The role of interference competition in a sustained population outbreak of the aspen leaf miner in Alaska

Direct density-dependence through intraspecific competition may be an important mechanism permitting sustained herbivore outbreaks. In theory, interference competition could allow a relatively stable number of herbivore individuals to survive while moderating host plant damage. This research examined the potential role of intraspecific competition in permitting a decadelong outbreak of the aspen leaf miner, Phyllocnistis populiella, on Populus tremuloides in interior Alaska. A combination of observational and experimental studies examined larval food requirements, food resources, and the impacts of P. populiella larval density on survival, mass, and leaf mining damage. These results were then compared to those from nine years of survey data examining the density of eggs and pupal chambers, as well as leaf mining damage. The number of P. populiella eggs per leaf surface often exceeded the number that could be supported through larval development. Consistent with the expectations of interference competition, the probability of larval survival displayed a decelerating decline with increasing density. Pupal mass of surviving individuals was not related to larval density suggesting little impact of exploitative competition. Mean percent of leaf area mined saturated between 65 and 75%. Taken together these results suggest that strong interference competition largely precludes exploitative competition in P. populiella larvae thereby allowing some individuals to survive and attain normal pupal size even when densities far surpass the carrying capacity of the resource. Interference competition also limits host plant damage thereby contributing to the preservation of a healthy resource base. By constraining both larval survival and host plant damage, interference competition may foster the maintenance of sustained outbreaks of P. populiella.