Eastern hemlock (Tsuga canadensis; ‘hemlock’) forests are currently threatened by the invasive hemlock woolly adelgid (Adelges tsugae; 'HWA'). Once HWA invades a landscape, its populations increase rapidly and quickly colonize nearby hemlock stands. The ecosystem-level impact of hemlock loss is particularly severe because hemlocks are being replaced by deciduous trees rather than another shade-tolerant native conifer. Another sessile hemipteran, the elongate hemlock scale (Fiorinia externa: ‘EHS’), is a sap-feeding insect that settles on the underside of hemlock needles; at high densities it is capable of damaging hemlock foliage. While both HWA and EHS can negatively affect hemlock health, HWA has a far stronger impact on multiple aspects of hemlock growth and fitness. While field surveys have suggested that HWA can also affect bud break, this hypothesis has never been tested. Although such life-history shifts can substantially affect both population- and community-level processes, we are unaware of any work assessing the effect of sap-feeding insects on tree phenology. We took advantage of an ongoing long-term experiment to assess whether HWA or EHS alters the timing of hemlock bud break. In early spring 2016, we tracked bud break on twenty trees in each of three treatments: HWA only (experimentally infested since 2014), EHS only (experimentally infested since 2014), or uninfested controls. After determining herbivore density on each tree, we conducted daily bud break counts on one previously-marked branch per tree. After completing the daily bud count for each tree, we also took a hemispherical photo above each tree in order to assess canopy closure.
Results/Conclusions
While there was no main effect of insect presence on bud break, this was largely due to low HWA densities in several of the sampled trees. When within-treatment insect density was regressed on bud break, there was a strong effect on HWA density on the timing of first and mean bud break: increasing HWA densities from zero to one insect/cm branch delayed bud break by 25 days. By contrast, there was no within-treatment relationship between EHS density and hemlock phenology. The 25-day phenological delay associated with even moderate HWA densities corresponded with an increase in canopy cover from ~22% to ~38%. Because conifers rely heavily on photosynthetic products produced prior to deciduous canopy leaf-out, this delay could significantly reduce carbon gain and exacerbate the effects of this insect on plant health.