that is, when regulation of the host population tends to be influenced by other density dependent forces.