The role of simulated spring water stress in interactions between eastern larch and larch casebearer

Water stress can influence the ability of plants to tolerate and resist herbivory and indirectly mediate inset fitness. Larch casebearer, Coleophora laricella Hübner (Lepidoptera: Coleophoridae), is an invasive defoliator in North America where it infests eastern larch, Larix laricina (Du Roi) K. Koch (Pinaceae). Anomalous outbreaks of larch casebearer have been detected each year since 2000 in Minnesota, USA. In Minnesota, eastern larch typically occurs in peatland bogs or fens with complex hydrology. Given the potential for global climate change to alter precipitation and the seasonal flooding dynamics of eastern larch stands, we investigated the role of simulated waterlogging and drought on eastern larch–larch casebearer interactions over two years. We quantified the growth, survival, and foliar monoterpene concentrations of juvenile eastern larches in response to varying watering regimens and challenge from larch casebearer. We also quantified how watering regimen and monoterpene concentrations affected the survival of fourth instar larch casebearers to adulthood. The growth and survival of eastern larch was negatively impacted by challenge from larch casebearer, waterlogging, and drought, though the strength of responses varied between years. The monoterpene concentrations in eastern larch foliage did not change in response to water stress or challenge from larch casebearer but the within-year concentrations of several monoterpenes decreased with time. No consistent patterns emerged in the response of larch casebearer to watering regimen or monoterpenes. In summary, it appears that watering stress and defoliation do not interact to impact growth and survival of juvenile eastern larches, but rather act independently.