Experimental Isolation of Oak Host Plants: Effects on Mortality, Survivorship, and Abundances of Leaf‐Mining Insects

We experimentally isolated oak host trees by transplanting them into an agricultural field to test for spatial and temporal differences in a suite of mortality factors and in larval survivorship of a guild of leaf—mining insects. While other mortality factors for leaf miners were unaffected by isolation, parasitism by hymenopteran insects decreased significantly with isolation. Decreased parasitism resulted in higher larval survivorship for leaf miners on isolated oak host trees. However, the increase in larval survivorship on isolated trees was not followed by higher abundances in either subsequent generations within a season or in the following year, because leaf—mining populations on at least small trees are maintained by continual re—immigration. Little or no in situ reproduction and colonization occur on small isolated trees. Larger trees may maintain autochthonous populations of leaf miners. When all oak species were examined together, we found no strong evidence of density compensation in which summed densities of insular faunas equal or exceed mainland densities. Overall population densities on all three isolated oak species did not differ from non—isolated oaks, but number of species was reduced only by one. However, we suggest that other apparent cases of density compensation in herbivorous arthropods may be a consequence of escape from parasitism and/or predation rather than niche expansion as a result of competitive release.