HIDE OR FIGHT - THE COMPETITIVE EVOLUTION OF CONCEALMENT AND ENCAPSULATION IN PARASITOID-HOST ASSOCIATIONS

I employ a mathematical model integrating the population and co-adapti ve dynamics of an insect host and its specialist parasitoid wasp to in vestigate the competitive evolution of hva forms of host resistance: c oncealment from adult parasitoid location and encapsulation of parasit oid eggs. When only one form of resistance is permitted to evolve, con cealment always evolves to frequencies equal to or higher than encapsu lation. When both forms of resistance evolve, the outcome depends on t he capacity of the parasitoid to evolve counter-measures. Evolution of tile host in the presence of the most virulent parasitoid clone resul ts in no differences between the equilibrium frequencies of the two fo rms of resistance, unless host clone densities are somehow unequally p erturbed. When the parasitoid is allowed to co-evolve, the frequency o f hosts concealing themselves exceeds the frequency of those capable o f encapsulating parasitoid eggs. At equilibrium, there is generally a negative correlation between resistance measures in the host populatio n. and a positive one for countermeasures 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 th e 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 com mon and;or parasitism rates relatively low; that is, when regulation o f the host population tends to be influenced bq other density dependen t forces.