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Climate Change Impacts on Forest Insects

Excerpt from: Régnière, J. 2009. Predicting insect continental distributions from species physiology. Unasylva. 60:37-42.

Insects constitute the most diverse form of animal life in terrestrial ecosystems. Most species are innocuous but essential components of natural ecosystems. Because they are cold-blooded, the rates of key physiological processes in their life cycles are determined by environmental conditions, especially temperature and precipitation. In general they have short generation times, high fecundity, and high mobility (either through their own faculties or aided by wind, animals, and humans). The effects of climate change on forest insects (reviewed in Moore and Allard 2008) must be considered in the context of increasing international trade and changing land use patterns.

The fossil record suggests that previous episodes of rapid global warming led to increased levels of insect herbivory (Currano et al. 2008). Similarly, insect herbivory levels are currently increasing (DeLucia et al. 2008), for example, in the birch forests of northern Europe (Wolf et al. 2008). The reasons include lower plant defenses and higher plant nutritional value in the presence of increased CO2 and O3 (Kopper and Lindroth 2003) and altered seasonal synchrony between plants, insect herbivores, and their natural enemies (van Asch and Visser 2007, Stireman et al. 2005).

Many insects are sensitive to extreme weather events (e.g., droughts, heat waves, cold spells). As a result of climate change and deforestation, tropical environments that harbor the bulk of Earth’s biodiversity could very well become too hot, dry, or fragmented for many insect species to persist (Williams et al. 2003). Species that exhibit highly evolved host plant interactions inhabit microhabitats and are at high risk of extinction, especially in tropical areas (Lewis 2006).

Changing distributions

The climates of temperate and subarctic regions are becoming increasingly hospitable to plant and insect life, raising concerns about the behavior of indigenous species and about the risk of invasion by exotic species, which could result in disruption of normal ecosystem functions. Many temperate-zone insect species have shifted their distributions in response to recent climate change. Examples are the pine processionary moth (Thaumetopoea pityocampa) in Europe (Battisti et al. 2006), winter moth (Operophtera brumata) and autumnal moth (Epirrita autumnata) in Scandinavia (Jepsen et al. 2008), and southern pine beetle (Dendroctonus frontalis) in North America (Tran et al. 2007). Some species that have historically been constrained in their distribution by geographical barriers, such as mountain ranges or large bodies of water, are likely to overcome these barriers and suddenly expand their range. For example, increased movements of warm air masses toward high latitudes have caused recent influxes of diamondback moth (Plutella xylostella) on the Norwegian islands of Svalbard in the Arctic Ocean, 800 km north of the edge of its current distribution in the western Russian Federation (Coulson et al. 2002).

The fate of specific insect species depends on their degree of specialization (host and habitat range), their mobility, and the factors constraining their distribution. For example, specialist butterfly species are declining in abundance in the United Kingdom, while generalist species are increasing (Thomas 2005, Franco et al. 2006). Insect species richness is increasing in the cool habitats of the planet (Andrew and Hughes 2005). Butterfly species found throughout the United Kingdom are decreasing most rapidly in the south while species with a southerly distribution are expanding northward (Conrad et al. 2004). Thus, the geographical range of insect species may be shifting by simultaneous expansion at the upper end and contraction at the lower end of their latitude and altitude limits (Parmesan et al. 1999).

Insect species are also changing their genetic makeup in response to climate change. While genetic change is a normal process in nature, exceptionally rapid alterations have been observed over short periods (in the order of a decade) in morphology related to flight capacity (Hill et al. 1999, Thomas et al. 2001), life history strategies, diapause (dormancy) induction (Burke et al. 2005), developmental physiology (Rank and Dahlhoff 2002), and cold tolerance (Calosi et al. 2008) in species that are changing their range.

Outbreak frequency

Conclusive evidence of changes in outbreak frequencies among forest insect pests in response to climate change is rare because such evidence must be based on long historical records and adequate knowledge of each species’ population dynamics. Considerable information has linked drought stress due to climate change and extensive damage by insects to pinyon pine (Pinus spp.) in the southwestern United States (Trotter et al. 2008).

There is evidence that the regular (eight- to 13-year) outbreak cycles of larch budmoth (Zeiraphera diniana) in Switzerland have stopped since the early 1970s (Esper et al. 2007). Outbreaks of spruce budworm (Choristoneura spp.) in eastern Canada seem to have increased in frequency and severity during the past 200 years (Simard et al. 2006). Climate change can affect the behavior of insect populations in their current range by altering the ecological interactions that regulate them. These effects are difficult to predict in large part because the population dynamics of only a few species are sufficiently understood (Harrington et al. 2001). Even for the most studied species, such as the spruce budworm in North America, the complexity of ecological interactions involved is almost overwhelming (Eveleigh et al. 2007).

Adapted for eXtension.org by Thomas DeGomez, University of Arizona

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