Effects of climate change on plant pests

This section of the report explores the potential effects of climate change on pests and hence on plant health, first in terms of broad trends and then by reviewing the effects on a selection of individual species or groups of species, provided as case studies.

Simulation of future pest risk

Simulation studies to determine future pest risks under climate-change scenarios have mostly employed species-distribution models, population-dynamics models, or hybrids of both (Table 2). Climatic factors considered in these studies include temperature, precipitation and humidity, but elevated CO2 is usually not considered (Eastburn, McElrone and Bilgin, 2011; Juroszek and von Tiedemann, 2015). The effects of climate change are probably easier to predict for those pest species that are mainly affected by temperature. Prediction is more difficult for pests whose reproduction and dispersal are strongly related to water availability, wind and crop management. This is also true for pests that are strongly affected by interactions with other organisms such as vectors of pathogens (Trebicki and Finlay, 2019), unless their interactions are well studied (Juroszek and von Tiedemann, 2013a) and thus predictable (see case study for Xylella fastidiosa).

The outcome of simulations is dependent on the materials and methods used, including the global climate model used, the emission scenarios, the regional climate model, and the specific pest model, together with the precise parameters used in the simulation (Miedaner and Juroszek, 2021a). All of these contribute to the outcome of pest risk projections (Gouache et al., 2013; Juroszek and von Tiedemann, 2013b; Launay et al., 2020) and should be borne in mind when reading and interpreting the results from simulation studies such as those listed in Table 2. In addition, it should be noted that the effect of climate change on pest risk can vary across a country (e.g. lowlands vs mountains, north vs south, summer vs winter, hot and wet vs cool and dry season), as recently highlighted by Miedaner and Juroszek (2021a).

According to Juroszek and von Tiedemann (2015), in general the projected change (increase or decrease) in pest risk will be more pronounced by the end of the twenty-first century than earlier in the century if increasing temperature is the main driver of results. This reflects the fact that global warming is projected to be greater by the end compared to the middle and the beginning of the twenty-first century (e.g. 3 °C vs 2 °C vs 1 °C global temperature increase, respectively).

The projected changes to pest risk vary according to geographical location (Sidorova and Voronina, 2020). For example, in an early simulation study of future pest risk driven by a climate-change scenario, an increased risk of rice blast disease, caused by the fungus Magnaporthe grisea, was predicted for cool, subtropical rice-growing regions such as Japan, whereas in the humid, warm tropics, such as in the Philippines, rice blast risk was predicted to decrease in the future (Luo et al., 1995, 1998). Regarding insect pests, projections by Kocmánková et al. (2011) suggest that the European corn borer (Ostrinia nubilalis) and Colorado potato beetle (Leptinotarsa decemlineata) will probably increase their ranges in many parts of Europe, colonize higher altitudes, and increase their annual number of generations, as a result of a projected temperature increase. On the other hand, climate warming may cause temperature increases which are near the upper lethal limit of some insect species, especially during the summer in temperate climates (Bale and Hayward, 2010; Harvey et al., 2020) and in the already very warm tropics (Deutsch et al., 2008). This variation in impact with geographical location means that generalizations should be treated with extreme caution and researchers need to be very careful when extrapolating their results (Juroszek et al., 2020).

Recently, Seidl et al. (2017) published a comprehensive, global analysis of available results (more than 1 600 single observations) and concluded that around two-thirds of all observations show that the risk of abiotic (e.g. fire, drought) and biotic (e.g. insect pests, pathogens) stress factors will increase in forestry worldwide. Warmer and drier conditions favour disturbances by insects, whereas warmer and wetter conditions favour disturbances from pathogens. The same trend is expected for many crop diseases (e.g. Juroszek and von Tiedemann, 2015), insect pests (e.g. Choudhary, Kumari and Fand, 2019) and weeds (e.g. Clements, DiTommaso and Hyvönen, 2014), with increasing pest risk in most cases. Thus, preventive, mitigation and adaptation measures are needed in the future to reduce the projected increases in pest risk in agriculture, horticulture, forestry as well as in urban areas and national parks (Edmonds, 2013; Pautasso, 2013). There is currently an ongoing debate between conservationist movements and plant-health services on how to treat pest infestations in national parks and protected areas and the emotive subject of whether to intervene in currently unmanaged ecosystems.