Effects of browser exclusion on the willow leafblotch miner (Micrurapteryx salicifoliella): host plant availability, oviposition, and survival

Abstract

Herbivores can affect the quality and quantity of their food plants in ways that indirectly influence the food resources and habitat available to other herbivores present. This study examined indirect interactions between a mammalian browser and an insect folivore that feed on shared plant species during different seasons. On a boreal floodplain in interior Alaska, we investigated how a history of winter browsing by moose (Alces alces) affected the behavior and performance of the willow leafblotch miner moth, Micrurapteryx salicifoliella in summer. Excluding moose browsing for 8 years did not change plant density, but strongly increased canopy height, vegetative cover and overstory density. These vegetation changes led to slightly higher relative humidity and lower air and soil temperatures on moose exclosure relative to control plots. Excluding browsers did not alter the foliar quality (leaf area, water content, leaf mass per unit area, nitrogen concentration) of the three focal willow host species examined. Browsing-related effects on willow morphology and canopy structure did not influence patterns of oviposition by M. salicifoliella on host plants. However, larvae feeding within control plots exposed to vertebrate browsing were less likely to survive to pupation than those excluded from browsing, perhaps because larval predation was more frequent on the warmer and more open browsed plots. Both oviposition and subsequent larval survival were strongly affected by host species. Interestingly, the willow host species on which leaf miner larvae survived best did not correspond to that which received the highest egg abundance during oviposition. We conclude that a history of browsing reduced canopy height, cover and overstory density, which in turn affected the performance of the outbreak insect M. salicifoliella by reducing the proportion of larvae to survive to pupation.