THE IMPORTANCE OF REFUGES IN THE INTERACTION BETWEEN CONTARINIA-SORGHICOLA AND ITS PARASITIC WASP APROSTOCETUS-DIPLOSIDIS

1. The population dynamics and stability properties of the sorghum mid ge Contarinia sorghicola and its predominant parasitic wasp Aprostocet us diplosidis were explored with a modified version of the Nicholson-B ailey model using experimental data, that included age-structure effec ts, density dependence and a type II functional response. 2. A Leslie matrix was used to model the non-linear transitions between age-classe s within the population of each species and the interaction between th ese populations. 3. The stability and persistence of the interaction b etween these two species depended on the presence of refuges resulting from mutual interference of the parasitoids. The parasitoid tends to avoid grains with conspecifics, independently of the number of midges present in each grain, and therefore a fraction of the hosts is always invulnerable. As opposed to a constant number of refuges, the fractio n of invulnerable hosts decreases as host populations become smaller. 4. The presence of unstable equilibria associated with stable ones sug gests that the spatial structure resulting from the patchy distributio n of the natural host-plant may play an important role in the persiste nce of the system. 5. The presence of refuges affected the ability of parasitoids to suppress host densities. When host populations increase d beyond a critical density ('refuge density'), a fraction of the popu lation became invulnerable to parasitism and therefore showed dynamics independent of parasitoids.