The dream team: Native and invasive pests team up to increase tree mortality due to climate change.
Written by Suvi Birch, edited by Abby Guinan
Summary
Both native and invasive insects cause defoliation and potentially tree mortality. Climate change can alter pest range and phenology, leading to increasing numbers of insect outbreaks. With multiple insect pests affecting some tree species in addition to climate stress, there is concern that repeated defoliation events could lead to a forest death spiral (see our Forest Pest Management Challenge). Ward & Aukema (2019) use spatial data and aerial surveys in forests in Minnesota, USA to assess how climate affects outbreaks of invasive larch casebearer (Coleophora laricella) and native eastern larch beetle (Dendroctonus simplex) on tamarack (Larix laricina) trees. Both insect pests had an advantage with winter warming, which likely enables greater survival over the winter. Larch casebearer outbreaks were more likely when spring was wet, while eastern larch beetle outbreaks were more likely during extended drought. Ward & Aukema found that trees were more vulnerable to mortality by the eastern larch beetle if they had been previously defoliated by larch casebearer. Thus, repeated defoliation by two insect pests could lead to rapid larch mortality.
Take home points
Even though the larch casebearer had been present in Minnesota for several decades, it had not shown signs of defoliation until 2000, which may be due to warming temperatures enabling larval overwintering.
Eastern larch beetle and larch casebearer interact to cause high rates of tree mortality. Defoliation by larch casebearer weakens tree defenses, making it easier for eastern larch beetle to colonize surviving trees and leading to high tree mortality.
Management implications
Beetle outbreaks are more likely if other outbreaks have occurred recently or nearby (within ~4 km and 2 years). Land managers should allocate extra resources to beetle detection and prevention near recent outbreaks.
Warmer winter temperatures are associated with reduced larval mortality and range shifts in defoliating insects. Managers should check for infestations in years when early winter months are particularly warm.
Keywords
Forest pest, Risk assessment, Competitiveness, Shifting seasons, Observation, Biotic disturbance, Coleophora laricella, Dendroctonus simplex