Herbivorous insects are often locally adapted to their host plants due to direct selection by the host plant itself or indirectly by the environment in which that host plant is found. This fitness advantage can be nullified by high levels of gene flow or potentially reversed by a change in the coevolutionary arms race between plant and herbivore. While local maladaptation in insect herbivores has been found to be rare, herbivorous insects in primary successional environments may be an exception. Extinction and recolonization dynamics are common after catastrophic disturbances. In these primary successional landscapes, frequent colonization and host plant shifts by herbivores to pioneer plant species may temporarily cause local maladaptation. We examined whether the aphid Macrosiphum albifrons that feeds on Lupinus lepidus var. lobbii, the most important colonizing plant on the Mount St. Helens (MSH) primary successional Pumice Plain, was locally adapted to its host. In a greenhouse experiment, aphid adults from MSH were placed on L. lepidus from the Pumice Plain and Olympic National Park. In a field experiment, aphids from MSH and two more distant source populations, Mount Hood and Santiam Pass, were placed on L. lepidus on the Pumice Plain. Number of aphid progeny were counted after 17 days as a measure of fitness.
Results/Conclusions

In the greenhouse experiment, L. lepidus source population significantly affected MSH aphid fitness in terms of progeny per adult, with MSH aphids having fewer progeny on MSH plants than on plants from Olympic National Park. In the field experiment, we found a similar pattern, with aphids from MSH having significantly fewer progeny than aphids from Mount Hood and Santiam Pass when grown on MSH plants. These results suggest that aphids are locally maladapted to L. lepidus on the Pumice Plain. In an additional experiment, L. lepidus plants from the Pumice Plain were grown in the lab and fertilized with nitrogen, phosphorous (P), or not at all. MSH aphid adults were placed on these plants and progeny per adult was measured after 17 days. Aphids performed significantly better on L. lepidus that had been fertilized with P, suggesting that MSH aphids may be P-limited.