Insect and disease outbreaks have long influenced vegetation within Northeastern forests; prior to European settlement, native species including the spruce budworm and forest tent caterpillar periodically led to extensive defoliation. These species continue to impact forests in the Northeast, however, insect and disease outbreaks have recently occurred at an increasing frequency with the introduction and establishment of nonnative insects and disease agents. Forest pests and pathogens are generally expected to become more damaging as the climate changes, because they are able to adapt more rapidly to new climatic conditions, migrate more quickly to suitable habitat, and reproduce at faster rates than host tree species (Ryan and Vose 2012, Weed et al. 2013).

Some of the non-native insect pests of greatest concern within Northeastern forests include the Lymantria dispar (gypsy moth), the hemlock woolly-adelgid, the emerald ash borer, the Asian-longhorned beetle, and the southern pine-beetle. Outbreaks of the nonnative gypsy moth have caused mortality in oak forests across the Northeast since the early 1900s. Recent research has demonstrated that gypsy moth defoliation is contributing to oak mortality across nearly all diameter classes, with mortalities being highest among younger, smaller trees (Morin & Liebhold 2015). The range of the hemlock woolly adelgid is generally limited by cold winter temperatures with mortality occurring at temperatures below –20 °F (–29 °C), however, less severe winters may increase hemlock woolly adelgid survival and spread (Dukes et al. 2009, Paradis et al. 2008, Trotter & Talbot 2013). The emerald ash borer has expanded its range within Northeastern forests, but the impacts of climate change on emerald ash borer populations are uncertain and dependent on several dynamics. Extreme variations in temperature have been demonstrated to limit its fecundity and survival, but may also hinder our ability to control populations through introduction of parasitoids (Wetherington et al. 2017). Southern pine beetle has also been detected in Connecticut, Massachusetts, and Rhode Island (Dodds et al. 2018), and winter temperatures by 2040 are expected to be warm enough to allow the beetle species to exist across the entire northeastern United States (Lesk et al. 2017).

Research on the relationship between forest pests and diseases and climate change highlights the potential for increased tree stress and mortality due to interactions involving other stressors such as extreme heat and drought (Sturrock et al. 2011, Trotter & Talbot 2013, Weed et al. 2013). For instance, the already widespread fungal pathogen, Armillaria, could expand further or become more aggressive due to a longer active season, enhanced colonization under warmer and drier conditions, and increased stress on host trees (Dukes et al. 2009, Kliejunas 2011, Sturrock et al. 2011).

Within oak forests, insect pests such as gypsy moth, southern pine beetle, and winter moth are expected to cause more frequent and severe damage under climate change, and new pests present unknown risks:

• Southern pine beetle (Dendroctonus frontalis): Expansion of southern pine beetle populations into the Northeast has also been detected and is expected to increase (Lesk et al. 2017, Dodds et al. 2018). Climates suitable for southern pine beetle are predicted to reach 71% of the red pine (Pinus resinosa) and 48% of jack pine (P. banksiana) forest ranges that extend across more than 706,000 km² in the northeastern United States and northwards (Heuss et al. 2019).
• Gypsy moth (Lymantria dispar dispar): Defoliation due to gypsy moth may hinder oak regeneration by leading to increased mortality in young, small diameter oaks (Morin & Liebhold 2015).
• Winter moth (Operophtera brumata): Expansion of winter moth outbreaks (which have thus far been temperature limited) is expected as average winter temperatures increase and as hybridization with the bruce spanworm improves the winter moth's cold tolerance (Havill et al. 2016, Brinkman 2017, Andersen et al. 2019).