Potential for Self-Defeating Biological Control? Variation in Herbivore Vulnerability Among Invasive Spartina Genotypes

Invasive species can experience strong selection in their new environments. Some populations of invasive Spartina spp. cordgrass in Pacific estuaries have been separated from the specialist planthopper Prokelisia marginata for many generations while virtually no native populations, in estuaries of the Atlantic and Gulf coasts of North America, have experienced this separation. Contemplating the implications for biological control, we compared native cordgrass populations for resistance and tolerance to the planthopper with invasive ones that have been separated from the herbivore from many generations. We found that plant genotypes varied more in their ability to resist and support planthoppers in a population that had been separated from the herbivore for many generations (in Willapa Bay, Washington, USA) than in one that had been consistently exposed to the herbivore (in San Francisco Bay, California, USA). In the former, some plant genotypes experienced !50% shoot mortality while others experienced none. In contrast, in the latter no genotype experienced !20% shoot mortality. Population growth rates of the herbivore paralleled this pattern among plant genotypes from the two populations. Extending previous observations, we found generally lower resistance and tolerance among six (nonnative) cordgrass populations that have been long-separated from the planthopper than in six (native) cordgrass populations that had been consistently exposed to it. One Willapa Bay genotype of S. alterniflora was quite tolerant of the planthopper while lacking resistance; the high densities of the insect that grew on this genotype did it virtually no harm. Plant genotypes with this combination of traits are insidious to biological control. Initially they could contribute to control by fostering herbivores that harm vulnerable genotypes. However, as these tolerant plant genotypes increase in frequency, the effectiveness of biological control would decrease. Prokelisia marginata has recently been released in Willapa Bay for biological control of S. alterniflora. Our findings of tolerant but nonresistant genetic cordgrass variants suggest the need for complementary chemical and/ or mechanical control of less vulnerable genotypes. Attention to the frequency and nature of genetic variation in vulnerability to natural enemies in target species is germane to both the science and the practice of biological control.