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Predicting Invasions of Nonindigenous Plants and Plant Pests
is known quantitatively about many of these interactions that using them in a predictive manner remains difficult.
Among the factors that influence an immigrant’s transition from persistence to invasion are the same abiotic and biotic forces that are faced by organisms during establishment. For example, the establishment of nonindigenous arthropods has often been attributed to their escape from natural enemies in their native habitat. However, knowing whether resident enemies in a newly colonized habitat will attack a nonindigenous arthropod and limit its demographic growth or spread is important if we wish to predict which immigrants will not only establish, but also proliferate and spread.
Cornell and Hawkins (1995) and Hawkins et al. (1997) compiled life tables for herbivorous insects and examined patterns in mortality caused by natural enemies to determine whether established nonindigenous species sustained lower mortality from resident natural enemies than did natives. In the first report, Cornell and Hawkins (1995) found that the invasion status of the herbivores was only weakly related to the mortality caused by natural enemies. Sources of mortality differed most strongly between early-stage and late-stage herbivore larvae and between insects that feed inside plants (endophytic) and insects that feed externally (exophytic). Hawkins et al. (1997) used slightly different methods to differentiate among causes of mortality in 63 resident and 20 invading species. Nonindigenous insects did not sustain lower overall mortality than natives from resident natural enemies, but larvae and pupae of nonindigenous species experienced more predation mortality and perhaps more pathogen mortality than natives, although differences were not statistically significant. It must be noted, however, that the analysis relied on data collected in life-table studies of economically important insect species; this suggests that the nonindigenous species included in the analysis had successfully made the transition from establishment to proliferation and spread. Knowing whether interactions between nonindigenous insects and native natural enemies are similar earlier in the invasion process will require additional research.
Evidence from existing studies, however, could be useful in developing testable predictions about the effects of native natural enemies on nonindigenous insects that have become established in the United States. For example, we might expect that nonindigenous insects that feed externally on plant foliage, where they are exposed, will be more likely to acquire a complement of native predators than insects with more protected or specialized feeding behaviors. We might also predict that native parasitoids that are habitat-specific, rather than host-specific, could eventually become an important cause of mortality among nonindigenous insects, especially those confined within leaf or phloem tissue. In a review of cases involving insects imported for weed control, endophytic herbivores were the group most likely to experience mortality from native parasitoids (Goeden and Louda 1976). Native endophytic leaf-miners sustained the highest mortality from parasitoids in the analysis by Hawkins et al. (1997). Bright (1996) noted