Trophic interactions and population persistence in the cherry fruit fly (Rhagoletis cingulata) and its natural enemy (Diachasma ferrugineum) in urban versus agricultural landcover

Urban and agricultural intensification represent extremes of human land-use with profound impacts on spatial distributions and movements of insect populations. Sensitivity to human-induced landcover change is expected to be greater when herbivores and their natural enemies are strict habitat specialists. We sampled the cherry fruit fly (Rhagoletis cingulata) and its associated larval parasitoid (Diachasma ferrugineum), both specialists, via sampling of black cherry (Prunus serotina) host fruits across a 360 km² Eastern Iowa landscape for three consecutive years. Results from our first year of data showed that, despite significantly more abundant urban cherry trees, urban cherry fruit fly densities were depressed compared to those in agricultural settings and the parasitoid was absent from urban sites. We quantified insect spatial distributions across years to test the predictions that, compared to agricultural landcover, urbanization will result in 1) consistently weaker associations between trophic levels, 2) decreased population persistence in the herbivore, and 3) continued parasitoid absence. We quantified herbivore and natural enemy densities on hosts, correlations between trophic levels, and colonization/ extinction rates. We found that urban cherry fruit fly populations were significantly less strongly correlated with host plant density in each season and less likely to persist on sites with fruit resources between years than their agricultural counterparts. Strikingly, their natural enemies were present in agricultural areas but absent on urban sites across seasons. Our results indicate that urban and agricultural landcover result in distinct spatial population structure for the herbivore with important consequences for community assembly at the parasitoid level.