Perhaps the most frequent biotic constraint imposed on plants in a new range has been attack by pathogens. There are spectacular examples in which a nonindigenous plant species has been destroyed by an indigenous parasite. In such cases, establishment or naturalization is out of the question. The fate of cacao in West Africa is illustrative. Cacao (Theobroma cacao) is native to the Amazon Basin. It is successfully cultivated in West Africa but only if scrupulously protected from the cacao swollen-shoot virus (CSSV), an indigenous virus in West Africa that attacks native West African relatives of cacao. A native scale insect serves as the vector of the virus. Cacao, which has no natural resistance in cacao to CCSV, can be cultivated in West Africa only as long as movement of the scale insect from tree to tree is diligently prevented and a quarantine and destruction protocol for infected trees is rigidly enforced (Jeger and Thresh 1993). A further example of host range extension of an endemic pathogen to a nonindigenous plant species is now occurring in the United States. Multiflora rose (Rosa multiflora) was advancing unchecked in its range expansion across the United States from east to west until it came into contact with native roses infected by rose rosette disease (RRD). This endemic disease is caused by a yet uncharacterized agent that is transmitted by a native mite (Phyllocoptes fructiphylus). Because RRD is highly pathogenic to multiflora rose, the incidence and impact of multiflora rose is diminishing in many midwestern and eastern states (Epstein and Hill 1999).

Those examples involve spectacular action by resident pathogens, but we do not know how often such phenomena occur. Some generalist parasites thwart establishment of nonindigenous species. Texas root rot fungus, Phymatrotrichum omnivorum, is a generalist soil parasite that has remained indigenous in the U.S. Southwest and adjacent Mexico. It attacks at least 2000 plant taxa in more than 40 families and is highly virulent in many of these hosts. The parasite attacks so many nonindigenous woody ornamentals that it is a major deterrent to the introduction of woody horticultural species in the Southwest. Some species, such as Ulmus americana, will not persist in the range of Texas root rot, because of their vulnerability to it (Mack 2002).

Hypothetically, establishment of nonindigenous insects could be affected by interference or exploitation competition with resident species. In interference competition, one species reduces the fitness of another through an action, such as fighting or allelopathy, that is not directly related to resource availability or abundance. Exploitation competition occurs when the rate of resource availability or supply determines the rate of change among populations of different species. Resource availability to the immigrants will depend on the standing crop of the resource (which is a function of the feeding behavior of resident competitors), the productivity of the resource, and the rate at which the resource is removed by the